Surveying Nearby Brown Dwarfs with HGCA: Direct Imaging Discovery of a Faint, High-Mass Brown Dwarf Orbiting HD 176535 A

Abstract

Brown dwarfs with well-measured masses, ages and luminosities provide direct benchmark tests of substellar formation and evolutionary models. We report the first results from a direct imaging survey aiming to find and characterize substellar companions to nearby accelerating stars with the assistance of the Hipparcos-Gaia Catalog of Accelerations (HGCA). In this paper, we present a joint high-contrast imaging and astrometric discovery of a substellar companion to HD 176535 A, a K3.5V main-sequence star aged approximately $3.59_{-1.15}^{+0.87}$ Gyrs at a distance of $36.99\pm 0.03$ pc. In advance of our high-contrast imaging observations, we combined precision HARPS RVs and HGCA astrometry to predict the potential companion's location and mass. We thereafter acquired two nights of KeckAO/NIRC2 direct imaging observations in the $L^{\prime }$ band, which revealed a companion with a contrast of $\Delta L_p^{\prime }=9.20\pm 0.06$ mag at a projected separation of $\approx$0.${}^{\prime \prime }35$ ($\approx$13 AU) from the host star. We revise our orbital fit by incorporating our dual-epoch relative astrometry using the open-source MCMC orbit fitting code $\mathtt{orvara}$. HD 176535 B is a new benchmark dwarf useful for constraining the evolutionary and atmospheric models of high-mass brown dwarfs. We found a luminosity of $\log ({\mathrm{L}}_{\mathrm{bol}}/{\mathrm{L}}_{\odot })=-5.26\pm 0.07$ and a model-dependent effective temperature of $980\pm 35$ K for HD 176535 B. Our dynamical mass suggests that some substellar evolutionary models may be underestimating luminosity for high-mass T dwarfs. Given its angular separation and luminosity, HD 176535 B would make a promising candidate for Aperture Masking Interferometry with JWST and GRAVITY/KPIC, and further spectroscopic characterization with instruments like the CHARIS/SCExAO/Subaru integral field spectrograph.

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