TOI-5375 B: A Very Low Mass Star at the Hydrogen-Burning Limit Orbiting an Early M-type Star

The TESS mission detected a companion orbiting TIC 71268730, categorized it as a planet candidate, and designated the system TOI-5375. Our follow-up analysis using radial velocity data from the Habitable-zone Planet Finder (HPF), photometric data from Red Buttes Observatory (RBO), and speckle imaging with NN-EXPLORE Exoplanet Stellar Speckle Imager (NESSI) determined that the companion is a very low mass star (VLMS) near the hydrogen-burning mass limit with a mass of 0.080$\pm 0.002M_{\text{Sun}}$ ($83.81\pm 2.10M_J$), a radius of 0.1114${}_{-0.0050}^{+0.0048}{R}_{\text{Sun}}$ (1.0841${}_{0.0487}^{0.0467}{R}_J$), and brightness temperature of $2600\pm 70$ K. This object orbits with a period of 1.721553$\pm 0.000001$ days around an early M dwarf star ($0.62\pm 0.016M_{\text{Sun}}$). TESS photometry shows regular variations in the host star’s TESS light curve, which we interpreted as activity-induced variation of $\sim$2%, and used this variability to measure the host star’s stellar rotation period of 1.9716${}_{-0.0083}^{+0.0080}$ days. The TOI-5375 system provides tight constraints on stellar models of low-mass stars at the hydrogen-burning limit and adds to the population in this important region.