arXiv subjects

Stephen P. Schmidt

Publications and source records attributed to Stephen P. Schmidt.

17 recordsLinked to original sources

arXiv2026

A JWST transiting survey of FGK stellar limb darkening: empirical evidence for quadratic laws and atmospheric model comparisons

We present a study of stellar limb-darkening using JWST transit observations of seven exoplanets orbiting FGK host stars, spanning 4200-6800 K. The wide wavelength coverage and high S/N of NIRISS/SOSS and NIRSpec/PRISM enable precise constraints on the wavelength-dependent limb-darkening. Using Bayesian model selection, we find that the quadratic limb-darkening law is statistically preferred over higher-order laws for most FGK stars, recovers consistent intensity profiles for $>$95% of the stellar disk area, and introduces a minimal bias in the derived transit depths of only $\sim$14 ppm (1$\sigma$) for 6/7 targets. This contrasts with previous studies relying on stellar models. We compare the empirically derived quadratic coefficients to predictions from the PHOENIX, MPS-ATLAS, MURaM, and Stagger stellar atmosphere grids. We introduce a quadratic limb-darkening parameterization in terms of limb intensity ($\ell$) and curvature at mid-$\mu$ ($\delta$), finding that empirical FGK limb darkening is generally more linear than models predict ($\delta \lesssim 0.1$). We identify wavelength-independent offsets between data and model quadratic coefficients, minimized by adopting $\mu_{\rm min} = 0.2$ in intensity calculations; we attribute this in part to models overpredicting limb-darkening near the limb where the plane-parallel approximation breaks down. For spherical PHOENIX models, we derive a $\mu$ rescaling method using a $\tau = 1$ photospheric radius. With these corrections, residual offsets are minimized and all stellar models achieve statistically acceptable fits. We provide recommended limb-darkening offset priors for use in JWST transit analyses, enabling more accurate constraints on exoplanet transmission spectra while accounting for residual stellar model uncertainties.

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arXiv2026

A Clearer View of HAT-P-1 b: JWST NIRSpec G395H Reveals Water, Carbon Dioxide, and Possibly Hydrogen Sulfide

As part of JWST's Exoplanet Grand Tour Survey, we use panchromatic transmission spectroscopy to connect HAT-P-1 b's previously studied optical and near-infrared atmosphere to the longer-wavelength molecular bands accessible with JWST. We present JWST NIRSpec G395H transmission spectroscopy of the hot Jupiter HAT-P-1 b over 2.7--5.3~$\mu$m, and combine the new spectrum with archival HST STIS and WFC3 observations for a 0.3--5.3~$\mu$m atmospheric analysis. We independently reduce the JWST data with the Eureka!, FIREFLy, and Tswift pipelines, finding mutually consistent transmission spectra across the G395H bandpass. Atmospheric retrievals yield strong evidence for H$_2$O and CO$_2$ with Bayes factors of $\log_{10}B_{\mathrm{H_2O}}=8.9$ and $\log_{10}B_{\mathrm{CO_2}}=52.3$, while providing tentative evidence for H$_2$S ($\log_{10}B_{\mathrm{H_2S}}=1.4$). The joint H$_2$O and CO$_2$ constraints favor an atmosphere near chemical equilibrium, with $\log_{10} \text{M/H}=0.99^{+0.19}_{-0.14}$, corresponding to $\sim10\times$ Solar or $\sim9\times$ relative to the near-solar metallicity host star, and a 3$\sigma$ upper limit of C/O $<0.52$. Because H$_2$O and CO$_2$ provide a metallicity comparatively insensitive to vertical mixing in this temperature regime, their combined detection suggests the composition is dominated by bulk enrichment rather than strong disequilibrium transport. We find no significant evidence for clouds; instead, the persistence of molecular structure across the spectrum argues against strong cloud muting. The tentative H$_2$S signal, if confirmed, would further suggest limited photochemical processing at the pressures probed. Together, the molecular inventory, enriched metallicity, and low C/O ratio point to an oxygen-rich atmosphere and establish HAT-P-1 b as a benchmark for comparative studies of hot-Jupiter atmospheric composition.

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arXiv2026

Asymmetric Aerosol Distribution on the Terminators of the Warm Saturn WASP-69 b Revealed by JWST NIRISS/SOSS

How aerosols form, are transported, and cycle between condensation and evaporation across exoplanet temperature regimes remains poorly understood. Recent models and observations suggest that warm giant planets near $800$--$1000$ K may span a transition between homogeneous and longitudinally heterogeneous aerosol distributions. We present a robust detection of aerosol asymmetry in a giant planet with $T_{\rm eq}\lesssim1000$ K, using the $0.86$--$2.82~\mu$m JWST NIRISS/SOSS transmission spectrum of WASP-69 b. The evening limb shows prominent 1.4 $\mu$m H$_2$O absorption ($\Delta\mathrm{BIC}_{\rm H_2O}=+22.7$), whereas H$_2$O is not detected on the morning limb ($\Delta\mathrm{BIC}_{\rm H_2O}=-8.7$). Atmospheric retrievals reveal significant aerosol opacity on both limbs, with high-altitude, optically thick clouds muting molecular features on the morning limb and lower cloud opacity allowing H$_2$O to emerge on the evening limb. The evening terminator is hotter by $304^{+62}_{-91}$ K, consistent with morning-limb condensates partially evaporating during transport toward the evening limb. This mechanism is independently verified with 3D general circulation models. Stellar contamination or aerosols dominated by photochemical haze do not readily explain the asymmetry. From a limb-resolved analysis, we infer a stellar-to-superstellar atmospheric metallicity, with $\rm[M/H]=0.11^{+0.40}_{-0.46}$ from the equilibrium retrieval and [O/H]$=1.38^{+0.44}_{-0.79}$ from the free retrieval. We also detect an escaping metastable-helium tail extending to $3.08^{+0.50}_{-0.45}\,R_p$. WASP-69 b anchors the cooler edge of the emerging population of planets with asymmetric aerosol distributions and suggests that substantial aerosol opacity may persist on both limbs across this transition.

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arXiv2026

Mitigating Charge Migration in JWST NIRISS Reveals That KELT-7 b is a Metal-enriched Ultra-hot Jupiter Orbiting a Young Metal-rich Star

We present the first panchromatic JWST transmission spectrum of an ultra-hot Jupiter, combining NIRISS and NIRSpec observations to constrain KELT-7\,b's atmospheric properties. We show evidence for charge migration in our NIRISS SOSS observation between 1--1.5~$\mu$m, a wavelength range crucial to test for enhanced H$^-$ previously inferred from HST WFC3/IR G141 observations. We mitigate charge migration by fitting the ramp after extracting 1D stellar spectra at the group level. This ``late-ramp-fit'' method accurately calculates KELT-7\,b's transmission spectrum between 1--1.5~$\mu$m at higher signal-to-noise. Using the transit-derived stellar mean density during stellar property inference reveals that KELT-7 is a $640\pm100$ Myr-old, $[\text{Fe}/\text{H}]=0.46\pm0.02$ star. Combined with NIRSpec and re-reduced WFC3/UVIS G280 data, our free retrieval analysis shows strong evidence for H$_2$O, CO$_2$, and TiO among high-temperature species, but not H$^-$ or clouds. Unaccounted-for systematics may therefore bias longer-wavelength WFC3/IR G141 transit depths shallower. Our free retrieval, two equilibrium retrievals, and self-consistent grid fit all prefer a high metallicity but find discrepant C/O ratios. Agglomerated together, we constrain a super-stellar $\text{M/H}=92^{+24}_{-23}\times$~Solar and C/O~$\leq0.9$, suggesting enhanced metal accretion in the later stages of KELT-7\,b's formation. Our GCMs explain the observed lack of limb asymmetry with superrotating jet-driven efficient horizontal mixing. The stark contrast between our panchromatic analysis and prior analyses on subsets of these data demonstrates the value of broad wavelength coverage for the comprehensive study of exoplanet atmospheres.

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arXiv2026

Most Hot Jupiters Were Cool Giant Planets for More Than 1 Gyr

The origin of hot Jupiters is the oldest problem in exoplanet astrophysics. Hot Jupiters formed in situ or via disk migration should be in place just a few Myr after the formation of their host stars. On the other hand, hot Jupiters formed via eccentricity excitation and tidal damping as a result of planet--planet scattering or Kozai-Lidov oscillations may take 1 Gyr or more to arrive at their observed locations. We propose that the relative ages of hot Jupiters inside, near, and outside the bias-corrected peak of the observed hot Jupiter period distribution can be used to distinguish between these possibilities. Though the lack of precise and accurate age inferences for isolated hot Jupiter host stars makes this test difficult to implement, comparisons between the Galactic velocity dispersions of the hot Jupiter subpopulations enable this investigation. To transform relative age offsets into absolute age offsets, we calibrate the monotonically increasing solar neighborhood age--velocity dispersion relation using an all-sky sample of subgiants with precise ages and a metallicity distribution matched to that of hot Jupiter hosts. We find that the inside-peak and near-peak subpopulations are older than the outside-peak subpopulation, with the inside-peak subpopulation slightly older than the near-peak subpopulation. We conclude that at least 40\% but not more than 70\% of the hot Jupiter population must have formed via a late-time, peak-populating process like high-eccentricity migration that typically occurs more than 1.5 Gyr after system formation.

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arXiv2025

Hot Jupiters are Inflated Primarily by Shallow Heating

The unexpectedly large radii of transiting hot Jupiters have led to many proposals for the physical mechanisms responsible for heating their interiors. While it has been shown that hot Jupiters reinflate as their host stars brighten due to heating deep in planetary interiors, young hot Jupiters also exhibit signs of delayed cooling possibly related to heating closer to their surfaces. To investigate this ambiguity, we enhance our previously published hot Jupiter thermal evolution model by adding a parameter that allows for both deep heating and delayed cooling. We fit our thermal evolution models to a homogeneous, physically self-consistent catalog of accurate and precise hot Jupiter system properties in a hierarchical Bayesian framework. We find that hot Jupiters' interior cooling rates are reduced on average by 95\%--98\% compared to simpler anomalous heating models. The most plausible explanation for this inference is substantial shallow heating just below their radiative--convective boundaries that enables reinflation with much weaker deep heating. Shallow heating by Ohmic dissipation and/or temperature advection are therefore important components of accurate models of hot Jupiter atmospheres, especially in circulation models. If hot Jupiters are inflated primarily by shallow heating as we propose, then we predict that atmospheric circulation-related observables should increase with temperature in the range $T_{\text{eq}}~\lesssim1500~\text{K}$, peak in the range $1500~\text{K}~\lesssim~T_{\text{eq}}~\lesssim~1800~\text{K}$, and decrease in the range $T_{\text{eq}}~\gtrsim~1800~\text{K}$.

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arXiv2025

A Comprehensive Reanalysis of K2-18 b's JWST NIRISS+NIRSpec Transmission Spectrum

Sub-Neptunes are the most common type of planet in our galaxy. Interior structure models suggest that the coldest sub-Neptunes could host liquid water oceans underneath their hydrogen envelopes -- sometimes called ``hycean'' planets. JWST transmission spectra of the $\sim$ 250 K sub-Neptune K2-18 b were recently used to report detections of CH$_4$ and CO$_2$, alongside weaker evidence of (CH$_3$)$_2$S (dimethyl sulfide, or DMS). Atmospheric CO$_2$ was interpreted as evidence for a liquid water ocean, while DMS was highlighted as a potential biomarker. However, these notable claims were derived using a single data reduction and retrieval modeling framework, which did not allow for standard robustness tests. Here we present a comprehensive reanalysis of K2-18 b's JWST NIRISS SOSS and NIRSpec G395H transmission spectra, including the first analysis of the second-order NIRISS SOSS data. We incorporate multiple well-tested data reduction pipelines and retrieval codes, spanning 60 different data treatments and over 250 atmospheric retrievals. We confirm the detection of CH$_4$ ($\approx 4σ$), with a volume mixing ratio range $-2.14 \leq \log_{10} \mathrm{CH_4} \leq -0.53$, but we find no statistically significant or reliable evidence for CO$_2$ or DMS. Finally, we assess the retrieved atmospheric composition using photochemical-climate and interior models, demonstrating that our revised composition of K2-18\,b can be explained by an oxygen-poor mini-Neptune without requiring a liquid water surface or life.

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arXiv2025

A Large Catalog of DA White Dwarf Characteristics Using SDSS and Gaia Observations

We present a catalog of 8545 and 19,257 unique DA white dwarfs observed in SDSS Data Release 19 and previous SDSS data releases, respectively. This is the largest catalog of both spectroscopic and photometric measurements of DA white dwarfs available to date, and we make this catalog and all code used to create it publicly available. We measure the apparent radial velocity, spectroscopic effective temperature and surface gravity, and photometric effective temperature and radius for all objects in our catalog. We validate our measurements against other published white dwarf catalogs. For apparent radial velocities, surface gravities, and effective temperatures measured from spectra with signal-to-noise ratios $>50$, our measurements agree with published SDSS white dwarf catalogs to within 7.5 km/s, 0.060 dex, and $2.4\%$, respectively. For radii and effective temperatures measured with Gaia photometry, our measurements agree with other published Gaia datasets to within $0.0005$ $R_\odot$ and $3\%$, respectively. We use this catalog to investigate systematic discrepancies between white dwarfs observed in SDSS-V and previous generations of SDSS. For objects observed in both SDSS-V and previous generations, we uncover systematic differences between measured spectroscopic parameters depending on which set of survey data is used. On average, the measured apparent radial velocity of a DA white dwarf is $11.5$ km/s larger and the surface gravity is $0.015$ dex smaller when a white dwarf's spectroscopic parameters are measured using SDSS-V data compared to using data from previous generations of SDSS. These differences may be due to changes in the wavelength solution across survey generations.

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arXiv2025

Overcast mornings and clear evenings in hot Jupiter exoplanet atmospheres

Aerosols is an old topic in the young field of exoplanet atmospheres. Understanding what they are, how they form, and where they go has long provided a fertile playground for theorists. For observers, however, aerosols have been a multi-decade migraine, as their chronic presence hides atmospheric features. For hot Jupiters, the large day-night temperature contrast drives inhomogeneous thermal structures and aerosol distribution, leading to different limb properties probed by transit spectra. We present JWST NIRISS/SOSS spectra of morning and evening limbs for nine gas giants with equilibrium temperatures of ~800-1700 K. By measuring feature size of the 1.4 $μ$m water band for both limbs, we found three planets (WASP-39 b, WASP-94 Ab, and WASP-17 b) show prominent ($>$5$σ$) limb-limb atmospheric opacity difference with muted morning and clear evening limbs. The heavily muted water features on morning limbs indicate high-altitude (0.1 to 0.01 mbar) aerosols. To simultaneously have clear evening limbs requires processes with timescales ($\sim$day) comparable to advection to remove these lofted grains, and we found that both downwelling flow and dayside cloud evaporation could be plausible mechanisms. We hypothesize an empirical boundary--termed the "asymmetry horizon"--in temperature-gravity space that marks the transition where inhomogeneous aerosol coverage begins to emerge. Heterogeneous aerosol coverage is common among hot Jupiters. If unrecognized, limb averaging suppresses spectral features, mimicking high-mean-molecular-weight atmospheres, inflating inferred metallicity by up to 2 dex, and underestimating limb temperatures by as much as half. Finally, we introduce the Limb Spectroscopy Metric (LSM) to predict limb spectral feature size based on planet parameters.

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arXiv2025

Cloudy mornings and clear evenings on a gas giant exoplanet

The spectra of exoplanet atmospheres are affected by aerosols (clouds and hazes) of uncertain origin. Proposed aerosol formation mechanisms include gas condensation or photochemical reactions. We measure the transmission spectrum of the tidally locked gas giant exoplanet WASP-94A b and identify asymmetry in its atmosphere. The morning limb is cooler and cloudy, while the evening limb is hotter and exhibits gaseous H$_2$O absorption features. We interpret this difference as due to the formation of cloud droplets near the morning limb, which evaporate during circulation to the evening limb. The dominant aerosols are clouds cycling between the day and night sides of the atmosphere, not photochemical hazes. The resulting asymmetry can severely bias chemical abundance measurements, unless limb-resolved spectroscopy is available.

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arXiv2025

A Revised Density Estimate for the Largest Known Exoplanet, HAT-P-67 b

Low-density ($ρ< 0.1 \rm{~g~cm^{-3}}$) hot Saturns are expected to quickly ($<100$ Myr) lose their atmospheres due to stellar irradiation, explaining their rarity. HAT-P-67 b seems to be an exception, with $ρ< 0.09 \rm{~g~cm^{-3}}$ and maintaining its atmosphere to well after 1 Gyr. We present a photometric and spectroscopic follow-up of HAT-P-67 b to determine how it avoided mass loss. HAT-P-67 b orbits a $V=10.1$ evolved F-type star in a 4.81 day orbit. We present new radial velocity observations of the system from the NEID spectrograph on the WIYN 3.5m Telescope from a follow-up campaign robust to stellar activity. We characterize the activity using photometry and activity indicators, revealing a stellar rotation period ($5.40\pm0.09$ d) near HAT-P-67 b's orbital period. We mitigate the stellar activity using a constrained quasi-periodic Gaussian process through a joint fit of archival ground-based photometry, TESS photometry, and our NEID observations, obtaining a planetary mass of $M_p = 0.45 \pm 0.15~M_{\rm J}$. Combined with a radius measurement of $R_p=2.140 \pm 0.025~R_{\rm J}$, this yields a density of $ρ_p = 0.061^{+0.020}_{-0.021} \rm{~g~cm^{-3}}$, making HAT-P-67 b the second lowest-density hot giant known to date. We find the recent evolution of the host star caused mass loss for HAT-P-67 b to only recently occur. The planet will be tidally disrupted/engulfed in $\sim 150-500$ Myr, shortly after losing its atmosphere. With rapid atmospheric mass loss, a large, helium leading tail, and upcoming observations with the Hubble Space Telescope, HAT-P-67 b is an exceptional target for future studies, for which an updated mass measurement provides important context.

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arXiv2024

Detection of the Temperature Dependence of the White Dwarf Mass-Radius Relation with Gravitational Redshifts

Models predict that the well-studied mass-radius relation of white dwarf stars depends on the temperature of the star, with hotter white dwarfs having larger masses at a given radius than cooler stars. In this paper, we use a catalog of 26,041 DA white dwarfs observed in Sloan Digital Sky Survey Data Releases 1-19. We measure the radial velocity, effective temperature, surface gravity, and radius for each object. By binning this catalog in radius or surface gravity, we average out the random motion component of the radial velocities for nearby white dwarfs to isolate the gravitational redshifts for these objects and use them to directly measure the mass-radius relation. For gravitational redshifts measured from binning in either radius or surface gravity, we find strong evidence for a temperature-dependent mass-radius relation, with warmer white dwarfs consistently having greater gravitational redshifts than cool objects at a fixed radius or surface gravity. For warm white dwarfs, we find that their mean radius is larger and mean surface gravity is smaller than those of cool white dwarfs at 5.2σ and 6.0σ significance, respectively. Selecting white dwarfs with similar radii or surface gravities, the significance of the difference in mean gravitational redshifts between the warm and cool samples is >6.1σ and >3.6σ for measurements binned in radius and surface gravity, respectively, in the direction predicted by theory. This is an improvement over previous implicit detections, and our technique can be expanded to precisely test the white dwarf mass-radius relation with future surveys.

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arXiv2024

Resonant and Ultra-short-period Planet Systems are at Opposite Ends of the Exoplanet Age Distribution

Exoplanet systems are thought to evolve on secular timescales over billions of years. This evolution is impossible to directly observe on human timescales in most individual systems. While the availability of accurate and precise age inferences for individual exoplanet host stars with ages $τ$ in the interval $1~\text{Gyr}\lesssim~τ~\lesssim10~\text{Gyr}$ would constrain this evolution, accurate and precise age inferences are difficult to obtain for isolated field dwarfs like the host stars of most exoplanets. The Galactic velocity dispersion of a thin disk stellar population monotonically grows with time, and the relationship between age and velocity dispersion in a given Galactic location can be calibrated by a stellar population for which accurate and precise age inferences are possible. Using a sample of subgiants with precise age inferences, we calibrate the age--velocity dispersion relation in the Kepler field. Applying this relation to the Kepler field's planet populations, we find that Kepler-discovered systems plausibly in second-order mean-motion resonances have $1~\text{Gyr}\lesssim~τ~\lesssim2~\text{Gyr}$. The same is true for systems plausibly in first-order mean-motion resonances, but only for systems likely affected by tidal dissipation inside their innermost planets. These observations suggest that many planetary systems diffuse away from initially resonant configurations on secular timescales. Our calibrated relation also indicates that ultra-short-period (USP) planet systems have typical ages in the interval $5~\text{Gyr}\lesssim~τ~\lesssim6~\text{Gyr}$. We propose that USP planets tidally migrated from initial periods in the range $1~\text{d}\lesssim~P~\lesssim2~\text{d}$ to their observed locations at $P<1~\text{d}$ over billions of years and trillions of cycles of secular eccentricity excitation and inside-planet damping.

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arXiv2023

A Lithium Depletion Age for the Carina Association

The dispersed remnants of stellar nurseries, stellar associations provide unparalleled samples of coeval stars critical for studies of stellar and planetary formation and evolution. The Carina Stellar Association is one of the closest stellar associations to Earth, and yet measurements of its age have varied from 13 to 45 Myr. We aim to update the age of Carina using the Lithium Depletion Boundary method. We obtain new measurements of the Li 6708 Angstrom, absorption feature in likely members using optical spectra from the Goodman HTS on SOAR and NRES on LCO. We detect the depletion boundary at M_K ~= 6.8 (M5), which corresponds to an age of 41(+3,-5) Myr. The age is consistent within uncertainties across six different models, including those that account for magnetic fields and spots. We also estimate the age through analysis of the group's overall variability, and by comparing the association members' CMD to stellar evolutionary models using a Gaussian Mixture Model, recovering ages consistent with the LDB. The resulting age agrees with the older end of previous age measurements and is consistent with the lithium depletion age for the neighboring Tucana-Horologium Moving Group.

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arXiv2023

Verification of Gaia DR3 Single-lined Spectroscopic Binary Solutions With Three Transiting Low-mass Secondaries

While secondary mass inferences based on single-lined spectroscopic binary (SB1) solutions are subject to $\sin{i}$ degeneracies, this degeneracy can be lifted through the observations of eclipses. We combine the subset of Gaia Data Release (DR) 3 SB1 solutions consistent with brown dwarf-mass secondaries with the Transiting Exoplanet Survey Satellite (TESS) Object of Interest (TOI) list to identify three candidate transiting brown dwarf systems. Ground-based precision radial velocity follow-up observations confirm that TOI-2533.01 is a transiting brown dwarf with $M=72^{+3}_{-3}~M_{\text{Jup}}= 0.069^{+0.003}_{-0.003}~M_\odot$ orbiting TYC 2010-124-1 and that TOI-5427.01 is a transiting very low-mass star with $M=93^{+2}_{-2}~M_{\text{Jup}}=0.088^{+0.002}_{-0.002}~M_\odot$ orbiting UCAC4 515-012898. We validate TOI-1712.01 as a very low-mass star with $M=82^{+7}_{-7}~M_{\text{Jup}}=0.079^{+0.007}_{-0.007}~M_\odot$ transiting the primary in the hierarchical triple system BD+45 1593. Even after accounting for third light, TOI-1712.01 has radius nearly a factor of two larger than predicted for isolated stars with similar properties. We propose that the intense instellation experienced by TOI-1712.01 diminishes the temperature gradient near its surface, suppresses convection, and leads to its inflated radius. Our analyses verify Gaia DR3 SB1 solutions in the low Doppler semiamplitude limit, thereby providing the foundation for future joint analyses of Gaia radial velocities and Kepler, K2, TESS, and PLAnetary Transits and Oscillations (PLATO) light curves for the characterization of transiting massive brown dwarfs and very low-mass stars.

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arXiv2022

TESS Hunt for Young and Maturing Exoplanets (THYME) IX: a 27 Myr extended population of Lower-Centaurus Crux with a transiting two-planet system

We report the discovery and characterization of a nearby (~ 85 pc), older (27 +/- 3 Myr), distributed stellar population near Lower-Centaurus-Crux (LCC), initially identified by searching for stars co-moving with a candidate transiting planet from TESS (HD 109833; TOI 1097). We determine the association membership using Gaia kinematics, color-magnitude information, and rotation periods of candidate members. We measure it's age using isochrones, gyrochronology, and Li depletion. While the association is near known populations of LCC, we find that it is older than any previously found LCC sub-group (10-16 Myr), and distinct in both position and velocity. In addition to the candidate planets around HD 109833 the association contains four directly-imaged planetary-mass companions around 3 stars, YSES-1, YSES-2, and HD 95086, all of which were previously assigned membership in the younger LCC. Using the Notch pipeline, we identify a second candidate transiting planet around HD 109833. We use a suite of ground-based follow-up observations to validate the two transit signals as planetary in nature. HD 109833 b and c join the small but growing population of <100 Myr transiting planets from TESS. HD 109833 has a rotation period and Li abundance indicative of a young age (< 100 Myr), but a position and velocity on the outskirts of the new population, lower Li levels than similar members, and a CMD position below model predictions for 27 Myr. So, we cannot reject the possibility that HD 109833 is a young field star coincidentally nearby the population.

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arXiv2022

TESS Hunt for Young and Maturing Exoplanets (THYME) VI: an 11 Myr giant planet transiting a very low-mass star in Lower Centaurus Crux

Mature super-Earths and sub-Neptunes are predicted to be $\simeq$Jovian radius when younger than 10 Myr. Thus, we expect to find 5-15$R_\oplus$ planets around young stars even if their older counterparts harbor none. We report the discovery and validation of TOI 1227 b, a $0.85\pm0.05R_J$ (9.5$R_\oplus$) planet transiting a very low-mass star ($0.170\pm0.015M_\odot$) every 27.4 days. TOI~1227's kinematics and strong lithium absorption confirm it is a member of a previously discovered sub-group in the Lower Centaurus Crux OB association, which we designate the Musca group. We derive an age of 11$\pm$2 Myr for Musca, based on lithium, rotation, and the color-magnitude diagram of Musca members. The TESS data and ground-based follow-up show a deep (2.5\%) transit. We use multiwavelength transit observations and radial velocities from the IGRINS spectrograph to validate the signal as planetary in nature, and we obtain an upper limit on the planet mass of $\simeq0.5 M_J$. Because such large planets are exceptionally rare around mature low-mass stars, we suggest that TOI 1227 b is still contracting and will eventually turn into one of the more common $<5R_\oplus$ planets.

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