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Abstract The elemental and isotopic abundances of volatiles like carbon, oxygen, and nitrogen may trace a planet’s formation location relative to H₂O, CO₂, CO, NH₃, and N₂“snowlines,” or the distance from the star at which these volatile elements sublimate. By comparing the C/O and¹²C/¹³C ratios measured in giant exoplanet atmospheres to complementary measurements of their host stars, we can determine whether the planet inherited stellar abundances from formation inside the volatile snowlines, or nonstellar C/O and¹³C enrichment characteristic of formation beyond the snowlines. To date, there are still only a handful of exoplanet systems where we can make a direct comparison of elemental and isotopic CNO abundances between an exoplanet and its host star. Here, we present a¹²C/¹³C abundance analysis for host star WASP-77A (whose hot Jupiter’s¹²C/¹³C abundance was recently measured). We use MARCS stellar atmosphere models and the radiative transfer code TurboSpectrum to generate synthetic stellar spectra for isotopic abundance calculations. We find a¹²C/¹³C ratio of 51 ± 6 for WASP-77A, which is subsolar (∼91) but may still indicate¹³C enrichment in its companion planet WASP-77A b (¹²C/¹³C = 26 ± 16, previously reported). Together with the inventory of carbon and oxygen abundances in both the host and companion planet, these chemical constraints point to WASP-77A b’s formation beyond the H₂O and CO₂snowlines and provide chemical evidence for the planet’s migration to its current location ∼0.024 au from its host star. 

Abstract The chemical abundance measurements of host stars and their substellar companions provide a powerful tool to trace the formation mechanism of the planetary systems. We present a detailed high-resolution spectroscopic analysis of a young M-type star, DH Tau A, which is located in the Taurus molecular cloud belonging to the Taurus-Auriga star-forming region. This star is host to a low-mass companion, DH Tau b, and both the star and the companion are still in their accreting phase. We apply our technique to measure the abundances of carbon and oxygen using carbon- and oxygen-bearing molecules, such as CO and OH, respectively. We determine a near-solar carbon-to-oxygen abundance ratio of C/O = 0.555 ± 0.063 for the host star DH Tau A. We compare this stellar abundance ratio with that of the companion from our previous study ( $\mathrm{C}/\mathrm{O}={0.54}_{-0.05}^{+0.06}$), which also has a near-solar value. This confirms the chemical homogeneity in the DH Tau system, which suggests a formation scenario for the companion consistent with a direct and relatively fast gravitational collapse rather than a slow core accretion process. 

Abstract We present an in-depth, high-resolution spectroscopic analysis of the M dwarf K2-18, which hosts a sub-Neptune exoplanet in its habitable zone. We show our technique to accurately normalize the observed spectrum, which is crucial for a proper spectral fitting. We also introduce a new automatic, line-by-line, model-fitting code, AutoSpecFit, which performs an iterativeχ²minimization process to measure individual elemental abundances of cool dwarfs. We apply this code to the star K2-18, and measure the abundance of 10 elements: C, O, Na, Mg, Al, K, Ca, Sc, Ti, and Fe. We find these abundances to be moderately supersolar, except for Fe, with a slightly subsolar abundance. The accuracy of the inferred abundances is limited by the systematic errors due to uncertain stellar parameters. We also derive the abundance ratios associated with several planet-building elements such as Al/Mg, Ca/Mg, Fe/Mg, and (a solar-like) C/O = 0.568 ± 0.026, which can be used to constrain the chemical composition and the formation location of the exoplanet. On the other hand, the planet K2-18 b has attracted considerable interest, given the JWST measurements of its atmospheric composition. Early JWST studies reveal an unusual chemistry for the atmosphere of this planet, which is unlikely to be driven by formation in a disk of unusual composition. The comparison between the chemical abundances of K2-18 b from future JWST analyses and those of the host star can provide fundamental insights into the formation of this planetary system. 

Abstract Using Keck Planet Imager and Characterizer high-resolution (R∼ 35,000) spectroscopy from 2.29 to 2.49μm, we present uniform atmospheric retrievals for eight young substellar companions with masses of ∼10–30M_Jup, orbital separations spanning ∼50–360 au, andT_effbetween ∼1500 and 2600 K. We find that all companions have solar C/O ratios and metallicities to within the 1σ–2σlevel, with the measurements clustered around solar composition. Stars in the same stellar associations as our systems have near-solar abundances, so these results indicate that this population of companions is consistent with formation via direct gravitational collapse. Alternatively, core accretion outside the CO snowline would be compatible with our measurements, though the high mass ratios of most systems would require rapid core assembly and gas accretion in massive disks. On a population level, our findings can be contrasted with abundance measurements for directly imaged planets withm< 10M_Jup, which show tentative atmospheric metal enrichment compared to their host stars. In addition, the atmospheric compositions of our sample of companions are distinct from those of hot Jupiters, which most likely form via core accretion. For two companions withT_eff∼ 1700–2000 K (κAnd b and GSC 6214–210 b), our best-fit models prefer a nongray cloud model with >3σsignificance. The cloudy models yield 2σ−3σlowerT_efffor these companions, though the C/O and [C/H] still agree between cloudy and clear models at the 1σlevel. Finally, we constrain¹²CO/¹³CO for three companions with the highest signal-to-noise ratio data (GQ Lup b, HIP 79098b, and DH Tau b) and report $v\sin i$and radial velocities for all companions.