Searching for GEMS: Discovery and Characterization of Two Brown Dwarfs Around M Dwarfs

Abstract

Brown dwarfs bridge the gap between stars and planets, providing valuable insight into both planetary and stellar-formation mechanisms. Yet the census of transiting brown-dwarf companions, in particular around M-dwarf stars, remains incomplete. We report the discovery of two transiting brown dwarfs around low-mass hosts using a combination of space- and ground-based photometry along with near-infrared radial velocities. We characterize TOI-5389Ab ( $68.{0}_{-2.2}^{+2.2}\,{M}_{{\rm{J}}}$ ) and TOI-5610b ( $40.{4}_{-1.0}^{+1.0}\,{M}_{{\rm{J}}}$ ), two moderately massive brown dwarfs orbiting early M-dwarf hosts ( T _eff = 3569 ± 59 K and 3618 ± 59 K, respectively). For TOI-5389Ab, the best fitting parameters are period P = 10.40046 ± 0.00002 days, radius ${R}_{{\rm{BD}}}=0.82{4}_{-0.031}^{+0.033}$ R _J , and low eccentricity $e=0.096{2}_{-0.0046}^{+0.0027}$ . In particular, this constitutes one of the most extreme substellar-stellar companion-to-host mass ratios of q = 0.150. For TOI-5610b, the best-fitting parameters are period P = 7.95346 ± 0.00002 days, radius ${R}_{{\rm{BD}}}=0.88{7}_{-0.031}^{+0.031}$ R _J , and moderate eccentricity $e=0.35{4}_{-0.012}^{+0.011}$ . Both targets are expected to have shallow, but potentially observable, occultations: ≲500 ppm in the Johnson K band. A statistical analysis of M-dwarf/BD systems reveals for the first time that those at short orbital periods ( P < 13 days) exhibit a dearth of 13 M _J < M _BD < 40 M _J companions ( q < 0.1) compared to those at slightly wider separations.

Keywords

• Brown dwarfs
• Transits
• Photometry
• Transit photometry
• Spectroscopy
• Substellar companion stars