Scientific Publications

As part of our engagement and scientific efforts, we're presenting some of the key results from the Exoplanet Science Initiative below.

Papers Published in 2018

  • 2018 - "Buildup of Abiotic Oxygen and Ozone in Moist Atmospheres of Temperate Terrestrial Exoplanets and its Impact on the Spectral Fingerprint in Transit Observations"
    Kleinböhl, Armin; Willacy, Karen; Friedson, A. James; Chen, Pin; Swain, Mark R.

    We investigate the abiotic production of oxygen and its photochemical byproduct ozone through water vapor photolysis in moist atmospheres of temperate terrestrial exoplanets. The amount of water vapor available for photolysis in the middle atmosphere of a planet can be limited by an atmospheric cold-trap, the formation of which largely depends on the amount of non-condensable gases. We study this effect using a photochemical model coupled to a 1D radiative-convective equilibrium model in atmospheres with N2, CO2 and H2O as the main constituents. We find that in atmospheres with a low N2 inventory, water vapor mixing ratios in the middle atmosphere can be over two orders of magnitude higher compared to atmospheres with an Earth-like N2 inventory. Without a strong surface sink, the non-condensable oxygen can build up rapidly, drying out the upper atmosphere. With a moderate surface sink, the planet can approach a steady state with significant oxygen mixing ratios in which oxygen production is balanced by surface uptake. We use a radiative transfer model to study the spectroscopic fingerprint of these atmospheres in transit observations. Spectral signatures of abiotic oxygen and ozone can be of comparable magnitude as in spectra of Earth seen as an exoplanet. Middle atmospheric water vapor is unlikely to be a usable indicator of the abiotic origin of oxygen because of the influence of oxygen on the water vapor distribution. This suggests that atmospheric oxygen and ozone cannot be used as binary bioindicators and their interpretation will likely require atmospheric and planetary models.

  • 2018 - "Three small planets transiting the bright young field star K2-233"
    David, Trevor; Crossfield, Ian; Benneke; Björn; Petigura, Erik; Gonzales, Erica; Schlieder, Joshua; Yu, Liang, Isaacson, Howard; Howard, Andrew; Ciardi, David; Mamajek, Eric; Hillenbrand, Lynne; Cody, Ann Marie; Riedel, Adric; Schwengeler, Hans Martin; Tanner, Christopher; Ende, Martin

    We report the detection of three small transiting planets around the young K3 dwarf K2-233 (2MASS J15215519-2013539) from observations during Campaign 15 of the K2 mission. The star is relatively nearby (d = 69 pc) and bright (V = 10.7 mag, Ks = 8.4 mag), making the planetary system an attractive target for radial velocity follow-up and atmospheric characterization with the James Webb Space Telescope. The inner two planets are hot super-Earths (Rb = 1.40 ± 0.06 R⊕, Rc = 1.34 ± 0.08 R⊕), while the outer planet is a warm sub-Neptune (Rd = 2.6 ± 0.1 R⊕). We estimate the stellar age to be 360 (+490, −140) Myr based on rotation, activity, and kinematic indicators. The K2-233 system is particularly interesting given recent evidence for inflated radii in planets around similarly-aged stars, a trend potentially related to photo-evaporation, core-cooling, or both mechanisms.

  • 2018 - "Discovery of a Transiting Adolescent Sub-Neptune Exoplanet in the Cas-Tau Association with K2"
    David, Trevor J.; Mamajek, Eric E.; Vanderburg, Andrew; Schlieder, Joshua E.; Bristow, Makennah; Petigura, Erik A.; Ciardi, David R.; Crossfield, Ian J. M.; Isaacson, Howard T.; Cody, Ann Marie; Stauffer, John R.; Hillenbrand, Lynne A.; Bieryla, Allyson; Latham, David W.; Fulton, Benjamin J.; Rebull, Luisa M.; Beichman, Chas; Gonzales, Erica J.; Hirsch, Lea A.; Howard, Andrew W.; Vasisht, Gautam; Ygouf, Marie

    The role of stellar age in the measured properties and occurrence rates of exoplanets is not well understood. This is in part due to a paucity of young planets and the uncertainties in age-dating for most exoplanet host stars. Exoplanets belonging to coeval stellar populations, young or old, are particularly useful as benchmarks for studies aiming to constrain the evolutionary timescales relevant for planets. Such timescales may concern orbital migration, gravitational contraction, or atmospheric photo-evaporation, among other mechanisms. Here we report the serendipitous discovery of a transiting sub-Neptune from K2 photometry of the low-mass star EPIC 247267267. From multiple age indicators we estimate the age of the star to be in the range of 50-500 Myr at 68% confidence. However, based on its kinematics, we propose the star belongs to the Cas-Tau association, which we suggest has an age of 46 $\pm$ 8 Myr. Our newly derived age estimate for the association is based on a color-magnitude diagram analysis of high-probability members near the main sequence turnoff. The size of EPIC 247267267 b ($R_P = 3.0 \pm 0.5 R_\oplus$) combined with its youth make it an intriguing case study for photo-evaporation models, which predict enhanced atmospheric mass loss during early evolutionary stages.

  • 2018 - "A gap in the planetesimal disc around HD 107146 and asymmetric warm dust emission revealed by ALMA"
    Marino, S.; Carpenter, J.; Wyatt, M. C.; Booth, M.; Casassus, S.; Faramaz, V.; Guzman, V.; Hughes, A. M.; Kennedy, A. Isella G. M.; Matrà, L.; Ricci, L.; Corder, S.

    While detecting low mass exoplanets at tens of au is beyond current instrumentation, debris discs provide a unique opportunity to study the outer regions of planetary systems. Here we report new ALMA observations of the 80-200 Myr old Solar analogue HD 107146 that reveal the radial structure of its exo-Kuiper belt at wavelengths of 1.1 and 0.86 mm. We find that the planetesimal disc is broad, extending from 40 to 140 au, and it is characterised by a circular gap extending from 60 to 100 au in which the continuum emission drops by about 50%. We also report the non-detection of the CO J=3-2 emission line, confirming that there is not enough gas to affect the dust distribution. To date, HD 107146 is the only gas-poor system showing multiple rings in the distribution of millimeter sized particles. These rings suggest a similar distribution of the planetesimals producing small dust grains that could be explained invoking the presence of one or more perturbing planets. Because the disk appears axisymmetric, such planets should be on circular orbits. By comparing N-body simulations with the observed visibilities we find that to explain the radial extent and depth of the gap, it would be required the presence of multiple low mass planets or a single planet that migrated through the disc. Interior to HD 107146's exo-Kuiper belt we find extended emission with a peak at ~20 au and consistent with the inner warm belt that was previously predicted based on 22$\mu$m excess as in many other systems. This warm belt is the first to be imaged, although unexpectedly suggesting that it is asymmetric. This could be due to a large belt eccentricity or due to clumpy structure produced by resonant trapping with an additional inner planet.

  • 2018 - "Differences in the Gas and Dust Distribution in the Transitional Disk of a Sun-like Young Star, PDS 70"
    Long, Zachary C.; Akiyama, Eiji; Sitko, Michael; Fernandes, Rachel B.; Assani, Korash; Grady, Carol A.; Cure, Michel; Danchi, William C.; Dong, Ruobing; Fukagawa, Misato; Hasegawa, Yasuhiro; Hashimoto, Jun; Henning, Thomas; Inutsuka, Shu-Ichiro; Kraus, Stefan; Kwon, Jungmi; Lisse, Carey M.; Baobabu Liu, Hauyu; Mayama, Satoshi; Muto, Takayuki; Nakagawa, Takao; Takami, Michihiro; Tamura, Motohide; Currie, Thayne; Wisniewski, John P.; Yang, Yi

    We present ALMA 0.87 mm continuum, HCO+ J = 4–3 emission line, and CO J = 3–2 emission line data of the disk of material around the young, Sun-like star PDS 70. These data reveal the existence of a possible two-component transitional disk system with a radial dust gap of 0.″42 ± 0.″05, an azimuthal gap in the HCO+ J = 4–3 moment zero map, as well as two bridge-like features in the gas data. Interestingly these features in the gas disk have no analog in the dust disk making them of particular interest. We modeled the dust disk using the Monte Carlo radiative transfer code HOCHUNK3D using a two-disk component. We find that there is a radial gap that extends from 15 to 60 au in all grain sizes, which differs from previous work.

  • 2018 - "Detection of 40-48 GHz dust continuum linear polarization towards the Class 0 young stellar object IRAS 16293-2422"
    Liu, Hauyu Baobab; Hasegawa, Yasuhiro; Ching, Tao-Chung; Lai, Shih-Ping; Hirano, Naomi; Rao, Ramprasad

    We performed the new JVLA full polarization observations at 40-48 GHz (6.3-7.5 mm) towards the nearby ($d$ $=$147$\pm$3.4 pc) Class 0 YSO IRAS 16293-2422, and compare with the previous SMA observations reported by Rao et al. (2009; 2014). We observed the quasar J1407+2827 which is weakly polarized and can be used as a leakage term calibrator for $<$9 GHz observations, to gauge the potential residual polarization leakage after calibration. We did not detect Stokes Q, U, and V intensities from the observations of J1407+2827, and constrain (3-$\sigma$) the residual polarization leakage after calibration to be $\lesssim$0.3\%. We detect linear polarization from one of the two binary components of our target source, IRAS\,16293-2422\,B. The derived polarization position angles from our observations are in excellent agreement with those detected from the previous observations of the SMA, implying that on the spatial scale we are probing ($\sim$50-1000 au), the physical mechanisms for polarizing the continuum emission do not vary significantly over the wavelength range of $\sim$0.88-7.5 mm. We hypothesize that the observed polarization position angles trace the magnetic field which converges from large scale to an approximately face-on rotating accretion flow. In this scenario, magnetic field is predominantly poloidal on $>$100 au scales, and becomes toroidal on smaller scales. However, this interpretation remains uncertain due to the high dust optical depths at the central region of IRAS\,16293-2422\,B and the uncertain temperature profile. We suggest that dust polarization at wavelengths comparable or longer than 7\,mm may still trace interstellar magnetic field. Future sensitive observations of dust polarization in the fully optically thin regime will have paramount importance for unambiguously resolving the magnetic field configuration.

  • 2018 - "Eccentric Cavity, Triple Rings, Two-Armed Spirals, and Double Clumps of the MWC 758 Disk"
    Dong, Ruobing; Liu, Sheng-yuan; Eisner, Josh; Andrews, Sean; Fung, Jeffrey; Zhu, Zhaohuan; Chiang, Eugene; Hashimoto, Jun; Liu, Hauyu Baobab; Casassus, Simon; Esposito, Thomas; Hasegawa, Yasuhiro; Muto, Takayuki; Pavlyuchenkov, Yaroslav; Wilner, David; Akiyama, Eiji; Tamura, Motohide; Wisniewski, John

    Spatially resolved structures in protoplanetary disks hint at unseen planets. Previous imaging observations of the transitional disk around MWC 758 revealed an inner cavity, a ring-like outer disk, emission clumps, and spiral arms, all possibly generated by companions. We present ALMA dust continuum observations of MWC 758 at 0.87 millimeter (mm) wavelength with 43$\times$39 mas angular resolution (6.9$\times$6.2 AU) and 20 $\mu$Jy beam$^{-1}$ rms. The central sub-mm emission cavity is revealed to be eccentric; once deprojected, its outer edge can be well-fitted by an ellipse with an eccentricity of 0.1 and one focus on the star. The broad ring-like outer disk is resolved into three narrow rings with two gaps in between. The outer two rings tentatively show the same eccentricity and orientation as the innermost ring bounding the inner cavity. The two previously known dust emission clumps are resolved in both the radial and azimuthal directions, with radial widths equal to $\sim$4$\times$ the local scale height. Only one of the two spiral arms previously imaged in near-infrared (NIR) scattered light is revealed in ALMA dust emission, at a slightly larger stellocentric distance owing to projection effects. We also submit evidence of disk truncation at $\sim$100 AU based on comparing NIR imaging observations with models. The spirals, the north clump, and the truncated disk edge are all broadly consistent with the presence of one companion exterior to the spirals at roughly 100 AU.

  • 2018 - "Validation and Initial Characterization of the Long-period Planet Kepler-1654 b"
    Beichman, C. A.; Giles, H. A. C.; Akeson, R.; Ciardi, D.; Christiansen, J.; Isaacson, H.; Marcy, G. M.; Sinukoff, E.; Greene, T.; Fortney, J. J.; Crossfield, I.; Hu, R.; Howard, A. W.; Petigura, E. A.; Knutson, H. A.

    Fewer than 20 transiting Kepler planets have periods longer than one year. Our early search of the Kepler light curves revealed one such system, Kepler-1654b (originally KIC 8410697b), which shows exactly two transit events and whose second transit occurred only five days before the failure of the second of two reaction wheels brought the primary Kepler mission to an end. A number of authors have also examined light curves from the Kepler mission searching for long-period planets and identified this candidate. Starting in 2014 September, we began an observational program of imaging, reconnaissance spectroscopy, and precision radial velocity (RV) measurements that confirm with a high degree of confidence that Kepler-1654b is a bona fide transiting planet orbiting a mature G5V star (T eff = 5580 K, [Fe/H] = ‑0.08) with a semimajor axis of 2.03 au, a period of 1047.84 days, and a radius of 0.82 ± 0.02 R Jup. RV measurements using Keck’s HIRES spectrometer obtained over 2.5 years set a limit to the planet’s mass of >0.5 (3σ) M Jup. The bulk density of the planet is similar to that of Saturn or possibly lower. We assess the suitability of temperate gas giants like Kepler-1654b for transit spectroscopy with the James Webb Space Telescope, as their relatively cold equilibrium temperatures (T pl ∼ 200 K) make them interesting from the standpoint of exoplanet atmospheric physics. Unfortunately, these low temperatures also make the atmospheric scale heights small and thus transmission spectroscopy challenging. Finally, the long time between transits can make scheduling JWST observations difficult—as is the case with Kepler-1654b.

  • 2018 - "Signatures of Lithospheric Flexure and Elevated Heat Flow in Stereo Topography at Coronae on Venus"
    O'Rourke, Joseph G.; Smrekar, Suzanne E.

    Signatures of lithospheric flexure were previously identified at a dozen or more large coronae on Venus. Thin plate models fit to topographic profiles return elastic parameters, allowing derivation of mechanical thickness and surface heat flows given an assumed yield strength envelope. However, the low resolution of altimetry data from the NASA Magellan mission has hindered studying the vast majority of coronae, particularly those less than a few hundred kilometers in diameter. Here we search for flexural signatures around 99 coronae over ˜20% of the surface in Magellan altimetry data and stereo-derived topography that was recently assembled from synthetic aperture radar images. We derive elastic thicknesses of ˜2 to 30 km (mostly ˜5 to 15 km) with Cartesian and axisymmetric models at 19 coronae. We discuss the implications of low values that were also noted in earlier gravity studies. Most mechanical thicknesses are estimated as >19 km, corresponding to thermal gradients <24 K km-1. Implied surface heat flows >95 mW m-2—twice the global average in many thermal evolution models—imply that coronae are major contributors to the total heat budget or Venus is cooling faster than expected. Binomial statistics show that "Type 2" coronae with incomplete fracture annuli are significantly less likely to host flexural signatures than "Type 1" coronae with largely complete annuli. Stress calculations predict extensional faulting where nearly all profiles intersect concentric fractures. We failed to identify systematic variations in flexural parameters based on type, geologic setting, or morphologic class. Obtaining quality, high-resolution topography from a planet wide survey is vital to verifying our conclusions.

  • 2018 - "Hubble PanCET: An isothermal day-side atmosphere for the bloated gas-giant HAT-P-32Ab"
    Nikolov, N.; Sing, D. K.; Goyal, J.; Henry, G. W.; Wakeford, H. R.; Evans, T. M.; Lopez-Morales, M.; Garcia Munoz, A.; Ben-Jaffel, L.; Sanz-Forcada, J.; Ballester, G. E.; Kataria, T.; Barstow, J. K.; Bourrier, V.; Buchhave, L. A.; Cohen, O.; Deming, D.; Ehrenreich, D.; Knutson, H.; Lavvas, P; Lecavelier des Etangs, A.; Lewis, N. K.; Williamson, A. M. Mandell M. H.

    We present a thermal emission spectrum of the bloated hot Jupiter HAT-P-32Ab from a single eclipse observation made in spatial scan mode with the Wide Field Camera 3 (WFC3) aboard the Hubble Space Telescope (HST). The spectrum covers the wavelength regime from 1.123 to 1.644 microns which is binned into 14 eclipse depths measured to an averaged precision of 104 parts-per million. The spectrum is unaffected by a dilution from the close M-dwarf companion HAT-P-32B, which was fully resolved. We complemented our spectrum with literature results and performed a comparative forward and retrieval analysis with the 1D radiative-convective ATMO model. Assuming solar abundance of the planet atmosphere, we find that the measured spectrum can best be explained by the spectrum of a blackbody isothermal atmosphere with Tp = 1995 +/- 17K, but can equally-well be described by a spectrum with modest thermal inversion. The retrieved spectrum suggests emission from VO at the WFC3 wavelengths and no evidence of the 1.4 micron water feature. The emission models with temperature profiles decreasing with height are rejected at a high confidence. An isothermal or inverted spectrum can imply a clear atmosphere with an absorber, a dusty cloud deck or a combination of both. We find that the planet can have continuum of values for the albedo and recirculation, ranging from high albedo and poor recirculation to low albedo and efficient recirculation. Optical spectroscopy of the planet's day-side or thermal emission phase curves can potentially resolve the current albedo with recirculation degeneracy.

  • 2018 - "Phase curves of WASP-33b and HD 149026b and a New Correlation Between Phase Curve Offset and Irradiation Temperature"
    Zhang, Michael; Knutson, Heather A.; Kataria, Tiffany; Schwartz, Joel C.; Cowan, Nicolas B.; Showman, Adam P.; Burrows, Adam; Fortney, Jonathan J.; Todorov, Kamen; Desert, Jean-Michel; Agol, Eric; Deming, Drake

    We present new 3.6 and 4.5 $\mu m$ Spitzer phase curves for the highly irradiated hot Jupiter WASP-33b and the unusually dense Saturn-mass planet HD 149026b. As part of this analysis, we develop a new variant of pixel level decorrelation that is effective at removing intrapixel sensitivity variations for long observations (>10 hours) where the position of the star can vary by a significant fraction of a pixel. Using this algorithm, we measure eclipse depths, phase amplitudes, and phase offsets for both planets at 3.6 $\mu m$ and 4.5 $\mu m$. We use a simple toy model to show that WASP-33b's phase offset, albedo, and heat recirculation efficiency are largely similar to those of other hot Jupiters despite its very high irradiation. On the other hand, our fits for HD 149026b prefer a very high albedo and an unusually high recirculation efficiency. We also compare our results to predictions from general circulation models, and find that while neither are a good match to the data, the discrepancies for HD 149026b are especially large. We speculate that this may be related to its high bulk metallicity, which could lead to enhanced atmospheric opacities and the formation of reflective cloud layers in localized regions of the atmosphere. We then place these two planets in a broader context by exploring relationships between the temperatures, albedos, heat transport efficiencies, and phase offsets of all planets with published thermal phase curves. We find a striking relationship between phase offset and irradiation temperature--the former drops with increasing temperature until around 3400 K, and rises thereafter. Although some aspects of this trend are mirrored in the circulation models, there are notable differences that provide important clues for future modeling efforts.

Papers Published in 2017

  • 2017 - "An analytical formalism accounting for clouds and other 'surfaces' for exoplanet transmission spectroscopy"
    Bétrémieux, Yan; Swain, Mark R.

    Journal: Monthly Notices of the Royal Astronomical Society

    DOI: 10.1093/mnras/stx257

    Although the formalism of Lecavelier des Etangs et al. is extremely useful to understand what shapes transmission spectra of exoplanets, it does not include the effects of a sharp change in flux with altitude generally associated with surfaces and optically thick clouds. Recent advances in understanding the effects of refraction in exoplanet transmission spectra have, however, demonstrated that even clear thick atmospheres have such a sharp change in flux due to a refractive boundary. We derive a more widely applicable analytical formalism by including first-order effects from all these 'surfaces' to compute an exoplanet's effective radius, effective atmospheric thickness and spectral modulation for an atmosphere with a constant scaleheight. We show that the effective radius cannot be located below these 'surfaces' and that our formalism matches the formalism of Lecavelier des Etangs et al. in the case of a clear atmosphere. Our formalism explains why clouds and refraction reduce the contrast of spectral features, and why refraction decreases the Rayleigh scattering slope as wavelength increases, but also shows that these are common effects of all 'surfaces'. We introduce the concept of a 'surface' cross-section, the minimum mean cross-section that can be observed, as an index to characterize the location of 'surfaces' and provide a simple method to estimate their effects on the spectral modulation of homogeneous atmospheres. We finally devise a numerical recipe that extends our formalism to atmospheres with a non-constant scaleheight and arbitrary sources of opacity, a potentially necessary step to interpret observations.

  • 2017 - "A Case for an Atmosphere on Super-Earth 55 Cancri e"
    Angelo, Isabel; Hu, Renyu

    One of the primary questions when characterizing Earth-sized and super-Earth-sized exoplanets is whether they have a substantial atmosphere like Earth and Venus or a bare-rock surface like Mercury. Phase curves of the planets in thermal emission provide clues to this question, because a substantial atmosphere would transport heat more efficiently than a bare-rock surface. Analyzing phase-curve photometric data around secondary eclipses has previously been used to study energy transport in the atmospheres of hot Jupiters. Here we use phase curve, Spitzer time-series photometry to study the thermal emission properties of the super-Earth exoplanet 55 Cancri e. We utilize a semianalytical framework to fit a physical model to the infrared photometric data at 4.5 μm. The model uses parameters of planetary properties including Bond albedo, heat redistribution efficiency (I.e., ratio between radiative timescale and advective timescale of the atmosphere), and the atmospheric greenhouse factor. The phase curve of 55 Cancri e is dominated by thermal emission with an eastward-shifted hotspot. We determine the heat redistribution efficiency to be {1.47}-0.25+0.30, which implies that the advective timescale is on the same order as the radiative timescale. This requirement cannot be met by the bare-rock planet scenario because heat transport by currents of molten lava would be too slow. The phase curve thus favors the scenario with a substantial atmosphere. Our constraints on the heat redistribution efficiency translate to an atmospheric pressure of ˜1.4 bar. The Spitzer 4.5 μm band is thus a window into the deep atmosphere of the planet 55 Cancri e.

  • 2017 - "Observing Exoplanets with High Dispersion Coronagraphy. I. The Scientific Potential of Current and Next-generation Large Ground and Space Telescopes"
    Wang, Ji; Mawet, Dimitri; Ruane, Garreth; Hu, Renyu; Benneke, Björn

    Direct imaging of exoplanets presents a formidable technical challenge owing to the small angular separation and high contrast between exoplanets and their host stars. High Dispersion Coronagraphy (HDC) is a pathway to achieve unprecedented sensitivity to Earth-like planets in the habitable zone. Here, we present a framework to simulate HDC observations and data analyses. The goal of these simulations is to perform a detailed analysis of the trade-off between raw star light suppression and spectral resolution for various instrument configurations, target types, and science cases. We predict the performance of an HDC instrument at Keck observatory for characterizing directly imaged gas-giant planets in near-infrared bands. We also simulate HDC observations of an Earth-like planet using next-generation ground-based (TMT) and spaced-base telescopes (HabEx and LUVOIR). We conclude that ground-based ELTs are more suitable for HDC observations of an Earth-like planet than future space-based missions owing to the considerable difference in collecting area. For ground-based telescopes, HDC observations can detect an Earth-like planet in the habitable zone around an M-dwarf star at 10-4 star light suppression level. Compared to the 10-7 planet/star contrast, HDC relaxes the star light suppression requirement by a factor of 103. For space-based telescopes, detector noise will be a major limitation at spectral resolutions higher than 104. Considering detector noise and speckle chromatic noise, R = 400 (1600) is the optimal spectral resolutions for HabEx (LUVOIR). The corresponding star light suppression requirement to detect a planet with planet/star contrast = 6.1× {10}-11 is relaxed by a factor of 10 (100) for HabEx (LUVOIR).

  • 2017 - "Noise-weighted angular differential imaging"
    Bottom, Michael; Ruane, Garreth; Mawet, Dimitri;

    Angular differential imaging (ADI) (Marois et al. 2006) is an observational technique in high contrast imaging where the telescope is used in pupil tracking mode so that the image of the sky rotates with respect to the optical surfaces. Bright "speckle" light caused by optical errors remains fixed in the image, while planets and disks rotate with the sky. The resulting dataset is then post-processed to remove the speckles, de-rotated to undo the sky motion, and median-collapsed to create a final data product. The postprocessing algorithms to remove the speckles are an active area of research and beyond the scope of this note. We consider the derotation and median-combination, where we show gains in signal-to-noise ratio are possible with a small change to the algorithm.

  • 2017 - "Quantifying the Impact of Spectral Coverage on the Retrieval of Molecular Abundances from Exoplanet Transmission Spectra"
    Chapman, John W.; Zellem, Robert T.; Line, Michael R.; Vasisht, Gautam; Bryden, Geoff; Willacy, Karen; Iyer, Aishwarya R.; Bean, Jacob; Cowan, Nicolas B.; Fortney, Jonathan J.; Griffith, Caitlin A.; Kataria, Tiffany; Kempton, Eliza M.-R.; Kreidberg, Laura; Moses, Julianne I.; Stevenson, Kevin B.; Swain, Mark R.

    Using forward models for representative exoplanet atmospheres and a radiometric instrument model, we generated synthetic observational data to explore how well the major C- and O-bearing chemical species (CO, CO2, CH4, and H2O), important for determining atmospheric opacity and radiation balance, can be constrained by transit measurements as a function of spectral wavelength coverage. This work features simulations for a notional transit spectroscopy mission and compares two cases for instrument spectral coverage (wavelength coverage from 0.5-2.5 μm and 0.5-5 μm). The simulation is conducted on a grid with a range of stellar magnitudes and incorporates a full retrieval of atmospheric model parameters. We consider a range of planets from sub-Neptunes to hot Jupiters and include both low and high mean molecular weight atmospheres. We find that including the 2.5-5 μm wavelength range provides a significant improvement in the degree of constraint on the retrieved molecular abundances: up to ˜3 orders of magnitude for a low mean molecular weight atmosphere (μ = 2.3) and up to a factor of ˜6 for a high mean molecular weight atmosphere (μ = 28). These decreased uncertainties imply that broad spectral coverage between the visible and the mid-infrared is an important tool for understanding the chemistry and composition of exoplanet atmospheres. This analysis suggests that the James Webb Space Telescope's (JWST) Near-Infrared Spectrograph (NIRSpec) 0.6-5 μm prism spectroscopy mode, or similar wavelength coverage with possible future missions, will be an important resource for exoplanet atmospheric characterization.

  • 2017 - "Magnetically Induced Disk Winds and Transport in the HL Tau Disk"
    Hasegawa, Yasuhiro; Okuzumi, Satoshi; Flock, Mario; Turner, Neal J.

    The mechanism of angular momentum transport in protoplanetary disks is fundamental to understanding the distributions of gas and dust in the disks. The unprecedented ALMA observations taken toward HL Tau at high spatial resolution and subsequent radiative transfer modeling reveal that a high degree of dust settling is currently achieved in the outer part of the HL Tau disk. Previous observations, however, suggest a high disk accretion rate onto the central star. This configuration is not necessarily intuitive in the framework of the conventional viscous disk model, since efficient accretion generally requires a high level of turbulence, which can suppress dust settling considerably. We develop a simplified, semi-analytical disk model to examine under what condition these two properties can be realized in a single model. Recent, non-ideal MHD simulations are utilized to realistically model the angular momentum transport both radially via MHD turbulence and vertically via magnetically induced disk winds. We find that the HL Tau disk configuration can be reproduced well when disk winds are properly taken into account. While the resulting disk properties are likely consistent with other observational results, such an ideal situation can be established only if the plasma β at the disk midplane is β 0 ≃ 2 × 104 under the assumption of steady accretion. Equivalently, the vertical magnetic flux at 100 au is about 0.2 mG. More detailed modeling is needed to fully identify the origin of the disk accretion and quantitatively examine plausible mechanisms behind the observed gap structures in the HL Tau disk.

  • 2017 - "A concordant scenario to explain FU Orionis from deep centimeter and millimeter interferometric observations"
    Liu, Hauyu Baobab; Vorobyov, Eduard I.; Dong, Ruobing; Dunham, Michael M.; Takami, Michihiro; Galván-Madrid, Roberto; Hashimoto, Jun; Kóspál, Ágnes; Henning, Thomas; Tamura, Motohide; Rodríguez, Luis F.; Hirano, Naomi; Hasegawa, Yasuhiro; Fukagawa, Misato; Carrasco-Gonzalez, Carlos; Tazzari, Marco

    Aims: The aim of this work is to constrain properties of the disk around the archetype FU Orionis object, FU Ori, with as good as 25 au resolution.

    Methods: We resolved FU Ori at 29-37 GHz using the Karl G. Jansky Very Large Array (JVLA) in the A-array configuration, which provided the highest possible angular resolution to date at this frequency band ( 0.07 arcsec). We also performed complementary JVLA 8-10 GHz observations, Submillimeter Array (SMA) 224 GHz and 272 GHz observations, and compared these with archival Atacama Large Millimeter Array (ALMA) 346 GHz observations to obtain the spectral energy distributions (SEDs).

    Results: Our 8-10 GHz observations do not find evidence for the presence of thermal radio jets, and constrain the radio jet/wind flux to at least 90 times lower than the expected value from the previously reported bolometric luminosity-radio luminosity correlation. The emission at frequencies higher than 29 GHz may be dominated by the two spatially unresolved sources, which are located immediately around FU Ori and its companion FU Ori S, respectively. Their deconvolved radii at 33 GHz are only a few au, which is two orders of magnitude smaller in linear scale than the gaseous disk revealed by the previous Subaru-HiCIAO 1.6 μm coronagraphic polarization imaging observations. We are struck by the fact that these two spatially compact sources contribute to over 50% of the observed fluxes at 224 GHz, 272 GHz, and 346 GHz. The 8-346 GHz SEDs of FU Ori and FU Ori S cannot be fit by constant spectral indices (over frequency), although we cannot rule out that it is due to the time variability of their (sub)millimeter fluxes. Conclusions: The more sophisticated models for SEDs considering the details of the observed spectral indices in the millimeter bands suggest that the >29 GHz emission is contributed by a combination of free-free emission from ionized gas and thermal emission from optically thick and optically thin dust components. We hypothesize that dust in the innermost parts of the disks (≲0.1 au) has been sublimated, and thus the disks are no longer well shielded against the ionizing photons. The estimated overall gas and dust mass based on SED modeling, can be as high as a fraction of a solar mass, which is adequate for developing disk gravitational instability. Our present explanation for the observational data is that the massive inflow of gas and dust due to disk gravitational instability or interaction with a companion/intruder, was piled up at the few-au scale due to the development of a deadzone with negligible ionization. The piled up material subsequently triggered the thermal instability and the magnetorotational instability when the ionization fraction in the inner sub-au scale region exceeded a threshold value, leading to the high protostellar accretion rate.

  • 2017 - "Systematic Analysis of Spectral Energy Distributions and the Dust Opacity Indices for Class 0 Young Stellar Objects"
    Li, Jennifer I-Hsiu; Liu, Hauyu Baobab; Hasegawa, Yasuhiro; Hirano, Naomi

    We are motivated by the recent measurements of dust opacity indices (β) around young stellar objects (YSOs), which suggest that efficient grain growth may have occurred earlier than the Class I stage. The present work makes use of abundant archival interferometric observations at submillimeter, millimeter, and centimeter wavelength bands to examine grain growth signatures in the dense inner regions (<1000 au) of nine Class 0 YSOs. A systematic data analysis is performed to derive dust temperatures, optical depths, and dust opacity indices based on single-component modified blackbody fittings to the spectral energy distributions (SEDs). The fitted dust opacity indices (β) are in a wide range of 0.3-2.0 when single-component SED fitting is adopted. Four out of the nine observed sources show β lower than 1.7, the typical value of the interstellar dust. Low dust opacity index (or spectral index) values may be explained by the effect of dust grain growth, which makes β < 1.7. Alternatively, the very small observed values of β may be interpreted by the presence of deeply embedded and hot inner disks, which only significantly contribute to the observed fluxes at long wavelength bands. This possibility can be tested by the higher angular resolution imaging observations of ALMA or more detailed sampling of SEDs in the millimeter and centimeter bands. The β values of the remaining five sources are close to or consistent with 1.7, indicating that grain growth would start to significantly reduce the values of β no earlier than the late Class 0 stage for these YSOs.

  • 2017 - "The Shadow Knows: Using Shadows to Investigate the Structure of the Pretransitional Disk of HD 100453"
    Long, Zachary C.; Fernandes, Rachel B.; Sitko, Michael; Wagner, Kevin; Muto, Takayuki; Hashimoto, Jun; Follette, Katherine; Grady, Carol A.; Fukagawa, Misato; Hasegawa, Yasuhiro; Kluska, Jacques; Kraus, Stefan; Mayama, Satoshi; McElwain, Michael W.; Oh, Daehyon; Tamura, Motohide; Uyama, Taichi; Wisniewski, John P.; Yang, Yi

    We present Gemini Planet Imager polarized intensity imagery of HD 100453 in Y, J, and K1 bands that reveals an inner gap (9-18 au), an outer disk (18-39 au) with two prominent spiral arms, and two azimuthally localized dark features that are also present in Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) total intensity images. Spectral energy distribution fitting further suggests that the radial gap extends to 1 au. The narrow, wedge-like shape of the dark features appears similar to predictions of shadows cast by an inner disk that is misaligned with respect to the outer disk. Using the Monte Carlo radiative transfer code HOCHUNCK3D, we construct a model of the disk that allows us to determine its physical properties in more detail. From the angular separation of the features, we measure the difference in inclination between the disks (45°) and their major axes, PA = 140° east of north for the outer disk, and 100° for the inner disk. We find an outer-disk inclination of 25° ± 10° from face-on, in broad agreement with the Wagner et al. measurement of 34°. SPHERE data in J and H bands indicate a reddish disk, which indicates that HD 100453 is evolving into a young debris disk.

  • 2017 - "Chondrule Accretion with a Growing Protoplanet"
    Matsumoto, Yuji; Oshino, Shoichi; Hasegawa, Yasuhiro; Wakita, Shigeru

    Chondrules are primitive materials in the solar system. They were formed in about the first 3 Myr of the solar system's history. This timescale is longer than that of Mars formation, and it is conceivable that protoplanets, planetesimals, and chondrules might have existed simultaneously in the solar nebula. Due to protoplanets' perturbation on the planetesimal dynamics and chondrule accretion on them, all the formed chondrules are unlikely to be accreted by the planetesimals. We investigate the amount of chondrules accreted by planetesimals in such a condition. We assume that a protoplanet is in oligarchic growth, and we perform analytical calculations of chondrule accretion by both a protoplanet and planetesimals. Through the oligarchic growth stage, planetesimals accrete about half of the formed chondrules. The smallest planetesimals get the largest amount of chondrules, compared with the amount accreted by more massive planetesimals. We perform a parameter study and find that this fraction is not greatly changed for a wide range of parameter sets.

  • 2017 - "Effects of Grain Growth on Molecular Abundances in Young Stellar Objects"
    Harada, Nanase; Hasegawa, Yasuhiro; Aikawa, Yuri; Hirashita, Hiroyuki; Liu, Haoyu Baobab; Hirano, Naomi

    Recent observations suggested that the growth of dust grains may have already occurred in class 0/I young stellar objects (YSOs). Since chemical reactions on dust grain surfaces are important in determining molecular abundances, the dust size growth may affect chemical compositions in YSOs significantly. In this work, we aim to determine how grain growth affects chemical abundances. We use a time-dependent gas-grain chemical model for a star-forming core to calculate the gas-phase and grain-surface chemical abundances with variation of surface areas of grains to imitate grain growth. We also perform parameter studies in which the initial molecular abundances vary. Our results show that a smaller extent of the surface areas caused by grain growth changes the dominant form of sulfur-bearing molecules by decreasing H2S abundances and increasing SO and/or SO2 abundances. We also find that complex organic molecules such as CH3CN decrease in abundances with larger grain sizes, while the abundance of other species such as CH3OCH3 is dependent on other parameters such as the initial conditions. Comparisons with observations of a class 0 protostar, IRAS 16293-2422, indicate that the observed abundance ratios between sulfur-bearing molecules H2S, SO, and SO2 can be reproduced very well when dust grains grow to a maximum grain size of a max = 10-100 μm.

  • 2017 - "Diffusion of Oxygen Isotopes in Thermally Evolving Planetesimals and Size Ranges of Presolar Silicate Grains"
    Wakita, Shigeru; Nozawa, Takaya; Hasegawa, Yasuhiro

    Presolar grains are small particles found in meteorites through their isotopic compositions, which are considerably different from those of materials in the solar system. If some isotopes in presolar grains diffused out beyond their grain sizes when they were embedded in parent bodies of meteorites, their isotopic compositions could be washed out, and hence the grains could no longer be identified as presolar grains. We explore this possibility for the first time by self-consistently simulating the thermal evolution of planetesimals and the diffusion length of 18O in presolar silicate grains. Our results show that presolar silicate grains smaller than ˜0.03 μm cannot keep their original isotopic compositions even if the host planetesimals experienced a maximum temperature as low as 600 °C. Since this temperature corresponds to that experienced by petrologic type 3 chondrites, isotopic diffusion can constrain the size of presolar silicate grains discovered in such chondrites to be larger than ˜0.03 μm. We also find that the diffusion length of 18O reaches ˜0.3-2 μm in planetesimals that were heated up to 700-800°C. This indicates that, if the original size of presolar grains spans a range from ˜0.001 μm to ˜0.3 μm like that in the interstellar medium, then the isotopic records of the presolar grains may be almost completely lost in such highly thermalized parent bodies. We propose that isotopic diffusion could be a key process to control the size distribution and abundance of presolar grains in some types of chondrites.

  • 2017 - "Planetesimal Collisions as a Chondrule Forming Event"
    Wakita, Shigeru; Matsumoto, Yuji; Oshino, Shoichi; Hasegawa, Yasuhiro

    Chondritic meteorites contain unique spherical materials named chondrules: sub-mm sized silicate grains once melted in a high temperature condition in the solar nebula. We numerically explore one of the chondrule forming processes-planetesimal collisions. Previous studies have found that impact jetting via protoplanet-planetesimal collisions can make chondrules with 1% of the impactors' mass, when the impact velocity exceeds 2.5 km s-1. Based on the mineralogical data of chondrules, undifferentiated planetesimals would be more suitable for chondrule-forming collisions than potentially differentiated protoplanets. We examine planetesimal-planetesimal collisions using a shock physics code and find two things: one is that planetesimal-planetesimal collisions produce nearly the same amount of chondrules as protoplanet-planetesimal collisions (˜1%). The other is that the amount of produced chondrules becomes larger as the impact velocity increases when two planetesimals collide with each other. We also find that progenitors of chondrules can originate from deeper regions of large targets (planetesimals or protoplanets) than small impactors (planetesimals). The composition of targets is therefore important, to fully account for the mineralogical data of currently sampled chondrules.

  • 2017 - "The Complete transmission spectrum of WASP-39b with a precise water constraint"
    Wakeford, Hannah R.; Sing, David K.; Deming, Drake; Lewis, Nikole K.; Goyal, Jayesh; Wilson, Tom J.; Barstow, Joanna; Kataria, Tiffany; Drummond, Benjamin; Evans, Thomas M.; Carter, Aarynn L.; Nikolov, Nikolay; Knutson, Heather A.; Ballester, Gilda E.; Mandell, Avi M.

    WASP-39b is a hot Saturn-mass exoplanet with a predicted clear atmosphere based on observations in the optical and infrared. Here we complete the transmission spectrum of the atmosphere with observations in the near-infrared (NIR) over three water absorption features with the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) G102 (0.8-1.1 microns) and G141 (1.1-1.7 microns) spectroscopic grisms. We measure the predicted high amplitude H2O feature centered at 1.4 microns, and the smaller amplitude features at 0.95 and 1.2 microns, with a maximum water absorption amplitude of 2.4 planetary scale heights. We incorporate these new NIR measurements into previously published observational measurements to complete the transmission spectrum from 0.3-5 microns. From these observed water features, combined with features in the optical and IR, we retrieve a well constrained temperature Teq = 1030(+30,-20) K, and atmospheric metallicity 151 (+48,-46)x solar which is relatively high with respect to the currently established mass-metallicity trends. This new measurement in the Saturn-mass range hints at further diversity in the planet formation process relative to our solar system giants.

  • 2017 - "The Science Case for an Extended Spitzer Mission"
    Yee, Jennifer C.; Fazio, Giovanni G.; Benjamin, Robert; Kirkpatrick, J. Davy; Malkan, Matt A.; Trilling, David; Carey, Sean; Ciardi, David R.; Apai, Daniel; Ashby, M. L. N.; Ballard, Sarah; Bean, Jacob L.; Beatty, Thomas; Berta-Thompson, Zach; Capak, P.; Charbonneau, David; Chesley, Steven; Cowan, Nicolas B.; Crossfield, Ian; Cushing, Michael C.; de Wit, Julien; Deming, Drake; Dickinson, M.; Dittmann, Jason; Dragomir, Diana; Dressing, Courtney; Emery, Joshua; Faherty, Jacqueline K.; Gagne, Jonathan; Gaudi, B. Scott; Gillon, Michael; Grillmair, Carl J.; Harris, Alan; Hora, Joseph; Ingalls, James G.; Kataria, Tiffany; Kreidberg, Laura; Krick, Jessica E.; Lowrance, Patrick J.; Mahoney, William A.; Metchev, Stanimir A.; Mommert, Michael; Migo Mueller, Michael; Shvartzvald, Yossi; Smith, Howard; Stevenson, Kevin B.; Teplitz, H. I.; Willner, S. P.

    Although the final observations of the Spitzer Warm Mission are currently scheduled for March 2019, it can continue operations through the end of the decade with no loss of photometric precision. As we will show, there is a strong science case for extending the current Warm Mission to December 2020. Spitzer has already made major impacts in the fields of exoplanets (including microlensing events), characterizing near Earth objects, enhancing our knowledge of nearby stars and brown dwarfs, understanding the properties and structure of our Milky Way galaxy, and deep wide-field extragalactic surveys to study galaxy birth and evolution. By extending Spitzer through 2020, it can continue to make ground-breaking discoveries in those fields, and provide crucial support to the NASA flagship missions JWST and WFIRST, as well as the upcoming TESS mission, and it will complement ground-based observations by LSST and the new large telescopes of the next decade. This scientific program addresses NASA's Science Mission Directive's objectives in astrophysics, which include discovering how the universe works, exploring how it began and evolved, and searching for life on planets around other stars.

  • 2017 - "The Very Low Albedo of WASP-12b from Spectral Eclipse Observations with Hubble"
    Bell, Taylor J.; Nikolov, Nikolay; Cowan, Nicolas B.; Barstow, Joanna K.; Barman, Travis S.; Crossfield, Ian J. M.; Gibson, Neale P.; Evans, Thomas M.; Sing, David K.; Knutson, Heather A.; Kataria, Tiffany; Lothringer, Joshua D.; Benneke, Björn; Schwartz, Joel C.

    We present an optical eclipse observation of the hot Jupiter WASP-12b using the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. These spectra allow us to place an upper limit of {A}g< 0.064 (97.5% confidence level) on the planet's white light geometric albedo across 290-570 nm. Using six wavelength bins across the same wavelength range also produces stringent limits on the geometric albedo for all bins. However, our uncertainties in eclipse depth are ˜40% greater than the Poisson limit and may be limited by the intrinsic variability of the Sun-like host star-the solar luminosity is known to vary at the 10-4 level on a timescale of minutes. We use our eclipse depth limits to test two previously suggested atmospheric models for this planet: Mie scattering from an aluminum-oxide haze or cloud-free Rayleigh scattering. Our stringent nondetection rules out both models and is consistent with thermal emission plus weak Rayleigh scattering from atomic hydrogen and helium. Our results are in stark contrast with those for the much cooler HD 189733b, the only other hot Jupiter with spectrally resolved reflected light observations; those data showed an increase in albedo with decreasing wavelength. The fact that the first two exoplanets with optical albedo spectra exhibit significant differences demonstrates the importance of spectrally resolved reflected light observations and highlights the great diversity among hot Jupiters.

  • 2017 - "An ultrahot gas-giant exoplanet with a stratosphere"
    Evans, Thomas M.; Sing, David K.; Kataria, Tiffany; Goyal, Jayesh; Nikolov, Nikolay; Wakeford, Hannah R.; Deming, Drake; Marley, Mark S.; Amundsen, David S.; Ballester, Gilda E.; Barstow, Joanna K.; Ben-Jaffel, Lotfi; Bourrier, Vincent; Buchhave, Lars A.; Cohen, Ofer; Ehrenreich, David; García Muñoz, Antonio; Henry, Gregory W.; Knutson, Heather; Lavvas, Panayotis; Lecavelier Des Etangs, Alain; Lewis, Nikole K.; López-Morales, Mercedes; Mandell, Avi M.; Sanz-Forcada, Jorge; Tremblin, Pascal; Lupu, Roxana

    Infrared radiation emitted from a planet contains information about the chemical composition and vertical temperature profile of its atmosphere. If upper layers are cooler than lower layers, molecular gases will produce absorption features in the planetary thermal spectrum. Conversely, if there is a stratosphere-where temperature increases with altitude-these molecular features will be observed in emission. It has been suggested that stratospheres could form in highly irradiated exoplanets, but the extent to which this occurs is unresolved both theoretically and observationally. A previous claim for the presence of a stratosphere remains open to question, owing to the challenges posed by the highly variable host star and the low spectral resolution of the measurements. Here we report a near-infrared thermal spectrum for the ultrahot gas giant WASP-121b, which has an equilibrium temperature of approximately 2,500 kelvin. Water is resolved in emission, providing a detection of an exoplanet stratosphere at 5σ confidence. These observations imply that a substantial fraction of incident stellar radiation is retained at high altitudes in the atmosphere, possibly by absorbing chemical species such as gaseous vanadium oxide and titanium oxide.

  • 2017 - "VLT/FORS2 comparative transmission spectroscopy II: Confirmation of a cloud deck and Rayleigh scattering in WASP-31b, but no potassium"
    Gibson, Neale P.; Nikolov, Nikolay; Sing, David K.; Barstow, Joanna K.; Evans, Thomas M.; Kataria, Tiffany; Wilson, Paul A.

    We present transmission spectroscopy of the hot-Jupiter WASP-31b using the FOcal Reducer and low dispersion Spectrograph 2 (FORS2) on the Very Large Telescope during two primary transits. The observations cover a wavelength range of ≈400-840 nm. The light curves are corrupted by significant systematics, but these were to first-order invariant with wavelength and could be removed using a common-mode correction derived from the white light curves. We reach a precision in the transit depth of ≈140 ppm in 15 nm bins, although the precision varies significantly over the wavelength range. Our FORS2 observations confirm the cloud deck previously inferred using Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS). We also re-analyse the HST/STIS data using a Gaussian process model, finding excellent agreement with earlier measurements. We reproduce the Rayleigh scattering signature at short wavelengths (≲5300 Å) and the cloud deck at longer wavelengths. However, our FORS2 observations appear to rule out the large potassium feature previously detected using STIS, yet it is recovered from the HST/STIS data, although with reduced amplitude and significance (≈2.5σ). The discrepancy between our results and the earlier STIS detection of potassium (≈4.3σ) is either a result of telluric contamination of the ground-based observations, or an underestimate of the uncertainties for narrow-band features in HST/STIS when using linear basis models to account for the systematics. Our results further demonstrate the use of ground-based multi-object spectrographs for the study of exoplanet atmospheres, and highlight the need for caution in our interpretation of narrow-band features in low-resolution spectra of hot Jupiters.

  • 2017 - "Atmospheric Circulation and Cloud Evolution on the Highly Eccentric Extrasolar Planet HD 80606b"
    Lewis, N. K.; Parmentier, V.; Kataria, T.; de Wit, J.; Showman, A. P.; Fortney, J. J.; Marley, M. S.

    Observations of the highly-eccentric (e~0.9) hot-Jupiter HD 80606b with Spitzer have provided some of best probes of the physics at work in exoplanet atmospheres. By observing HD 80606b during its periapse passage, atmospheric radiative, advective, and chemical timescales can be directly measured and used to constrain fundamental planetary properties such as rotation period, tidal dissipation rate, and atmospheric composition (including aerosols). Here we present three-dimensional general circulation models for HD 80606b that aim to further explore the atmospheric physics shaping HD 80606b's observed Spitzer phase curves. We find that our models that assume a planetary rotation period twice that of the pseudo-synchronous rotation period best reproduce the phase variations observed for HD~80606b near periapse passage with Spitzer. Additionally, we find that the rapid formation/dissipation and vertical transport of clouds in HD 80606b's atmosphere near periapse passage likely shapes its observed phase variations. We predict that observations near periapse passage at visible wavelengths could constrain the composition and formation/advection timescales of the dominant cloud species in HD 80606b's atmosphere. The time-variable forcing experienced by exoplanets on eccentric orbits provides a unique and important window on radiative, dynamical, and chemical processes in planetary atmospheres and an important link between exoplanet observations and theory.

  • 2017 - "HAT-P-26b: A Neptune-mass exoplanet with a well-constrained heavy element abundance"
    Wakeford, Hannah R.; Sing, David K.; Kataria, Tiffany; Deming, Drake; Nikolov, Nikolay; Lopez, Eric D.; Tremblin, Pascal; Amundsen, David S.; Lewis, Nikole K.; Mandell, Avi M.; Fortney, Jonathan J.; Knutson, Heather; Benneke, Björn; Evans, Thomas M.

    A correlation between giant-planet mass and atmospheric heavy elemental abundance was first noted in the past century from observations of planets in our own Solar System and has served as a cornerstone of planet-formation theory. Using data from the Hubble and Spitzer Space Telescopes from 0.5 to 5 micrometers, we conducted a detailed atmospheric study of the transiting Neptune-mass exoplanet HAT-P-26b. We detected prominent H2O absorption bands with a maximum base-to-peak amplitude of 525 parts per million in the transmission spectrum. Using the water abundance as a proxy for metallicity, we measured HAT-P-26b's atmospheric heavy element content (4.8-4.0+21.5 times solar). This likely indicates that HAT-P-26b's atmosphere is primordial and obtained its gaseous envelope late in its disk lifetime, with little contamination from metal-rich planetesimals.

  • 2017 - "High-temperature condensate clouds in super-hot Jupiter atmospheres"
    Wakeford, H. R.; Visscher, C.; Lewis, N. K.; Kataria, T.; Marley, M. S.; Fortney, J. J.; Mandell, A. M.

    Deciphering the role of clouds is central to our understanding of exoplanet atmospheres, as they have a direct impact on the temperature and pressure structure, and observational properties of the planet. Super-hot Jupiters occupy a temperature regime similar to low-mass M-dwarfs, where minimal cloud condensation is expected. However, observations of exoplanets such as WASP-12b (Teq ˜ 2500 K) result in a transmission spectrum indicative of a cloudy atmosphere. We re-examine the temperature and pressure space occupied by these super-hot Jupiter atmospheres, to explore the role of the initial Al- and Ti-bearing condensates as the main source of cloud material. Due to the high temperatures, a majority of the more common refractory material is not depleted into deeper layers and would remain in the vapour phase. The lack of depletion into deeper layers means that these materials with relatively low cloud masses can become significant absorbers in the upper atmosphere. We provide condensation curves for the initial Al- and Ti-bearing condensates which may be used to provide quantitative estimates of the effect of metallicity on cloud masses, as planets with metal-rich hosts potentially form more opaque clouds because more mass is available for condensation. Increased metallicity also pushes the point of condensation to hotter, deeper layers in the planetary atmosphere further increasing the density of the cloud. We suggest that planets around metal-rich hosts are more likely to have thick refractory clouds, and discuss the implication on the observed spectra of WASP-12b.

  • 2017 - "Spitzer Phase Curve Constraints for WASP-43b at 3.6 and 4.5 μm"
    Stevenson, Kevin B.; Line, Michael R.; Bean, Jacob L.; Désert, Jean-Michel; Fortney, Jonathan J.; Showman, Adam P.; Kataria, Tiffany; Kreidberg, Laura; Feng, Y. Katherina

    Previous measurements of heat redistribution efficiency (the ability to transport energy from a planet's highly irradiated dayside to its eternally dark nightside) show considerable variation between exoplanets. Theoretical models predict a positive correlation between heat redistribution efficiency and temperature for tidally locked planets; however, recent Hubble Space Telescope (HST) WASP-43b spectroscopic phase curve results are inconsistent with current predictions. Using the Spitzer Space Telescope, we obtained a total of three phase curve observations of WASP-43b (P = 0.813 days) at 3.6 and 4.5 μm. The first 3.6 μm visit exhibits spurious nightside emission that requires invoking unphysical conditions in our cloud-free atmospheric retrievals. The two other visits exhibit strong day-night contrasts that are consistent with the HST data. To reconcile the departure from theoretical predictions, WASP-43b would need to have a high-altitude, nightside cloud/haze layer blocking its thermal emission. Clouds/hazes could be produced within the planet's cool, nearly retrograde mid-latitude flows before dispersing across its nightside at high altitudes. Since mid-latitude flows only materialize in fast-rotating (≲ 1 day) planets, this may explain an observed trend connecting measured day-night contrast with planet rotation rate that matches all current Spitzer phase curve results. Combining independent planetary emission measurements from multiple phases, we obtain a precise dayside hemisphere H2O abundance (2.5× {10}-5{--}1.1× {10}-4 at 1σ confidence) and, assuming chemical equilibrium and a scaled solar abundance pattern, we derive a corresponding metallicity estimate that is consistent with being solar (0.4-1.7). Using the retrieved global CO+CO2 abundance under the same assumptions, we estimate a comparable metallicity of 0.3-1.7× solar. This is the first time that precise abundance and metallicity constraints have been determined from multiple molecular tracers for a transiting exoplanet.

  • 2017 - "HST PanCET program: A Cloudy Atmosphere for the Promising JWST Target WASP-101b"
    Wakeford, H. R.; Stevenson, K. B.; Lewis, N. K.; Sing, D. K.; López-Morales, M.; Marley, M.; Kataria, T.; Mandell, A.; Ballester, G. E.; Barstow, J.; Ben-Jaffel, L.; Bourrier, V.; Buchhave, L. A.; Ehrenreich, D.; Evans, T.; García Muñoz, A.; Henry, G.; Knutson, H.; Lavvas, P.; Lecavelier des Etangs, A.; Nikolov, N.; Sanz-Forcada, J.

    We present results from the first observations of the Hubble Space Telescope (HST) Panchromatic Comparative Exoplanet Treasury program for WASP-101b, a highly inflated hot Jupiter and one of the community targets proposed for the James Webb Space Telescope (JWST) Early Release Science (ERS) program. From a single HST Wide Field Camera 3 observation, we find that the near-infrared transmission spectrum of WASP-101b contains no significant H2O absorption features and we rule out a clear atmosphere at 13σ. Therefore, WASP-101b is not an optimum target for a JWST ERS program aimed at observing strong molecular transmission features. We compare WASP-101b to the well-studied and nearly identical hot Jupiter WASP-31b. These twin planets show similar temperature-pressure profiles and atmospheric features in the near-infrared. We suggest exoplanets in the same parameter space as WASP-101b and WASP-31b will also exhibit cloudy transmission spectral features. For future HST exoplanet studies, our analysis also suggests that a lower count limit needs to be exceeded per pixel on the detector in order to avoid unwanted instrumental systematics.

  • 2017 - "Spitzer Secondary Eclipse Depths with Multiple Intrapixel Sensitivity Correction Methods Observations of WASP-13b, WASP-15b, WASP-16b, WASP-62b, and HAT-P-22b"
    Kilpatrick, Brian M.; Lewis, Nikole K.; Kataria, Tiffany; Deming, Drake; Ingalls, James G.; Krick, Jessica E.; Tucker, Gregory S.

    We measure the 4.5 μm thermal emission of five transiting hot Jupiters, WASP-13b, WASP-15b, WASP-16b, WASP-62b, and HAT-P-22b using channel 2 of the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. Significant intrapixel sensitivity variations in Spitzer IRAC data require careful correction in order to achieve precision on the order of several hundred parts per million (ppm) for the measurement of exoplanet secondary eclipses. We determine eclipse depths by first correcting the raw data using three independent data reduction methods. The Pixel Gain Map (PMAP), Nearest Neighbors (NNBR), and Pixel Level Decorrelation (PLD) each correct for the intrapixel sensitivity effect in Spitzer photometric time-series observations. The results from each methodology are compared against each other to establish if they reach a statistically equivalent result in every case and to evaluate their ability to minimize uncertainty in the measurement. We find that all three methods produce reliable results. For every planet examined here NNBR and PLD produce results that are in statistical agreement. However, the PMAP method appears to produce results in slight disagreement in cases where the stellar centroid is not kept consistently on the most well characterized area of the detector. We evaluate the ability of each method to reduce the scatter in the residuals as well as in the correlated noise in the corrected data. The NNBR and PLD methods consistently minimize both white and red noise levels and should be considered reliable and consistent. The planets in this study span equilibrium temperatures from 1100 to 2000 K and have brightness temperatures that require either high albedo or efficient recirculation. However, it is possible that other processes such as clouds or disequilibrium chemistry may also be responsible for producing these brightness temperatures.

  • 2017 - "A Direct Imaging Survey of Spitzer-detected Debris Disks: Occurrence of Giant Planets in Dusty Systems"
    Meshkat, Tiffany; Mawet, Dimitri; Bryan, Marta L.; Hinkley, Sasha; Bowler, Brendan P.; Stapelfeldt, Karl R.; Batygin, Konstantin; Padgett, Deborah; Morales, Farisa Y.; Serabyn, Eugene; Christiaens, Valentin; Brandt, Timothy D.; Wahhaj, Zahed

    We describe a joint high-contrast imaging survey for planets at the Keck and Very Large Telescope of the last large sample of debris disks identified by the Spitzer Space Telescope. No new substellar companions were discovered in our survey of 30 Spitzer-selected targets. We combine our observations with data from four published surveys to place constraints on the frequency of planets around 130 debris disk single stars, the largest sample to date. For a control sample, we assembled contrast curves from several published surveys targeting 277 stars that do not show infrared excesses. We assumed a double power-law distribution in mass and semimajor axis (SMA) of the form f(m,a)={{Cm}}α {a}β , where we adopted power-law values and logarithmically flat values for the mass and SMA of planets. We find that the frequency of giant planets with masses 5-20 M Jup and separations 10-1000 au around stars with debris disks is 6.27% (68% confidence interval 3.68%-9.76%), compared to 0.73% (68% confidence interval 0.20%-1.80%) for the control sample of stars without disks. These distributions differ at the 88% confidence level, tentatively suggesting distinctness of these samples. Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation.

  • 2017 - "Photopolarimetric characteristics of brown dwarfs bearing uniform cloud decks"
    Sanghavi, Suniti; Millar-Blanchaer, Maxwell; Shporer, Avi; Riedel, Adric; Tinyatont, Samaporn; Nilsson, Ricky; Kataria, Tiffany; Mawet, Dimitri

    It has long been known that an envelope of scattering particles like free electrons, atoms and molecules, or particulate aggregates like haze or cloud grains affect the intensity and polarization of radiation emitted by a rotating body (Chandrasekhar 1946; Harrington and Collins 1968, Sengupta and Marley 2010, Marley and Sengupta 2011, de Kok et al. 2011). Due to their high rotation rates, brown dwarfs (BDs) are expected to be considerably oblate. We present a conics-based radiative transfer scheme for computing the disc-resolved and disc-integrated polarized emission of an oblate body. Using this capability, we examine the photopolarimetric signal of BDs as a function of the scattering properties of its atmosphere like cloud optical thickness and cloud grain size as well as properties specific to the BD such as its oblateness and the orientation of its rotation axis relative to the observer. The polarizing effect of temperature inhomogeneity caused by gravity-darkening is considered distinctly from the effect of oblateness, revealing that resulting temperature gradients cause intensity differences that can amplify the disc-integrated polarization by a factor of 2. Our examination of the properties of scatterers suggests that the contested relative brightening in the J-band for cooler BDs in the L/T-transition can partly be explained by thick clouds bearing larger-sized grains. Grain-size affects both the intensity and polarization of emitted radiation - as grain-size increases relative to wavelength, the polarization caused by scattering decreases sharply, especially at infrared wavelengths where Rayleigh scattering due to atoms and molecules becomes negligible. We thus claim that the presence of scattering particles is a necessary but not sufficient condition for observing polarization of emitted light.

  • 2017 - "The Habitability of Icy Worlds in the Solar System"
    Steven D. Vance

    Multiple icy oceanic worlds exist in the solar system, with global inventories of liquid water often exceeding those on Earth. They may create conditions amenable to life, despite an absence of continents and the possibility for high pressure ices covering their rocky seafloors. Exploration of ocean worlds in the solar system offers the chance to anticipate what features of habitability to look for on watery exoworlds.

  • 2017 - "Forecasting the Impact of Stellar Activity on Transiting Exoplanet Spectra"
    Zellem, Robert T.; Swain, Mark R.; Roudier, Gael; Shkolnik, Evgenya L.; Creech-Eakman, Michelle J.; Ciardi, David R.; Line, Michael R.; Iyer, Aishwarya R.; Bryden, Geoffrey; Llama, Joe; Fahy, Kristen A.

    We predict the impact of stellar variability on transit observations. Supplementing these forecasts with Kepler-measured stellar variabilities for F-, G-, K-, and M-dwarfs, and predicted transit precisions by the James Webb Space Telescope's (JWST) NIRISS, NIRCam, and MIRI, we conclude that stellar activity does not impact infrared transiting exoplanet observations of most presently known or predicted TESS targets by current or near-future platforms, such as JWST, as activity-induced spectral changes are below the measurement precision.

  • Papers Published in 2016

  • 2016 - "Hypotheses for Near-Surface Exchange of Methane on Mars"
    Hu, Renyu; Bloom, A. Anthony; Gao, Peter; Miller, Charles E.; Yung, Yuk L.

    The Curiosity rover recently detected a background of 0.7 ppb and spikes of 7 ppb of methane on Mars. This in situ measurement reorients our understanding of the martian environment and its potential for life, as the current theories do not entail any geological source or sink of methane that varies sub-annually. In particular, the 10-fold elevation during the southern winter indicates episodic sources of methane that are yet to be discovered. Here we suggest a near-surface reservoir could explain this variability. Using the temperature and humidity measurements from the rover, we find that perchlorate salts in the regolith deliquesce to form liquid solutions, and deliquescence progresses to deeper subsurface in the season of the methane spikes. We therefore formulate the following three testable hypotheses. The first scenario is that the regolith in Gale Crater adsorbs methane when dry and releases this methane to the atmosphere upon deliquescence. The adsorption energy needs to be 36 kJ mol-1 to explain the magnitude of the methane spikes, higher than existing laboratory measurements. The second scenario is that microorganisms convert organic matter in the soil to methane when they are in liquid solutions. This scenario does not require regolith adsorption but entails extant life on Mars. The third scenario is that deep subsurface aquifers produce the bursts of methane. Continued in situ measurements of methane and water, as well as laboratory studies of adsorption and deliquescence, will test these hypotheses and inform the existence of the near-surface reservoir and its exchange with the atmosphere.

  • 2016 - "Transiting Exoplanet Studies and Community Targets for JWST's Early Release Science Program"
    Stevenson, Kevin B.; Lewis, Nikole K.; Bean, Jacob L.; Beichman, Charles; Fraine, Jonathan; Kilpatrick, Brian M.; Krick, J. E.; Lothringer, Joshua D.; Mandell, Avi M.; Valenti, Jeff A.; Agol, Eric; Angerhausen, Daniel; Barstow, Joanna K.; Birkmann, Stephan M.; Burrows, Adam; Charbonneau, David; Cowan, Nicolas B.; Crouzet, Nicolas; Cubillos, Patricio E.; Curry, S. M.; Dalba, Paul A.; de Wit, Julien; Deming, Drake; Désert, Jean-Michel; Doyon, René; Dragomir, Diana; Ehrenreich, David; Fortney, Jonathan J.; García Muñoz, Antonio; Gibson, Neale P.; Gizis, John E.; Greene, Thomas P.; Harrington, Joseph; Heng, Kevin; Kataria, Tiffany; Kempton, Eliza M.-R.; Knutson, Heather; Kreidberg, Laura; Lafrenière, David; Lagage, Pierre-Olivier; Line, Michael R.; Lopez-Morales, Mercedes; Madhusudhan, Nikku; Morley, Caroline V.; Rocchetto, Marco; Schlawin, Everett; Shkolnik, Evgenya L.; Shporer, Avi; Sing, David K.; Todorov, Kamen O.; Tucker, Gregory S.; Wakeford, Hannah R.

    The James Webb Space Telescope (JWST) will likely revolutionize transiting exoplanet atmospheric science, due to a combination of its capability for continuous, long duration observations and its larger collecting area, spectral coverage, and spectral resolution compared to existing space-based facilities. However, it is unclear precisely how well JWST will perform and which of its myriad instruments and observing modes will be best suited for transiting exoplanet studies. In this article, we describe a prefatory JWST Early Release Science (ERS) Cycle 1 program that focuses on testing specific observing modes to quickly give the community the data and experience it needs to plan more efficient and successful transiting exoplanet characterization programs in later cycles. We propose a multi-pronged approach wherein one aspect of the program focuses on observing transits of a single target with all of the recommended observing modes to identify and understand potential systematics, compare transmission spectra at overlapping and neighboring wavelength regions, confirm throughputs, and determine overall performances. In our search for transiting exoplanets that are well suited to achieving these goals, we identify 12 objects (dubbed "community targets") that meet our defined criteria. Currently, the most favorable target is WASP-62b because of its large predicted signal size, relatively bright host star, and location in JWST's continuous viewing zone. Since most of the community targets do not have well-characterized atmospheres, we recommend initiating preparatory observing programs to determine the presence of obscuring clouds/hazes within their atmospheres. Measurable spectroscopic features are needed to establish the optimal resolution and wavelength regions for exoplanet characterization. Other initiatives from our proposed ERS program include testing the instrument brightness limits and performing phase-curve observations. The latter are a unique challenge compared to transit observations because of their significantly longer durations. Using only a single mode, we propose to observe a full-orbit phase curve of one of the previously characterized, short-orbital-period planets to evaluate the facility-level aspects of long, uninterrupted time-series observations.

  • 2016 - "The sustainability of habitability on terrestrial planets: Insights, questions, and needed measurements from Mars for understanding the evolution of Earth-like worlds"
    Ehlmann, B. L.; Anderson, F. S.; Andrews-Hanna, J.; Catling, D. C.; Christensen, P. R.; Cohen, B. A.; Dressing, C. D.; Edwards, C. S.; Elkins-Tanton, L. T.; Farley, K. A.; Fassett, C. I.; Fischer, W. W.; Fraeman, A. A.; Golombek, M. P.; Hamilton, V. E.; Hayes, A. G.; Herd, C. D. K.; Horgan, B.; Hu, R.; Jakosky, B. M.; Johnson, J. R.; Kasting, J. F.; Kerber, L.; Kinch, K. M.; Kite, E. S.; Knutson, H. A.; Lunine, J. I.; Mahaffy, P. R.; Mangold, N.; McCubbin, F. M.; Mustard, J. F.; Niles, P. B.; Quantin-Nataf, C.; Rice, M. S.; Stack, K. M.; Stevenson, D. J.; Stewart, S. T.; Toplis, M. J.; Usui, T.; Weiss, B. P.; Werner, S. C.; Wordsworth, R. D.; Wray, J. J.; Yingst, R. A.; Yung, Y. L.; Zahnle, K. J.

    What allows a planet to be both within a potentially habitable zone and sustain habitability over long geologic time? With the advent of exoplanetary astronomy and the ongoing discovery of terrestrial-type planets around other stars, our own solar system becomes a key testing ground for ideas about what factors control planetary evolution. Mars provides the solar system's longest record of the interplay of the physical and chemical processes relevant to habitability on an accessible rocky planet with an atmosphere and hydrosphere. Here we review current understanding and update the timeline of key processes in early Mars history. We then draw on knowledge of exoplanets and the other solar system terrestrial planets to identify six broad questions of high importance to the development and sustaining of habitability (unprioritized): (1) Is small planetary size fatal? (2) How do magnetic fields influence atmospheric evolution? (3) To what extent does starting composition dictate subsequent evolution, including redox processes and the availability of water and organics? (4) Does early impact bombardment have a net deleterious or beneficial influence? (5) How do planetary climates respond to stellar evolution, e.g., sustaining early liquid water in spite of a faint young Sun? (6) How important are the timescales of climate forcing and their dynamical drivers? Finally, we suggest crucial types of Mars measurements (unprioritized) to address these questions: (1) in situ petrology at multiple units/sites; (2) continued quantification of volatile reservoirs and new isotopic measurements of H, C, N, O, S, Cl, and noble gases in rocks that sample multiple stratigraphic sections; (3) radiometric age dating of units in stratigraphic sections and from key volcanic and impact units; (4) higher-resolution measurements of heat flux, subsurface structure, and magnetic field anomalies coupled with absolute age dating. Understanding the evolution of early Mars will feed forward to understanding the factors driving the divergent evolutionary paths of the Earth, Venus, and thousands of small rocky extrasolar planets yet to be discovered.

  • 2016 - "Spiral Structure and Differential Dust Size Distribution in the LKHα 330 Disk"
    Akiyama, Eiji; Hashimoto, Jun; Liu, Hauyu Baobab; Li, Jennifer I-Hsiu; Bonnefoy, Michael; Dong, Ruobing; Hasegawa, Yasuhiro; Henning, Thomas; Sitko, Michael L.; Janson, Markus; Feldt, Markus; Wisniewski, John; Kudo, Tomoyuki; Kusakabe, Nobuhiko; Tsukagoshi, Takashi; Momose, Munetake; Muto, Takayuki; Taki, Tetsuo; Kuzuhara, Masayuki; Satoshi, Mayama; Takami, Michihiro; Ohashi, Nagayoshi; Grady, Carol A.; Kwon, Jungmi; Thalmann, Christian; Abe, Lyu; Brandner, Wolfgang; Brandt, Timothy D.; Carson, Joseph C.; Egner, Sebastian; Goto, Miwa; Guyon, Olivier; Hayano, Yutaka; Hayashi, Masahiko; Hayashi, Saeko S.; Hodapp, Klaus W.; Ishii, Miki; Iye, Masanori; Knapp, Gillian R.; Kandori, Ryo; Matsuo, Taro; Mcelwain, Michael W.; Miyama, Shoken; Morino, Jun-Ichi; Moro-Martin, Amaya; Nishimura, Tetsuo; Pyo, Tae-Soo; Serabyn, Eugene; Suenaga, Takuya; Suto, Hiroshi; Suzuki, Ryuji; Takahashi, Yasuhiro H.; Takato, Naruhisa; Terada, Hiroshi; Tomono, Daigo; Turner, Edwin L.; Watanabe, Makoto; Yamada, Toru; Takami, Hideki; Usuda, Tomonori; Tamura, Motohide

    Dust trapping accelerates the coagulation of dust particles, and, thus, it represents an initial step toward the formation of planetesimals. We report H-band (1.6 μm) linear polarimetric observations and 0.87 mm interferometric continuum observations toward a transitional disk around LkHα 330. As a result, a pair of spiral arms were detected in the H-band emission, and an asymmetric (potentially arm-like) structure was detected in the 0.87 mm continuum emission. We discuss the origin of the spiral arm and the asymmetric structure and suggest that a massive unseen planet is the most plausible explanation. The possibility of dust trapping and grain growth causing the asymmetric structure was also investigated through the opacity index (β) by plotting the observed spectral energy distribution slope between 0.87 mm from our Submillimeter Array observation and 1.3 mm from literature. The results imply that grains are indistinguishable from interstellar medium-like dust in the east side (β =2.0+/- 0.5) but are much smaller in the west side β ={0.7}-0.4+0.5, indicating differential dust size distribution between the two sides of the disk. Combining the results of near-infrared and submillimeter observations, we conjecture that the spiral arms exist at the upper surface and an asymmetric structure resides in the disk interior. Future observations at centimeter wavelengths and differential polarization imaging in other bands (Y-K) with extreme AO imagers are required to understand how large dust grains form and to further explore the dust distribution in the disk.

  • 2016 - "Super-Earths as Failed Cores in Orbital Migration Traps"
    Hasegawa, Yasuhiro

    I explore whether close-in super-Earths were formed as rocky bodies that failed to grow fast enough to become the cores of gas giants before the natal protostellar disk dispersed. I model the failed cores' inward orbital migration in the low-mass or type I regime to stopping points at distances where the tidal interaction with the protostellar disk applies zero net torque. The three kinds of migration traps considered are those due to the dead zone's outer edge, the ice line, and the transition from accretion to starlight as the disk's main heat source. As the disk disperses, the traps move toward final positions near or just outside 1 au. Planets at this location exceeding about 3 M ⊕ open a gap, decouple from their host traps, and migrate inward in the high-mass or type II regime to reach the vicinity of the star. I synthesize the population of planets that formed in this scenario, finding that a fraction of the observed super-Earths could have been failed cores. Most super-Earths that formed this way have more than 4 M ⊕, so their orbits when the disks dispersed were governed by type II migration. These planets have solid cores surrounded by gaseous envelopes. Their subsequent photoevaporative mass loss is most effective for masses originally below about 6 M ⊕. The failed core scenario suggests a division of the observed super-Earth mass-radius diagram into five zones according to the inferred formation history.

  • 2016 - "Detection of Linearly Polarized 6.9 mm Continuum Emission from the Class 0 Young Stellar Object NGC 1333 IRAS4A"
    Liu, Hauyu Baobab; Lai, Shih-Ping; Hasegawa, Yasuhiro; Hirano, Naomi; Rao, Ramprasad; Li, I.-Hsiu; Fukagawa, Misato; Girart, Josep M.; Carrasco-González, Carlos; Rodríguez, Luis F.

    Dust growth, a first step of planet formation, can change the mass and the shape of dust grains. The detection of linearly polarized dust continuum emission at 6.9 mm can serve as a probe of how dust shape evolves, following dust growth. We demonstrate this possibility by reporting new JVLA, high angular resolution observations toward a Class 0 YSO.

  • 2016 - "Forming Chondrites in a Solar Nebula with Magnetically Induced Turbulence"
    Hasegawa, Yasuhiro; Turner, Neal J.; Masiero, Joseph; Wakita, Shigeru; Matsumoto, Yuji; Oshino, Shoichi

    Chondritic meteorites provide valuable opportunities to investigate the origins of the solar system. We identify under what conditions chondrule formation and accretion can be realized in the solar nebula, making use of the currently available meteoritic date. The data include the present asteroid belt mass, the formation timescale of chondrules, the magnetic field strength of the nebula derived from chondrules in Semarkona.

  • 2016 - "Planetary System Formation in the Protoplanetary Disk around HL Tauri"
    Akiyama, Eiji; Hasegawa, Yasuhiro; Hayashi, Masahiko; Iguchi, Satoru

    ALMA long-baseline science verification campaign reveals the astonishing multiple gap structure in the circumstellar disk around HL Tau. We reanalyze the data and discuss how such a gap structure is generated. Assuming that the gaps are opened up by unseen massive bodies, gravitational instabilities may be a mechanism to form the bodies in the outer region of the disk. ALMA's unprecedented high spatial resolution observations will revolutionize our picture of planet formation.

  • 2016 - "Absence of Significant Cool Disks in Young Stellar Objects Exhibiting Repetitive Optical Outbursts"
    Liu, Hauyu Baobab; Galván-Madrid, Roberto; Vorobyov, Eduard I.; Kóspál, Ágnes; Rodríguez, Luis F.; Dunham, Michael M.; Hirano, Naomi; Henning, Thomas; Takami, Michihiro; Dong, Ruobing; Hashimoto, Jun; Hasegawa, Yasuhiro; Carrasco-González, Carlos

    Many low-mass YSOs may have experienced repetitive outburst events until they finally arrive at the main-sequence stage. We report SMA 1.3 mm high angular resolution observations toward the so- called EXors. We find that most of them have low dust masses while the previous IR observations suggest that they should have enough gas masses. Our observations therefore infer that most of EXors may not have cold dust, which would be valuable information to understand how repetitive outbursts occur around YSOs.

  • 2016 - "Chondrule Formation via Impact Jetting Triggered by Planetary Accretion"
    Hasegawa, Yasuhiro; Wakita, Shigeru; Matsumoto, Yuji; Oshino, Shoichi

    Chondrules are the primitive materials in the solar system, which enables one to explore the origins of the solar system. We investigate how and what amount of chondrules can be formed via impact jetting that can occur at planetesimal collisions. We find that formation of protoplanets leads to a large number of chondrule-forming impacts with a certain timescale, both of which are consistent with the chondritic data.

  • 2016 - "The MUSCLES Treasury Survey. III. X-Ray to Infrared Spectra of 11 M and K Stars Hosting Planets"
    Loyd, R. O. P.; France, Kevin; Youngblood, Allison; Schneider, Christian; Brown, Alexander; Hu, Renyu; Linsky, Jeffrey; Froning, Cynthia S.; Redfield, Seth; Rugheimer, Sarah; Tian, Feng

    The shape and the absolute flux of a star's emission control the atmospheric chemistry of exoplanets around it. Using new Hubble Space Telescope observations, panchromatic (X-ray to mid-IR) spectra of 11 nearby planet-hosting low-mass (M to K) stars are derived. This database of stellar spectra is the foundation of modeling atmospheric photochemistry and assessing potential biosignature gases for rocky exoplanets.

  • 2016 - "VLT FORS2 Comparative Transmission Spectroscopy: Detection of Na in the Atmosphere of WASP-39b from the Ground"
    Nikolov, Nikolay; Sing, David K.; Gibson, Neale P.; Fortney, Jonathan J.; Evans, Thomas M.; Barstow, Joanna K.; Kataria, Tiffany; Wilson, Paul A.

    We present transmission spectroscopy of the warm Saturn-mass exoplanet WASP-39b made with the Very Large Telescope FOcal Reducer and Spectrograph (FORS2) across the wavelength range 411-810 nm. The transit depth is measured with a typical precision of 240 parts per million (ppm) in wavelength bins of 10 nm on a V = 12.1 mag star. We detect the sodium absorption feature (3.2σ) and find evidence of potassium. The ground-based transmission spectrum is consistent with Hubble Space Telescope (HST) optical spectroscopy, supporting the interpretation that WASP-39b has a largely clear atmosphere. Our results demonstrate the great potential of the recently upgraded FORS2 spectrograph for optical transmission spectroscopy, with which we obtained HST-quality light curves from the ground.

  • 2016 - "HST Hot-Jupiter Transmission Spectral Survey: Clear Skies for Cool Saturn WASP-39b"
    Fischer, Patrick D.; Knutson, Heather A.; Sing, David K.; Henry, Gregory W.; Williamson, Michael W.; Fortney, Jonathan J.; Burrows, Adam S.; Kataria, Tiffany; Nikolov, Nikolay; Showman, Adam P.; Ballester, Gilda E.; Désert, Jean-Michel; Aigrain, Suzanne; Deming, Drake; Lecavelier des Etangs, Alain; Vidal-Madjar, Alfred

    We present the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) optical transmission spectroscopy of the cool Saturn-mass exoplanet WASP-39b from 0.29-1.025 μm, along with complementary transit observations from Spitzer IRAC at 3.6 and 4.5 μm. The low density and large atmospheric pressure scale height of WASP-39b make it particularly amenable to atmospheric characterization using this technique. We detect a Rayleigh scattering slope as well as sodium and potassium absorption features; this is the first exoplanet in which both alkali features are clearly detected with the extended wings predicted by cloud-free atmosphere models. The full transmission spectrum is well matched by a clear H2-dominated atmosphere, or one containing a weak contribution from haze, in good agreement with the preliminary reduction of these data presented in Sing et al. WASP-39b is predicted to have a pressure-temperature profile comparable to that of HD 189733b and WASP-6b, making it one of the coolest transiting gas giants observed in our HST STIS survey. Despite this similarity, WASP-39b appears to be largely cloud-free, while the transmission spectra of HD 189733b and WASP-6b both indicate the presence of high altitude clouds or hazes. These observations further emphasize the surprising diversity of cloudy and cloud-free gas giant planets in short-period orbits and the corresponding challenges associated with developing predictive cloud models for these atmospheres.

  • 2016 - "Detection of H2O and Evidence for TiO/VO in an Ultra-hot Exoplanet Atmosphere"
    Evans, Thomas M.; Sing, David K.; Wakeford, Hannah R.; Nikolov, Nikolay; Ballester, Gilda E.; Drummond, Benjamin; Kataria, Tiffany; Gibson, Neale P.; Amundsen, David S.; Spake, Jessica

    We present a primary transit observation for the ultra-hot (T eq ˜ 2400 K) gas giant expolanet WASP-121b, made using the Hubble Space Telescope Wide Field Camera 3 in spectroscopic mode across the 1.12-1.64 μm wavelength range. The 1.4 μm water absorption band is detected at high confidence (5.4σ) in the planetary atmosphere. We also reanalyze ground-based photometric light curves taken in the B, r‧, and z‧ filters. Significantly deeper transits are measured in these optical bandpasses relative to the near-infrared wavelengths. We conclude that scattering by high-altitude haze alone is unlikely to account for this difference and instead interpret it as evidence for titanium oxide and vanadium oxide absorption. Enhanced opacity is also inferred across the 1.12-1.3 μm wavelength range, possibly due to iron hydride absorption. If confirmed, WASP-121b will be the first exoplanet with titanium oxide, vanadium oxide, and iron hydride detected in transmission. The latter are important species in M/L dwarfs and their presence is likely to have a significant effect on the overall physics and chemistry of the atmosphere, including the production of a strong thermal inversion.

  • 2016 - "The Atmospheric Circulation of a Nine-hot-Jupiter Sample: Probing Circulation and Chemistry over a Wide Phase Space"
    Kataria, Tiffany; Sing, David K.; Lewis, Nikole K.; Visscher, Channon; Showman, Adam P.; Fortney, Jonathan J.; Marley, Mark S.

    We present results from an atmospheric circulation study of nine hot Jupiters that compose a large transmission spectral survey using the Hubble and Spitzer Space Telescopes. These observations exhibit a range of spectral behavior over optical and infrared wavelengths, suggesting diverse cloud and haze properties in their atmospheres. By utilizing the specific system parameters for each planet, we naturally probe a wide phase space in planet radius, gravity, orbital period, and equilibrium temperature. First, we show that our model "grid" recovers trends shown in traditional parametric studies of hot Jupiters, particularly equatorial superrotation and increased day-night temperature contrast with increasing equilibrium temperature. We show how spatial temperature variations, particularly between the dayside and nightside and west and east terminators, can vary by hundreds of kelvin, which could imply large variations in Na, K, CO and {{{CH}}}4 abundances in those regions. These chemical variations can be large enough to be observed in transmission with high-resolution spectrographs, such as ESPRESSO on VLT, METIS on the E-ELT, or MIRI and NIRSpec aboard JWST. We also compare theoretical emission spectra generated from our models to available Spitzer eclipse depths for each planet and find that the outputs from our solar-metallicity, cloud-free models generally provide a good match to many of the data sets, even without additional model tuning. Although these models are cloud-free, we can use their results to understand the chemistry and dynamics that drive cloud formation in their atmospheres.

  • 2016 - "A continuum from clear to cloudy hot-Jupiter exoplanets without primordial water depletion"
    Sing, David K.; Fortney, Jonathan J.; Nikolov, Nikolay; Wakeford, Hannah R.; Kataria, Tiffany; Evans, Thomas M.; Aigrain, Suzanne; Ballester, Gilda E.; Burrows, Adam S.; Deming, Drake; Désert, Jean-Michel; Gibson, Neale P.; Henry, Gregory W.; Huitson, Catherine M.; Knutson, Heather A.; Lecavelier Des Etangs, Alain; Pont, Frederic; Showman, Adam P.; Vidal-Madjar, Alfred; Williamson, Michael H.; Wilson, Paul A.

    Thousands of transiting exoplanets have been discovered, but spectral analysis of their atmospheres has so far been dominated by a small number of exoplanets and data spanning relatively narrow wavelength ranges (such as 1.1-1.7 micrometres). Recent studies show that some hot-Jupiter exoplanets have much weaker water absorption features in their near-infrared spectra than predicted. The low amplitude of water signatures could be explained by very low water abundances, which may be a sign that water was depleted in the protoplanetary disk at the planet's formation location, but it is unclear whether this level of depletion can actually occur. Alternatively, these weak signals could be the result of obscuration by clouds or hazes, as found in some optical spectra. Here we report results from a comparative study of ten hot Jupiters covering the wavelength range 0.3-5 micrometres, which allows us to resolve both the optical scattering and infrared molecular absorption spectroscopically. Our results reveal a diverse group of hot Jupiters that exhibit a continuum from clear to cloudy atmospheres. We find that the difference between the planetary radius measured at optical and infrared wavelengths is an effective metric for distinguishing different atmosphere types. The difference correlates with the spectral strength of water, so that strong water absorption lines are seen in clear-atmosphere planets and the weakest features are associated with clouds and hazes. This result strongly suggests that primordial water depletion during formation is unlikely and that clouds and hazes are the cause of weaker spectral signatures.

  • 2016 - "Ringed Structures of the HD 163296 Protoplanetary Disk Revealed by ALMA"
    Isella, Andrea; Guidi, Greta; Testi, Leonardo; Liu, Shangfei; Li, Hui; Li, Shengtai; Weaver, Erik; Boehler, Yann; Carperter, John M.; De Gregorio-Monsalvo, Itziar; Manara, Carlo F.; Natta, Antonella; Pérez, Laura M.; Ricci, Luca; Sargent, Anneila; Tazzari, Marco; Turner, Neal

    We present Atacama Large Millimeter and Submillimeter Array observations of the protoplanetary disk around the Herbig Ae star HD 163296 that trace the spatial distribution of millimeter-sized particles and cold molecular gas on spatial scales as small as 25 astronomical units (A.U.). The image of the disk recorded in the 1.3 mm continuum emission reveals three dark concentric rings that indicate the presence of dust depleted gaps at about 60, 100, and 160 A.U. from the central star. The maps of the 12CO, 13CO, and C 18O J =2 -1 emission do not show such structures but reveal a change in the slope of the radial intensity profile across the positions of the dark rings in the continuum image. By comparing the observations with theoretical models for the disk emission, we find that the density of CO molecules is reduced inside the middle and outer dust gaps. However, in the inner ring there is no evidence of CO depletion. From the measurements of the dust and gas densities, we deduce that the gas-to-dust ratio varies across the disk and, in particular, it increases by at least a factor 5 within the inner dust gap compared to adjacent regions of the disk. The depletion of both dust and gas suggests that the middle and outer rings could be due to the gravitational torque exerted by two Saturn-mass planets orbiting at 100 and 160 A.U. from the star. On the other hand, the inner dust gap could result from dust accumulation at the edge of a magnetorotational instability dead zone, or from dust opacity variations at the edge of the CO frost line. Observations of the dust emission at higher angular resolution and of molecules that probe dense gas are required to establish more precisely the origins of the dark rings observed in the HD 163296 disk.

  • 2016 - "Signatures of Young Planets in the Continuum Emission From Protostellar Disks"
    Isella, Andrea; Turner, Neal

    Many protostellar disks show central cavities, rings, or spiral arms that could be caused by low-mass stellar or planetary companions. Few of these features are yet conclusively tied to objects embedded in the disks, perhaps because of shortcomings in our understanding of what a disk interacting with a planet looks like at the wavelengths we observe. We approach these issues noting that even small features on the disk's surface cast shadows, because the starlight grazes the surface at low angles. We therefore focus on accurately computing the disk's thickness, and thus its temperature. The embedded planet has 20, 100, or 1000~M$_\oplus$, ranging from barely enough to perturb the disk gas surface density significantly, to able to clear a deep tidal gap. We compare models that are in hydrostatic equilibrium in the vertical direction to models where the disk pressure scale height varies as a power of the orbital radius. The hydrostatic balance makes a large difference to the disk's temperature and appearance. The planet-carved gap's outer wall puffs up, throwing a shadow across the disk beyond. The shadow appears in scattered light as an additional dark ring, which could be mistaken for a gap opened by another more distant planet. The shadow is so deep it largely hides the outer arm of the spiral wave launched by the planet. The massive planets yield temperature gradients such that additional more distant low-mass planets undergoing orbital migration will converge on a location within the shadow. Furthermore the temperature perturbations affect the shape, size and contrast of the features predicted at millimeter and centimeter wavelengths. We conclude that radiative heating and cooling are important ingredients for reliably predicting the appearance of protostellar disks with embedded planets.

  • 2016 - "A Characteristic Transmission Spectrum dominated by H2O applies to the majority of HST/WFC3 exoplanet observations"
    Iyer, Aishwarya R.; Swain, Mark R.; Zellem, Robert T.; Line, Michael R.; Roudier, Gael; Rocha, Graça; Livingston, John H.

    Water is ubiquitous among hot Jupiter exoplanets and about half of their atmospheres are covered by opacity sources such as clouds, haze/aerosols. 10 out of 19 hot Jupiters observed using HST/WFC3 within the last decade have shown a significant water detection in their atmospheres. Our method allows for coherent averaging of the individual transmission spectra of these planets to produce a Representative Spectrum that is characteristic of hot Jupiter atmospheres. Additionally, we are also able to quantify that about half of these atmospheres are being blocked by cloud, haze or aerosols that affect our estimate for the true water content in hot Jupiters.

  • 2016 - “A map of the large day-night temperature gradient of a super-Earth exoplanet”
    Demory, Brice-Olivier; Gillon, Michael; de Wit, Julien; Madhusudhan, Nikku; Bolmont, Emeline; Heng, Kevin; Kataria, Tiffany; Lewis, Nikole; Hu, Renyu; Krick, Jessica; Stamenković, Vlada; Benneke, Björn; Kane, Stephen; Queloz, Didier

    55 Cancri e, approximately 40 light-years away in the constellation Cancer, has a radius twice Earth’s, and a mass 8 times greater. The paper reports a longitudinal thermal brightness map of 55 Cancri e obtained by the Spitzer Space Telescope, the first such map observed for any super-Earth class of exoplanets. The map tells us that the temperature contrast between the planet's dayside and the nightside is greater than 1000 K, and efficient energy transport occurs in the planet's dayside.

  • Papers Published in 2015

  • 2015 - "Tracing the fate of carbon and the atmospheric evolution of Mars"
    Hu, Renyu; Kass, David M.; Ehlmann, Bethany L.; Yung, Yuk L.

    The climate of Mars likely evolved from a warmer, wetter early state to the cold, arid current state. However, no solutions for this evolution have previously been found to satisfy the observed geological features and isotopic measurements of the atmosphere. Here we show that a family of solutions exist, invoking no missing reservoirs or loss processes. Escape of carbon via CO photodissociation and sputtering enriches heavy carbon (13C) in the Martian atmosphere, partially compensated by moderate carbonate precipitation. The current atmospheric 13C/12C and rock and soil carbonate measurements indicate an early atmosphere with a surface pressure >1 bar. Only scenarios with large amounts of carbonate formation in open lakes permit higher values up to 1.8 bar. The evolutionary scenarios are fully testable with data from the MAVEN mission and further studies of the isotopic composition of carbonate in the Martian rock record through time.

  • 2015 - “XO-2b: A Hot Jupiter with a Variable Host Star That Potentially Affects Its Measured Transit Depth”
    Zellem, Robert T.; Griffith, Caitlin A.; Pearson, Kyle A.; Turner, Jake D.; Henry, Gregory W.; Williamson, Michael H.; Ryleigh Fitzpatrick, M.; Teske, Johanna K.; Biddle, Lauren I.

    We conduct a multi-year study of the transiting exoplanet XO-2b to measure its Rayleigh scattering slope to place upper limits on its optically-thick radius and better constrain its molecular abundances. We also find that with mulity-year ground-based monitoring that its host star is active, potentially at the level that could affect future high-precision measurements of the signal of the exoplanet and alter the interpretations about its atmospheric properties.

  • 2015 - "Viscous Instability Triggered by Layered Accretion in Protoplanetary Disks"
    Hasegawa, Yasuhiro; Takeuchi, Taku

    The properties of protoplanetary disks, the birthplace of planets, are determined by magnetically induced turbulence. The previous theoretical studies suggest that the degree of disk turbulence varies as a function of the distance from the central star. We find that a viscous instability can take place at the boundary between high- and low-turbulent regions, which can considerably affect disk evolution and planet formation.

  • 2015 - "Studying Atmosphere-dominated Hot Jupiter Kepler Phase Curves: Evidence that Inhomogeneous Atmospheric Reflection Is Common"
    Shporer, Avi; Hu, Renyu

    In this paper we have shown that gas giant exoplanets on short orbital periods have non- uniform cloud coverage. We concluded this from the detection of non-uniform reflected star light, in visible light, across different planet longitudes. This has several implications regarding the structure and chemical composition of planetary atmospheres of such exoplanets.