TOI-2155 b: A Massive Brown Dwarf or a Very Low-Mass Star?

Abstract

We present TOI-2155\,b, a massive transiting companion, discovered using data from NASA's Transiting Exoplanet Survey Satellite (TESS) mission and confirmed with ground-based RV measurements from the Tillinghast Reflector Echelle Spectrograph (TRES). We also analyze ground-based follow-up photometric data from the Wendelstein Observatory (WST), Las Cumbres Observatory Global Telescope (LCOGT), and Wild Boar Remote Observatory (WBR). TOI-2155\,b is a short-period companion with {$P=3.7246950\pm 0.0000014$}~days. The radius and mass of TOI-2155\,b are found to be $R_b=0.972_{-0.008}^{+0.009}{\mathrm{R}}_{\mathrm{J}}$ and $M_b=80.6_{-1.1}^{+1.0}{\mathrm{M}}_{\mathrm{J}}$, respectively, corresponding to a density of {${\rho }_b=109_{-3.3}^{+3.1}$ g cm${}^{-3}$}. The F-type subgiant host star has an effective temperature of $T_{\mathrm{eff}}=6085\pm 78$ K, a radius $R_{\thinstar} = 1.705^{+0.066}_{-0.064}$ ${\mathrm{R}}_{\odot }$ and a mass $M_{\star }=1.33\pm 0.008$~M${}_{\odot }$. With a mass close to the hydrogen-burning minimum mass, TOI-2155\,b lies at the boundary between brown dwarfs and low-mass stars. Its measured mass, radius, and density place it in a transitional region, where distinguishing between a massive brown dwarf and a very low-mass star is not straightforward. TOI-2155\,b therefore provides a valuable benchmark for testing evolutionary models of stellar and substellar structure near the hydrogen-burning limit.

Accepted to AJ