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

Abstract

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 similar T_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_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 and e = 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 detect ¹³CO (7.8σ significance) and tentatively detect H218O (3.7σ significance) in the companion's atmosphere and measure ¹²C/¹³C = 98₋₂₂⁺²⁸ and H₂¹⁶O/H₂¹⁸O = 240₋₈₀⁺¹⁴⁵ after accounting for systematic errors. From a simplified retrieval analysis of HIP 55507 A, we measure ¹²C/¹³C = 79₋₁₆⁺²¹ and C¹⁶O/C¹⁸O = 288₋₇₀⁺¹²⁵ for the primary star. These results demonstrate that HIP 55507 A and B have consistent ¹²C/¹³C and ¹⁶O/¹⁸O to the <1σ level, 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 measuring ¹³CO and H₂¹⁸O abundances in the atmospheres of substellar or even planetary-mass companions with similar spectral types.

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