TOI-3235 b: a transiting giant planet around an M4 dwarf star

Abstract

We present the discovery of TOI-3235 b, a short-period Jupiter orbiting an M-dwarf with a stellar mass close to the critical mass at which stars transition from partially to fully convective. TOI-3235 b was first identified as a candidate from TESS photometry, and confirmed with radial velocities from ESPRESSO, and ground-based photometry from HATSouth, MEarth-South, TRAPPIST-South, LCOGT, and ExTrA. We find that the planet has a mass of $0.665\pm 0.025{\mathrm{M}}_{\mathrm{J}}$ and a radius of $1.017\pm 0.044{\mathrm{R}}_{\mathrm{J}}$. It orbits close to its host star, with an orbital period of $2.5926\mathrm{d}$, but has an equilibrium temperature of $\approx 604\mathrm{K}$, well below the expected threshold for radius inflation of hot Jupiters. The host star has a mass of $0.3939\pm 0.0030{\mathrm{M}}_{\odot }$, a radius of $0.3697\pm 0.0018{\mathrm{R}}_{\odot }$, an effective temperature of $3389\mathrm{K}$, and a J-band magnitude of $11.706\pm 0.025$. Current planet formation models do not predict the existence of gas giants such as TOI-3235 b around such low-mass stars. With a high transmission spectroscopy metric, TOI-3235 b is one of the best-suited giants orbiting M-dwarfs for atmospheric characterization.

15 pages, 4 figures. Accepted for publication in APJL