TOI-7154b: A Close-in Massive Brown Dwarf in an Eccentric Orbit

Abstract

We report here the discovery and characterization of a high-mass transiting brown dwarf in a close-in orbit around its host star, TOI-7154. Initially, the host star was identified as an exoplanetary candidate from the TESS photometry data. Later, with the mass measurements from the RV follow-up using the PARAS-2 and TRES spectrographs, the companion is found to be sub-stellar in nature. TOI-7154, is a G-type main-sequence metal-rich star metallicity $[Fe/H]=0.154_{-0.075}^{+0.077}\text{dex}$, effective temperature $T_{\mathrm{eff}}=5564_{-110}^{+100}\text{K}$, mass $M_{\star }=0.939_{-0.043}^{+0.047}{M}_{\odot }$, radius $R_{\star }=0.949_{-0.030}^{+0.032}{R}_{\odot }$, and surface gravity $\log g=4.456_{-0.036}^{+0.036}$. With the joint analysis of the TESS photometry and the PARAS-2 and TRES radial velocities we found that TOI-7154b orbits its host star in $P=8.860073\pm 0.000029\text{d}$, eccentric ($e=0.2482\pm 0.0024$) orbit and its radius is smaller than that of Jupiter ($R_b=0.827_{-0.037}^{+0.040}{R}_{\mathrm{J}}$). With a mass near the hydrogen-burning boundary ($M_b=71.7_{-2.2}^{+2.4}{M}_{\mathrm{J}}$) which separates brown dwarfs from very low-mass stars, TOI-7154b occupies a critical position in the regime for probing the transition between sub-stellar and stellar objects. The system is very old, with its age estimated to be $7.2_{-3.6}^{+3.9}\text{Gyr}$ by MIST isochrones, while Galactic kinematics indicate an age of $\sim 4-5\text{Gyr}$. {Our tidal evolution simulations indicate a stellar dissipation factor of $Q_{\star }^{\prime }\lesssim 10^6$. Since the presence of any companion is currently ruled out by observations, the presence of eccentricity in this old system is, therefore, indicative of it having stellar-like fragmentation origins.

18 pages, 12 figures

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