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

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 (7.8 σ significance) and tentatively detect H 2 18 O (3.7 σ significance) in the companion's atmosphere and measure 12 CO / 13 CO = 98 − 22 + 28 and H 2 16 O / H 2 18 O = 240 − 80 + 145 after accounting for systematic errors. From a simplified retrieval analysis of HIP 55507 A, we measure 12 CO / 13 CO = 79 − 16 + 21 and C 16 O / C 18 O = 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 <1 σ 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 H 2 18 O abundances in the atmospheres of substellar or even planetary-mass companions with similar spectral types. [ABSTRACT FROM AUTHOR]