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1. Atmospheric Metallicity and C/O of HD 189733 b from High-resolution Spectroscopy

   https://doi.org/10.3847/1538-3881/ad1180  

   Finnerty, Luke ; Xuan, Jerry W. ; Xin, Yinzi ; Liberman, Joshua ; Schofield, Tobias ; Fitzgerald, Michael P. ; Agrawal, Shubh ; Baker, Ashley ; Bartos, Randall ; Blake, Geoffrey A. ; et al ( January 2024 , The Astronomical Journal)

   We present high-resolutionK-band emission spectra of the quintessential hot Jupiter HD 189733 b from the Keck Planet Imager and Characterizer. Using a Bayesian retrieval framework, we fit the dayside pressure–temperature profile, orbital kinematics, mass-mixing ratios of H₂O, CO, CH₄, NH₃, HCN, and H₂S, and the¹³CO/¹²CO ratio. We measure mass fractions of${\mathrm{logH}}_2\mathrm{O}=-{2.0}_{-0.4}^{+0.4}$and$\mathrm{logCO}=-{2.2}_{-0.5}^{+0.5}$, and place upper limits on the remaining species. Notably, we find logCH₄< −4.5 at 99% confidence, despite its anticipated presence at the equilibrium temperature of HD 189733 b assuming local thermal equilibrium. We make a tentative (∼3σ) detection of¹³CO, and the retrieved posteriors suggest a¹²C/¹³C ratio similar to or substantially less than the local interstellar value. The possible¹³C enrichment would be consistent with accretion of fractionated material in ices or in the protoplanetary disk midplane. The retrieved abundances correspond to a substantially substellar atmospheric C/O = 0.3 ± 0.1, while the carbon and oxygen abundances are stellar to slightly superstellar, consistent with core-accretion models which predict an inverse correlation between C/O and metallicity. The specific combination of low C/O and high metallicity suggests significant accretion of solid material may have occurred late in the formation process of HD 189733 b.

    

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2. Brown Dwarf Retrievals on FIRE!: Atmospheric Constraints and Lessons Learned from High Signal-to-noise Medium-resolution Spectroscopy of a T9 Dwarf

   https://doi.org/10.3847/1538-4357/ace32e  

   Hood, Callie E. ; Fortney, Jonathan J. ; Line, Michael R. ; Faherty, Jacqueline K. ( August 2023 , The Astrophysical Journal)

   Brown dwarf spectra offer vital testbeds for our understanding of the chemical and physical processes that sculpt substellar atmospheres. Recently, atmospheric retrieval approaches have been successfully applied to low-resolution (R∼ 100) spectra of L, T, and Y dwarfs, yielding constraints on the chemical abundances and temperature structures of these atmospheres. Medium-resolution (R∼ 10³) spectra of brown dwarfs offer additional insight, as molecular features are more easily disentangled and the thermal structure of the upper atmosphere is better probed. We present results from a GPU-based retrieval analysis of a high signal-to-noise, medium-resolution (R∼ 6000) FIRE spectrum from 0.85 to 2.5μm of the T9 dwarf UGPS J072227.51–054031.2. At 60× higher spectral resolution than previous brown dwarf retrievals, a number of novel challenges arise. We examine the effect of different opacity sources, in particular for CH₄. Furthermore, we find that flaws in the data like errors from order stitching can bias our constraints. We compare these retrieval results to those for anR∼ 100 spectrum of the same object, revealing how constraints on atmospheric abundances and temperatures improve by an order of magnitude or more with increased spectral resolution. In particular, we can constrain the abundance of H₂S, which is undetectable at lower spectral resolution. While these medium-resolution retrievals offer the potential of precise, stellar-like constraints on atmospheric abundances (∼0.02 dex), our retrieved radius is unphysically small ($R={0.50}_{-0.01}^{+0.01}$R_Jup), indicating shortcomings with our modeling framework. This work is an initial investigation into brown dwarf retrievals at medium spectral resolution, offering guidance for future ground-based studies and JWST observations.

    

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3. Validation of Elemental and Isotopic Abundances in Late-M Spectral Types with the Benchmark HIP 55507 AB System

   https://doi.org/10.3847/1538-4357/ad1243  

   Xuan, Jerry W. ; Wang, Jason ; Finnerty, Luke ; Horstman, Katelyn ; Grimm, Simon ; Peck, Anne E. ; Nielsen, Eric ; Knutson, Heather A. ; Mawet, Dimitri ; Isaacson, Howard ; et al ( February 2024 , The Astrophysical Journal)

   M dwarfs are common host stars to exoplanets but often lack atmospheric abundance measurements. Late-M dwarfs are also good analogs to the youngest substellar companions, which share similarT_eff∼ 2300–2800 K. We present atmospheric analyses for the M7.5 companion HIP 55507 B and its K6V primary star with Keck/KPIC high-resolution (R∼ 35,000)K-band spectroscopy. First, by including KPIC relative radial velocities between the primary and secondary in the orbit fit, we improve the dynamical mass precision by 60% and find$M_B={88.0}_{-3.2}^{+3.4}{M}_{\mathrm{Jup}}$, putting HIP 55507 B above the stellar–substellar boundary. We also find that HIP 55507 B orbits its K6V primary star with$a={38}_{-3}^{+4}$au ande= 0.40 ± 0.04. From atmospheric retrievals of HIP 55507 B, we measure [C/H] = 0.24 ± 0.13, [O/H] = 0.15 ± 0.13, and C/O = 0.67 ± 0.04. Moreover, we strongly detect¹³CO (7.8σsignificance) and tentatively detect${\mathrm{H}}_2^{18}\mathrm{O}$(3.7σsignificance) in the companion’s atmosphere and measure${}^{12}\mathrm{CO}{/}^{13}\mathrm{CO}={98}_{-22}^{+28}$and${\mathrm{H}}_2^{16}\mathrm{O}/{\mathrm{H}}_2^{18}\mathrm{O}={240}_{-80}^{+145}$after accounting for systematic errors. From a simplified retrieval analysis of HIP 55507 A, we measure${}^{12}\mathrm{CO}{/}^{13}\mathrm{CO}={79}_{-16}^{+21}$and${\mathrm{C}}^{16}\mathrm{O}/{\mathrm{C}}^{18}\mathrm{O}={288}_{-70}^{+125}$for the primary star. These results demonstrate that HIP 55507 A and B have consistent¹²C/¹³C and¹⁶O/¹⁸O to the <1σlevel, as expected for a chemically homogeneous binary system. Given the similar flux ratios and separations between HIP 55507 AB and systems with young substellar companions, our results open the door to systematically measuring¹³CO and${\mathrm{H}}_2^{18}\mathrm{O}$abundances in the atmospheres of substellar or even planetary-mass companions with similar spectral types.

    

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4. Retrieving the C and O Abundances of HR 7672 AB: A Solar-type Primary Star with a Benchmark Brown Dwarf

   https://doi.org/10.3847/1538-3881/ac56e2  

   Wang, Ji ; Kolecki, Jared R. ; Ruffio, Jean-Baptiste ; Wang, Jason J. ; Mawet, Dimitri ; Baker, Ashley ; Bartos, Randall ; Blake, Geoffrey A. ; Bond, Charlotte Z. ; Calvin, Benjamin ; et al ( March 2022 , The Astronomical Journal)

   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. 

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5. An Atmospheric Retrieval of the Brown Dwarf Gliese 229B

   https://doi.org/10.3847/1538-4357/ac9cc9  

   Calamari, Emily ; Faherty, Jacqueline K. ; Burningham, Ben ; Gonzales, Eileen ; Bardalez-Gagliuffi, Daniella ; Vos, Johanna M. ; Gemma, Marina ; Whiteford, Niall ; Gaarn, Josefine ( December 2022 , The Astrophysical Journal)

   We present results from an atmospheric retrieval analysis of Gl 229B using the Brewster retrieval code. We find the best fit model to be cloud-free, consistent with the T dwarf retrieval work of Line et al.; Zalesky et al. and Gonzales et al. Fundamental parameters (mass, radius, log(L_Bol/L_Sun), log(g)) determined from our model agree within 1σto SED-derived values, except forT_effwhere our retrievedT_effis approximately 100 K cooler than the evolutionary model-based SED value. We find a retrieved mass of${50}_{-9}^{+12}$M_Jup, however, we also find that the observables of Gl 229B can be explained by a cloud-free model with a prior on mass at the dynamical value, 70M_Jup. We are able to constrain abundances for H₂O, CO, CH₄, NH₃, Na and K and find a supersolar C/O ratio as compared to its primary, Gl 229A. We report an overall subsolar metallicity due to atmospheric oxygen depletion, but find a solar [C/H], which matches that of the primary. We find that this work contributes to a growing trend in retrieval-based studies, particularly for brown dwarfs, toward supersolar C/O ratios and discuss the implications of this result on formation mechanisms and internal physical processes, as well as model biases.

    

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