TOI-1278 B: SPIRou Unveils a Rare Brown Dwarf Companion in Close-in Orbit around an M Dwarf

We present the discovery of an 18.5 ± 0.5 M_Jup brown dwarf (BD) companion to the M0V star TOI-1278. The system was first identified through a percent-deep transit in Transiting Exoplanet Survey Satellite photometry; further analysis showed it to be a grazing transit of a Jupiter-sized object. Radial velocity (RV) follow-up with the SPIRou near-infrared high-resolution velocimeter and spectropolarimeter in the framework of the 300-night SPIRou Legacy Survey carried out at the Canada-France-Hawaii Telescope led to the detection of a Keplerian RV signal with a semi-amplitude of 2306 ± 10 m s^-1 in phase with the 14.5 day transit period, with a slight but nonzero eccentricity. The intermediate-mass ratio (M_⋆/M_comp ~ 31) is unique for having such a short separation (0.095 ± 0.001 au) among known M-dwarf systems. Interestingly, M-dwarf-BD systems with similar mass ratios exist with separations of tens to thousands of astronomical unit.