arXiv subjects

Chih-Chun Hsu

Publications and source records attributed to Chih-Chun Hsu.

At least 19 recordsLinked to original sources

arXiv2026

Atmospheric characterization of six ultra-hot Jupiters from $K$-band high-resolution spectroscopy

We present new Keck/KPIC high-resolution spectroscopic detections of three ultra-hot Jupiters (UHJs) in the $K$ band: WASP-189b ($\rm SNR = 7.2$), MASCARA-1b ($\rm SNR = 8.6$), and TOI-1518b ($\rm SNR = 7.1$), as well as a tentative detection of KELT-9b ($\rm SNR = 5.0$). We perform a uniform set of atmospheric retrieval analysis on these objects, as well as previously reported KPIC observations of WASP-33b ($\rm SNR = 11.2$) and KELT-20b ($\rm SNR = 10.5$), We perform atmospheric retrievals for the pressure-temperature ($P-T$) profile, orbital velocity parameters, $v\sin i$, and abundances of CO, H$_2$O, OH, and Fe, with parameterized mixing profiles to account for the expected vertical abundance variations of H$_2$O and OH. We also perform a set of retrievals assuming chemical equilibrium, which are generally in good agreement with the free retrievals. Except for \knb, the retrieved spectra are dominated by CO emission features, with additional weak H$_2$O or OH features consistent with thermal dissociation of H$_2$O. \knb, which is significantly hotter, appears to have very weak molecular features. Dissociation limits our ability to reliably constrain H$_2$O or OH abundances from $K$ band data alone, resulting in poor constraints on the C/O ratio. For all objects, the atmospheric abundances from detected carbon and oxygen species are $1-10\times$ solar. These results highlight the importance of wide spectral coverage for high-resolution retrievals. Additional observations to expand phase and wavelength coverage are needed to better constrain oxygen species and possible spatial inhomogeneities from dissociation.

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arXiv2026

Evidence of Radius Inflation Based on 50 Transiting Brown Dwarfs and Low-mass Stellar Companions

Statistical assessment of stellar parameters enables validation and improvements in theoretical models. We compiled a sample of 85 transiting stellar and substellar companions, with masses ranging from $\sim$13-100 $M_\mathrm{Jup}$. We focus on analyzing 50 transiting companions with robust ages and model constraints, and evaluate the degree of radius inflation versus mass, separation, equilibrium temperature, and metallicity. Our evaluation of the differences between measured and model radii indicates an $8.7\pm1.9$% radius inflation for the full sample, at $4.6σ$ discrepancy, validating the existence of radius inflation at a population level in transiting brown dwarfs. For brown dwarfs at separations $\leq 0.05$ au, we find an even higher radius inflation, with a median inflation of $16\pm6$% at $2.7σ$, and the inflation decreases toward wider separations, likely due to reduced stellar irradiation. Finally, we provide our compilation for the community to use.

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arXiv2026

Distinct Rotational Evolution of Giant Planets and Brown Dwarf Companions

We present a rotational velocity (vsini) survey of 32 stellar/substellar objects and giant planets using Keck/KPIC high-resolution spectroscopy, including 6 giant planets (2-7 M$_\mathrm{Jup}$) and 25 substellar/stellar companions (12-88 M$_\mathrm{Jup}$). Adding companions with spin measurements from the literature, we construct a curated spin sample for 43 benchmark stellar/substellar companions and giant planets and 54 free-floating brown dwarfs and planetary mass objects. We compare their spins, parameterized as fractional breakup velocities at 10 Myr, assuming constant angular momentum evolution. We find the first clear evidence that giant planets exhibit distinct spins versus low-mass brown dwarf companions (10 to 40 M$_\mathrm{Jup}$) at 4-4.5 $σ$ significance assuming inclinations aligned with their orbits, while under randomly oriented inclinations the significance is at 1.6-2.1 $σ$. Our findings hold when considering various assumptions about planets, and the mass ratio below 0.8% gives a clean cut for rotation between giant planets and brown dwarf companions. The higher fractional breakup velocities of planets can be interpreted as less angular momentum loss through circumplanetary disk braking during the planet formation phase. Brown dwarf companions exhibit evidence of slower rotation compared to isolated brown dwarfs, while planets and planetary mass objects show similar spins. Finally, our analysis of specific angular momentum versus age of 221 stellar/substellar objects below 0.1 M$_{\odot}$ with spin measurements in the literature indicates that the substellar objects of 5-40 M$_\mathrm{Jup}$ retain much higher angular momenta compared to stellar and substellar objects of 40-100 M$_\mathrm{Jup}$ after 10 Myr, when their initial angular momenta were set.

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arXiv2026

TOI-4616 b: a benchmark Earth-sized planet transiting a nearby M4 dwarf

Rocky exoplanets are particularly abundant around M-type stars. Their small radii and low luminosities provide favourable conditions for detecting transiting terrestrial planets and probing their atmospheric properties. We report the discovery and statistical validation of TOI-4616 b, an Earth-sized planet transiting a nearby mid-M dwarf observed by the Transiting Exoplanet Survey Satellite (TESS). We confirm the planetary nature of the signal and determine the system parameters by combining TESS photometry with ground-based multi-band transit observations, high-resolution imaging, and optical and near-infrared spectroscopy. The host star lies at a distance of 28.10 +(-) 0.07 pc and has a radius of 0.1889 +(-)0.0096 solar radii, a mass of 0.1881 +(-) 0.0094 solar masses, and an effective temperature of 3150 +(-) 75 K. TOI-4616 b has a radius of 1.22 Earth radii and an orbital period of 1.55 days. The planet receives an incident flux of approximately 40 times that of Earth, corresponding to an equilibrium temperature of about 525 K. This places TOI-4616 b in a regime intermediate between Earth-sized planets orbiting early M dwarfs and those around ultra-cool hosts. Statistical validation with the TRICERATOPS framework, supported by high-resolution imaging and chromatic transit constraints, yields a false-positive probability of 0.0135, below the recommended validation threshold of 0.015, confirming TOI-4616 b as a validated planet. Owing to its proximity to Earth, well-constrained stellar properties, and extensive multi-band follow-up, TOI-4616 b constitutes a valuable benchmark system for comparative studies of terrestrial planets around mid-M dwarfs and for future atmospheric investigations.

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arXiv2026

Possible stratospheric emission in the warm Neptune GJ 436 b from high-resolution spectroscopy

We present high spectral resolution $L$ band (2.91--3.85 $μ$m) observations of the warm Neptune GJ 436 b from Keck II/KPIC. KPIC's single-mode fiber feed reduces the $L$ band sky background by a factor of 100, significantly improving sensitivity compared to a seeing-limited spectrometer and enabling a tentative ($\rm SNR = 3-4$) cross-correlation detection of GJ 436 b with a thermally inverted atmospheric model. In contrast with recent results from $JWST$ and high-resolution transmission spectroscopy, our retrieval analysis prefers the presence of H$_2$O, and possibly CH$_4$, molecular features in emission. The broad-band continuum flux associated with the maximum-likelihood model is substantially higher than expected based on both the $\sim670\rm\ K$ equilibrium temperature of GJ 436 b and previous results from low-resolution spectroscopy. We demonstrate that the loss of continuum information during the processing of high-resolution spectra makes our analysis effectively insensitive to the absolute continuum level of the planet, and that scaling the maximum-likelihood model to match the broad-band flux measured from low-resolution observations of GJ 436 b results in a detection of similar strength in cross-correlation. These results could be explained by a thermal inversion arising above a haze layer in the upper atmosphere of \gjb. Further observations, ideally post-eclipse in order to break the $K_p - Δv_{sys}$ degeneracy, are needed to clarify this possible detection. This work demonstrates the potential of $L$ band high-resolution spectroscopy for characterizing significantly smaller and cooler exoplanets compared with hot Jupiters.

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arXiv2025

Dynamical Architectures of S-type Transiting Planets in Binaries II: A Dichotomy in Orbital Alignment of Small Planets in Close Binary Systems

Stellar multiplicity plays a crucial role in shaping planet formation and dynamical evolution. We present a survey of 54 TESS Objects of Interest (TOIs) within 300 pc that exhibit significant Hipparcos-Gaia astrometric accelerations. We identified 35 TOIs with stellar companions at projected separations between $0.1^{\prime\prime}$ to $2^{\prime\prime}$ (or $10-200$ AU). We also identified 12 TOIs that could host planetary-mass or brown dwarf companions, including 6 that are newly discovered. Furthermore, we perform three-dimensional orbital characterization for 12 binaries hosting confirmed planets or planet candidates, allowing us to constrain the line-of-sight mutual inclination, $ΔI_{\mathrm{los}}$, between the planetary and binary orbits. Combining our sample with previous measurements, we apply Bayesian hierarchical analysis to a total of 26 binary systems with S-type transiting planets ($r_p<5R_{\oplus}$). Specifically, we fit the $ΔI_{\mathrm{los}}$ distribution with both single (Rayleigh) and mixture models (two-component Rayleigh and Rayleigh-isotropic mixture). We find the mixture models are strongly favored ($\log Z\gtrsim13.9$, or $\approx$5$σ$), indicating the observed planet-binary $ΔI_{\mathrm{los}}$ values likely originate from two underlying populations: one nearly aligned ($σ_1 = 2^{\circ}.4^{+0.7}_{-0.9}$) and one with more scattered mutual inclinations ($σ_2 = 23^{\circ}.6^{+8.8}_{-7.1}$). Alternatively, the misaligned systems can be equally well described by an isotropic distribution of inclinations. This observed dichotomy likely reflects different dynamical histories. Notably, the misaligned population only emerges in systems with stellar periastron distances $>40$ AU while systems with close-in or eccentric stellar companions (periastron distances $<40$ AU) preserve planet-binary alignment.

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arXiv2025

An Ancient Brown Dwarf Transiting a Metal-Poor Thick Disk Star

We report the discovery of TOI-7019b, the first transiting brown dwarf (BD) known to orbit a star that is part of the Milky Way's ancient thick disk, as defined chemically ([Fe/H] $= -0.79 \pm 0.05$ dex, [$α$/Fe] $= +0.26 \pm 0.05$ dex, [M/H] $= -0.59 \pm 0.06$ dex) and kinematically ($v_{\perp} \approx 150 \pm 1$ km s$^{-1}$). We estimate a system age $τ= 12 \pm 2$ Gyr by fitting the host star's spectrum and spectral energy distribution to alpha-enhanced isochrones, and independently using the age-metallicity relation of the thick disk. This makes TOI-7019 by far the most metal-poor and ancient BD host known to date. We measure a BD mass of $61.3 \pm 2.1$ $M_{\rm J}$ and radius of $0.82 \pm 0.02$ $R_{\rm J}$ from a joint analysis of transit photometry and radial velocity measurements, along with an orbital period of $48.2592 \pm 0.0001$ days and an orbital eccentricity of $0.403 \pm 0.002$. The measured radius appears $12.3\% \pm 2.8\%$ larger than predicted relative to standard evolutionary models for old, metal-poor brown dwarfs, hinting at missing physics like the magnetic inhibition of convection. TOI-7019b lowers the probed metallicity regime for transiting BDs by over a factor of two, making it a benchmark system to test evolutionary models in the low-metallicity regime. Future measurements of TOI-7019b's atmosphere will test whether a brown dwarf's atmospheric composition tracks its host star's abundances, as expected for binary-like co-formation.

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arXiv2025

Chemical and Isotopic Homogeneity Between the L Dwarf CD-35 2722 B and its Early M Host Star

CD-35 2722 B is an L dwarf companion to the nearby, $\sim 50-200$ Myr old M1 dwarf CD-35 2722 A. We present a detailed analysis of both objects using high-resolution ($R \sim 35,000$) $K$ band spectroscopy from the Keck Planet Imager and Characterizer (KPIC) combined with archival photometry. With a mass of $30^{+5}_{-4} M_{\mathrm{Jup}}$ (planet-to-host mass ratio 0.05) and projected separation of $67\pm4$ AU from its host, CD-35 2722 B likely formed via gravitational instability. We explore whether the chemical composition of the system tells a similar story. Accounting for systematic uncertainties, we find $\mathrm{[M/H]}=-0.16^{+0.03}_{-0.02} \mathrm{(stat)} \pm 0.25 \mathrm{(sys)}$ dex and $^{12}\mathrm{C}/^{13}\mathrm{C}=132^{+20}_{-14}$ for the host, and $\mathrm{[M/H]}=0.27^{+0.07}_{-0.06} (\mathrm{stat}) \pm 0.12 (\mathrm{sys})$ dex, $^{12}\mathrm{CO}/^{13}\mathrm{CO}=159^{+33}_{-24} \mathrm{(stat)}^{+40}_{-33} \mathrm{(sys)}$, and $\mathrm{C/O} = 0.55 \pm 0.01 (\mathrm{stat}) \pm 0.04 (\mathrm{sys})$ for the companion. The chemical compositions for the brown dwarf and host star agree within the $1.5σ$ level, supporting a scenario where CD-35 2722 B formed via gravitational instability. We do not find evidence for clouds on CD-35 2722 B despite it being a photometrically red mid-L dwarf and thus expected to be quite cloudy. We retrieve a temperature structure which is more isothermal than models and investigate its impact on our measurements, finding that constraining the temperature structure to self-consistent models does not significantly impact our retrieved chemical properties. Our observations highlight the need for data from complementary wavelength ranges to verify the presence of aerosols in likely cloudy L dwarfs.

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arXiv2025

Observation of undepleted phosphine in the atmosphere of a low-temperature brown dwarf

The atmospheres of low-temperature brown dwarfs and gas giant planets are expected to contain the phosphine molecule, PH$_3$ However, previous observations have shown much lower abundances of this molecule than predicted by atmospheric chemistry models. We report JWST spectroscopic observations of phosphine in the atmosphere of the brown dwarf Wolf 1130C. Multiple absorption lines due to phosphine are detected around 4.3 $μ$m, from which we calculate a phosphine abundance of 0.100$\pm$0.009 parts per million. This abundance is consistent with disequilibrium atmospheric chemistry models that reproduce the phosphine abundances in Jupiter and Saturn, and is much higher than abundances previously reported for other brown dwarfs or exoplanets.

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arXiv2025

The watery atmosphere of HD~209458~b revealed by joint $K$- and $L$-band high-resolution spectroscopy

We present a joint analysis of high-resolution $K$- and $L$-band observations of the benchmark hot Jupiter \hdb\ from the Keck Planet Imager and Characterizer (KPIC). One half night of observations were obtained in each bandpass covering similar pre-eclipse phases. The two epochs were then jointly analyzed using our atmospheric retrieval pipeline based on \petit\ to constrain the atmospheric pressure-temperature profile and chemical composition. Consistent with recent results from \textit{JWST} observations at lower spectral resolution, we obtain an oxygen-rich composition for \hdb\ ($\rm C/O < 10^{-3}$ at 95\% confidence) and a lower limit on the volatile metallicity similar to the solar value ($\rm [(C+O)/H] > -0.2$ at 95\% confidence). Leveraging the large spectral grasp of the multi-band observations, we constrain the H$_2$O mixing ratio to $\rm \log H_2O_{VMR} > -3.1$ at 95\% confidence, and obtain 95\% upper limits on the atmospheric mixing ratios of CO ($<10^{-4.8}$), CH$_4$ ($<10^{-4.5}$), NH$_3$ ($<10^{-5.8}$), H$_2$S ($<10^{-3.3}$), and HCN ($<10^{-5.6}$). The limits on CH$_4$, NH$_3$, and HCN are consistent with recent results from \textit{JWST} transmission spectroscopy, demonstrating the value of multi-band ground-based high resolution spectroscopy for precisely constraining trace species abundances in exoplanet atmospheres. The retrieved low-C/O, moderate-metallicity composition for \hdb\ is consistent with formation scenarios involving late accretion of substantial quantities of oxygen-rich refractory solids and/or ices.

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arXiv2025

Fringing analysis and forward modeling of Keck Planet Imager and Characterizer (KPIC) spectra

The Keck Planet Imager and Characterizer (KPIC) combines high contrast imaging with high resolution spectroscopy (R~35,000 in K band) to study directly imaged exoplanets and brown dwarfs in unprecedented detail. KPIC aims to spectrally characterize substellar companions through measurements of planetary radial velocities, spins, and atmospheric composition. Currently, the dominant source of systematic noise for KPIC is fringing, or oscillations in the spectrum as a function of wavelength. The fringing signal can dominate residuals by up to 10% of the continuum for high S/N exposures, preventing accurate wavelength calibration, retrieval of atmospheric parameters, and detection of planets with flux ratios less than 1% of the host star. To combat contamination from fringing, we first identify its three unique sources and adopt a physically informed model of Fabry-Perot cavities to apply to post-processed data. We find this strategy can effectively model the fringing in observations of A0V/F0V stars, reducing the residual systematics caused by fringing by a factor of 2. Next, we wedge two of the transmissive optics internal to KPIC to eliminate two sources of fringing and confirm the third source as the entrance window to the spectrograph. Finally, we apply our previous model of the Fabry-Perot cavity to new data taken with the wedged optics to reduce the amplitude of the residuals by a factor of 10.

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arXiv2025

The 3D Kinematics of the Orion Nebula Cluster II: Mass-dependent Kinematics of the Inner Cluster

We present the kinematic analysis of $246$ stars within $4\mbox{$^\prime$}$ from the center of Orion Nebula Cluster (ONC), the closest massive star cluster with active star formation across the full mass range, which provides valuable insights into the formation and evolution of star clusters on an individual-star basis. High-precision radial velocities and surface temperatures are retrieved from spectra acquired by the NIRSPEC instrument used with adaptive optics (NIRSPAO) on the Keck II 10-m telescope. A three-dimensional kinematic map is then constructed by combining with the proper motions previously measured by the Hubble Space Telescope (HST) ACS/WFPC2/WFC3IR and Keck II NIRC2. The measured root-mean-squared velocity dispersion is $2.26\pm0.08~\mathrm{km}\,\mathrm{s}^{-1}$, significantly higher than the virial equilibrium's requirement of $1.73~\mathrm{km}\,\mathrm{s}^{-1}$, suggesting that the ONC core is supervirial, consistent with previous findings. Energy equipartition is not detected in the cluster. Most notably, the velocity of each star relative to its neighbors is found to be negatively correlated with stellar mass. Low-mass stars moving faster than their surrounding stars in a supervirial cluster suggest that the initial masses of forming stars may be related to their initial kinematic states. Additionally, a clockwise rotation preference is detected. A weak sign of inverse mass segregation is also identified among stars, excluding the Trapezium stars, though it could be a sample bias. Finally, this study reports the discovery of four new candidate spectroscopic binary systems.

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arXiv2025

Water dissociation and rotational broadening in the atmosphere of KELT-20 b from high-resolution spectroscopy

We present atmospheric retrievals from Keck/KPIC phase II observations of the ultra-hot Jupiter KELT-20/MASCARA-2~b. Previous free retrievals of molecular abundances for ultra-hot Jupiters have been impacted by significant model biases due to variations in vertical abundance profiles, which we address by including molecular dissociation into our retrieval framework as an additional free parameter. We measure the abundance of CO ($\rm \log CO_{MMR} = -2.5^{+0.6}_{-0.5}$) and obtain a lower limit on the abundance of H$_2$O ($\rm \log H{_2}O_{MMR} = -1.5^{+0.8}_{-1.0}$, $>-3.0$ at 95\% confidence) in the atmosphere of \keltb. These abundances yield an atmospheric $\rm C/O = 0.1^{+0.4}_{-0.1}$ ($\rm C/O < 0.9$ at 95\% confidence) and suggest a metallicity approximately solar to $10\times$ solar. H$_2$O is dissociated at pressures below $\log P_{\rm H_2O} = -1.2^{+0.5}_{-0.7}$ bar, roughly consistent with predictions from chemical equilibrium models, and suggesting that the retrieved composition is not a result of assumptions about the vertical mixing profiles. We also constrain the rotational velocity of \keltb\ to $v\sin i = 7.5\pm0.7$ \kms, suggesting the presence of a jet comparable to the sound speed in the direction of the planet's rotation, assuming the actual rotation of the planet is tidally locked.

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arXiv2025

HD 206893 B at High Spectral Resolution with the Keck Planet Imager and Characterizer (KPIC)

We present an atmospheric characterization and orbital analysis of HD 206893 B, an exceptionally red, L/T-transition substellar companion in a multiplanetary system, via Keck Planet Imager and Characterizer (KPIC) high-resolution (R $\sim$ 35,000) K-band spectroscopy. Using PHOENIX atmospheric models in a forward-model framework that fits the spectrum of the companion and diffracted starlight simultaneously, we detect HD 206893 B at $>8σ$ significance via cross-correlation in two epochs. We find an effective temperature for the companion of $1634^{+72}_{-38}$ K and a log(g) of $4.55^{+0.17}_{-0.22}$. Only accounting for statistical uncertainties, we measure the carbon-oxygen ratio (C/O) of this companion to be $0.57 \pm 0.02$, or near-solar while assuming solar metallicity. The C/O ratio we measure fits the tentative trend of $>4 M_{Jup}$ companions having near-solar C/O ratios while less massive companions have greater-than-solar C/O ratios. Using substellar evolution models, we find an age of $112^{+36}_{-22}$ Myr, a mass of $22.7^{+2.5}_{-1.7} M_{Jup}$, and a radius of $1.11 \pm 0.03 R_{Jup}$ for this companion. We also use KPIC radial velocity data to fit the orbit of HD 206893 B and analyze the orbital stability of this system. We find that the orbital stability is relatively independent of the mass of HD 206893 B, and favors an orbital configuration where B and its interior planetary companion, HD 206893 c, are co-planar. The measured C/O ratio coupled with the current architecture of the system cannot rule out a core accretion scenario, nor a disk fragmentation scenario regarding the formation pathway of HD 206893 B.

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arXiv2024

PDS 70b Shows Stellar-like Carbon-to-oxygen Ratio

The $\sim$5 Myr PDS 70 is the only known system with protoplanets residing in the cavity of the circumstellar disk from which they formed, ideal for studying exoplanet formation and evolution within its natal environment. Here we report the first spin constraint and C/O measurement of PDS 70b from Keck/KPIC high-resolution spectroscopy. We detected CO (3.8 $σ$) and H$_2$O (3.5 $σ$) molecules in the PDS 70b atmosphere via cross-correlation, with a combined CO and H$_2$O template detection significance of 4.2 $σ$. Our forward model fits, using BT-Settl model grids, provide an upper limit for the spin-rate of PDS 70b ($<$29 km s$^{-1}$). The atmospheric retrievals constrain the PDS 70b C/O ratio to ${0.28}^{+0.20}_{-0.12}$ ($<$0.63 under 95$\%$ confidence level) and a metallicity [C/H] of ${-0.2}^{+0.8}_{-0.5}$ dex, consistent with that of its host star. The following scenarios can explain our measured C/O of PDS 70b in contrast with that of the gas-rich outer disk (for which C/O $\gtrsim$ 1). First, the bulk composition of PDS 70b might be dominated by dust+ice aggregates rather than disk gas. Another possible explanation is that the disk became carbon-enriched $\textit{after}$ PDS 70b was formed, as predicted in models of disk chemical evolution and as observed in both very low mass star and older disk systems with $\textit{JWST}$/MIRI. Because PDS 70b continues to accrete and its chemical evolution is not yet complete, more sophisticated modeling of the planet and the disk, and higher quality observations of PDS 70b (and possibly PDS 70c), are necessary to validate these scenarios.

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arXiv2024

True mass and atmospheric composition of the non-transiting hot Jupiter HD 143105 b

We present Keck/KPIC phase II $K$-band observations of the non-transiting hot Jupiter HD 143105 b. Using a cross-correlation approach, we make the first detection of the planetary atmosphere at $K_p = 185^{+11}_{-13}\rm km\ s^{-1}$ and an inferior conjunction time 2.5 hours before the previously-published ephemeris. The retrieved $K_p$ value, in combination with orbital period, mass of the host star, and lack of transit detection, gives an orbital inclination of $78^{\circ+2}_{-12}$ and a true planet mass of 1.23$\pm0.10\rm\ M_J$. While the equilibrium temperature of HD 143105 b is in the transition regime between non-inverted and inverted atmospheres, our analysis strongly prefers a non-inverted atmosphere. Retrieval analysis indicates the atmosphere of HD 143105 b is cloud-free to approximately 1 bar and dominated by H$_2$O absorption ($\log \rm H_2O_{MMR} = -3.9^{+0.8}_{-0.5}$), placing only an upper limit on the CO abundance ($\log \rm CO_{MMR} < -3.7$ at 95% confidence). We place no constraints on the abundances of Fe, Mg, or $^{13}$CO. From these abundances, we place an upper limit on the carbon-to-oxygen ratio for HD 143105 b, $\rm C/O < 0.2$ at 95% confidence, and find the atmospheric metallicity is approximately $0.1\times$ solar. The low metallicity may be responsible for the lack of a thermal inversion, which at the temperature of HD 143105 b would likely require significant opacity from TiO and/or VO. With these results, HD 143105 b joins the small number of non-transiting hot Jupiters with detected atmospheres.

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arXiv2024

New Cold Subdwarf Discoveries from Backyard Worlds and a Metallicity Classification System for T Subdwarfs

We report the results of a spectroscopic survey of candidate T subdwarfs identified by the Backyard Worlds: Planet 9 program. Near-infrared spectra of 31 sources with red $J-W2$ colors and large $J$-band reduced proper motions show varying signatures of subsolar metallicity, including strong collision-induced H$_2$ absorption, obscured methane and water features, and weak K I absorption. These metallicity signatures are supported by spectral model fits and 3D velocities, indicating thick disk and halo population membership for several sources. We identify three new metal-poor T subdwarfs ([M/H] $\lesssim$ $-$0.5), CWISE J062316.19+071505.6, WISEA J152443.14$-$262001.8, and CWISE J211250.11-052925.2; and 19 new "mild" subdwarfs with modest metal deficiency ([M/H] $\lesssim$ $-$0.25). We also identify three metal-rich brown dwarfs with thick disk kinematics. We provide kinematic evidence that the extreme L subdwarf 2MASS J053253.46+824646.5 and the mild T subdwarf CWISE J113010.07+313944.7 may be part of the Thamnos population, while the T subdwarf CWISE J155349.96+693355.2 may be part of the Helmi stream. We define a metallicity classification system for T dwarfs that adds mild subdwarfs (d/sdT), subdwarfs (sdT), and extreme subdwarfs (esdT) to the existing dwarf sequence. We also define a metallicity spectral index that correlates with metallicities inferred from spectral model fits and iron abundances from stellar primaries of benchmark T dwarf companions. This expansion of the T dwarf classification system supports investigations of ancient, metal-poor brown dwarfs now being uncovered in deep imaging and spectroscopic surveys.

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arXiv2024

RV measurements of directly imaged brown dwarf GQ Lup B to search for exo-satellites

GQ Lup B is one of the few substellar companions with a detected cicumplanetary disk, or CPD. Observations of the CPD suggest the presence of a cavity, possibly formed by an exo-satellite. Using the Keck Planet Imager and Characterizer (KPIC), a high contrast imaging suite that feeds a high resolution spectrograph (1.9-2.5 microns, R$\sim$35,000), we present the first dedicated radial velocity (RV) observations around a high-contrast, directly imaged substellar companion, GQ Lup B, to search for exo-satellites. Over 11 epochs, we find a best and median RV error of 400-1000 m/s, most likely limited by systematic fringing in the spectra due to transmissive optics within KPIC. With this RV precision, KPIC is sensitive to exomoons 0.6-2.8% the mass of GQ Lup B ($\sim 30 M_{\text{Jup}}$) at separations between the Roche limit and $65 R_{\text{Jup}}$, or the extent of the cavity inferred within the CPD detected around GQ Lup B. Using simulations of HISPEC, a high resolution infrared spectrograph planned to debut at W.M. Keck Observatory in 2026, we estimate future exomoon sensitivity to increase by over an order of magnitude, providing sensitivity to less massive satellites potentially formed within the CPD itself. Additionally, we run simulations to estimate the amount of material that different masses of satellites could clear in a CPD to create the observed cavity. We find satellite-to-planet mass ratios of $q > 2 \times 10^{-4}$ can create observable cavities and report a maximum cavity size of $\sim 51 \, R_{\text{Jup}}$ carved from a satellite.

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