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Abstract With the launch of the JWST, we will obtain more precise data for exoplanets than ever before. However, these data can only inform and revolutionize our understanding of exoplanets when placed in the larger context of planet–star formation. Therefore, gaining a deeper understanding of their host stars is equally important and synergistic with the upcoming JWST data. We present detailed chemical abundance profiles of 17 FGK stars that will be observed in exoplanet-focused Cycle 1 JWST observer programs. The elements analyzed (C, N, O, Na, Mg, Si, S, K, and Fe) were specifically chosen as being informative to the composition and formation of planets. Using archival high-resolution spectra from a variety of sources, we perform an LTE equivalent width analysis to derive these abundances. We look to literature sources to correct the abundances for non-LTE effects, especially for O, S, and K, where the corrections are large (often >0.2 dex). With these abundances and the ratios thereof, we will begin to paint clearer pictures of the planetary systems analyzed by this work. With our analysis, we can gain insight into the composition and extent of migration of Hot Jupiters, as well as the possibility of carbon-rich terrestrial worlds. 

We present a reanalysis of the K2-106 transiting planetary system, with a focus on the composition of K2-106b, an ultra-short-period, super-Mercury candidate. We globally model existing photometric and radial velocity data and derive a planetary mass and radius for K2-106b of Mp = 8.53 ± 1.02 M⊕ and = - + Rp 1.71 0.057 RÅ 0.069 , which leads to a density of r = - + 9.4 p 1.5 1.6 g cm−3 , a significantly lower value than previously reported in the literature. We use planet interior models that assume a two-layer planet comprised of a liquid, pure Fe core and an iron-free, MgSiO3 mantle, and we determine that the range of the core mass fractions are consistent with the observed mass and radius. We use existing high-resolution spectra of the host star to derive the Fe/Mg/Si abundances ([Fe/ H] = −0.03 ± 0.01, [Mg/H] = 0.04 ± 0.02, [Si/H] = 0.03 ± 0.06) to infer the composition of K2-106b. We find that K2-106b has a density and core mass fraction ( - + 44 %15 12 ) consistent with that of Earth (CMF⊕ = 32%). Furthermore, its composition is consistent with what is expected, assuming that it reflects the relative refractory abundances of its host star. K2-106b is therefore unlikely to be a super-Mercury, as has been suggested in previous literature. 

By measuring the elemental abundances of a star, we can gain insight into the composition of its initial gas cloud— the formation site of the star and its planets. Planet formation requires metals, the availability of which is determined by the elemental abundance. In the case where metals are extremely deficient, planet formation can be stifled. To investigate such a scenario requires a large sample of metal-poor stars and a search for planets therein. This paper focuses on the selection and validation of a halo star sample. We select ∼17,000 metal-poor halo stars based on their Galactic kinematics, and confirm their low metallicities ([Fe/H] < −0.5), using spectroscopy from the literature. Furthermore, we perform high-resolution spectroscopic observations using LBT/PEPSI and conduct detailed metallicity ([Fe/H]) analyses on a sample of 13 previously-known halo stars that also have hot kinematics. We can use the halo star sample presented here to measure the frequency of planets and to test planet formation in extremely metal-poor environments. The result of the planet search and its implications will be presented and discussed in a companion paper by Boley et al. 

Abstract Direct imaging studies have mainly used low-resolution spectroscopy ( R ∼ 20–100) to study the atmospheres of giant exoplanets and brown dwarf companions, but the presence of clouds has often led to degeneracies in the retrieved atmospheric abundances (e.g., carbon-to-oxygen ratio, metallicity). This precludes clear insights into the formation mechanisms of these companions. The Keck Planet Imager and Characterizer (KPIC) uses adaptive optics and single-mode fibers to transport light into NIRSPEC ( R ∼ 35,000 in the K band), and aims to address these challenges with high-resolution spectroscopy. Using an atmospheric retrieval framework based on petitRADTRANS , we analyze the KPIC high-resolution spectrum (2.29–2.49 μ m) and the archival low-resolution spectrum (1–2.2 μ m) of the benchmark brown dwarf HD 4747 B ( m = 67.2 ± 1.8 M Jup , a = 10.0 ± 0.2 au, T eff ≈ 1400 K). We find that our measured C/O and metallicity for the companion from the KPIC high-resolution spectrum agree with those of its host star within 1 σ –2 σ . The retrieved parameters from the K -band high-resolution spectrum are also independent of our choice of cloud model. In contrast, the retrieved parameters from the low-resolution spectrum are highly sensitive to our chosen cloud model. Finally, we detect CO, H 2 O, and CH 4 (volume-mixing ratio of log(CH 4 ) = −4.82 ± 0.23) in this L/T transition companion with the KPIC data. The relative molecular abundances allow us to constrain the degree of chemical disequilibrium in the atmosphere of HD 4747 B, and infer a vertical diffusion coefficient that is at the upper limit predicted from mixing length theory. 

Abstract A benchmark brown dwarf (BD) is a BD whose properties (e.g., mass and chemical composition) are precisely and independently measured. Benchmark BDs are valuable in testing theoretical evolutionary tracks, spectral synthesis, and atmospheric retrievals for substellar objects. Here, we report results of atmospheric retrieval on a synthetic spectrum and a benchmark BD, HR 7672 B, with petitRADTRANS . First, we test the retrieval framework on a synthetic PHOENIX BT-Settl spectrum with a solar composition. We show that the retrieved C and O abundances are consistent with solar values, but the retrieved C/O is overestimated by 0.13–0.18, which is about four times higher than the formal error bar. Second, we perform retrieval on HR 7672 B using high spectral-resolution data ( R = 35,000) from the Keck Planet Imager and Characterizer and near-infrared photometry. We retrieve [C/H], [O/H], and C/O to be −0.24 ± 0.05, −0.19 ± 0.04, and 0.52 ± 0.02. These values are consistent with those of HR 7672 A within 1.5 σ . As such, HR 7672 B is among only a few benchmark BDs (along with Gl 570 D and HD 3651 B) that have been demonstrated to have consistent elemental abundances with their primary stars. Our work provides a practical procedure of testing and performing atmospheric retrieval, and sheds light on potential systematics of future retrievals using high- and low-resolution data.