TOI-1278 B: SPIRou unveils a rare Brown Dwarf Companion in Close-In Orbit around an M dwarf

TL;DR

This paper reports the discovery of a rare brown dwarf companion orbiting an M dwarf star, using TESS photometry and SPIRou radial velocity measurements, revealing a close-in orbit with unique system characteristics.

Contribution

It presents the first detailed characterization of a close-in brown dwarf around an M dwarf using near-infrared spectroscopy, highlighting its rarity and system properties.

Findings

Brown dwarf companion has a mass of 18.5 M_Jup.

Detected a 14.5-day orbital period with RV semi-amplitude of 2306 m/s.

System has a short separation of 0.095 AU with a unique mass ratio.

Abstract

We present the discovery of an $18.5\pm0.5$M$_{\rm Jup}$ brown dwarf (BD) companion to the M0V star TOI-1278. The system was first identified through a percent-deep transit in TESS 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 (SLS) carried out at the Canada-France-Hawaii Telescope (CFHT) led to the detection of a Keplerian RV signal with a semi-amplitude of $2306\pm10$ m/s in phase with the 14.5-day transit period, having a slight but non-zero eccentricity. The intermediate-mass ratio ($M_\star/M_{\rm{comp}} \sim31$) is unique for having such a short separation ($0.095\pm0.001$ AU) among known M-dwarf systems. Interestingly, M dwarf-brown dwarf systems with similar mass ratios exist with separations of tens to thousands of AUs.