Retrieving the C and O Abundances of HR 7672~AB: a Solar-Type Primary Star with a Benchmark Brown Dwarf

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 sub-stellar objects. Here, we report results of atmospheric retrieval on a synthetic spectrum and a benchmark BD -- HR 7672~B -- with \petit. 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 $\sim$4 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 (KPIC) and near infrared photometry. We retrieve [C/H], [O/H], and C/O to be $-0.24\pm0.05$, $-0.19\pm0.04$, and $0.52\pm0.02$. These values are consistent with those of HR 7672~A within 1.5-$\sigma$. 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.Comment: 29 pages, 17 figures, 5 tables, resubmitted to AAS journals after first revisio

A Reanalysis of the Composition of K2-106b: An Ultra-short-period Super-Mercury Candidate

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 M _p = 8.53 ± 1.02 M _⊕ and ${R}_{p}={1.71}_{-0.057}^{+0.069}\,{R}_{\oplus }$ , which leads to a density of ${\rho }_{p}={9.4}_{-1.5}^{+1.6}$ 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, MgSiO _3 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