Stellar Multiplicity of M Dwarfs with Short-period Giant Planets, and the Characterization of TOI-5628Ab

Abstract

Binary stars are ubiquitous, yet it remains unclear how wide-orbit stellar companions influence the formation of hot Jupiters, particularly around M dwarfs. Here, we first report the discovery of TOI-5628Ab, a giant planet transiting a mid-type M dwarf ($M_{\ast }=0.36\pm 0.02M_{\odot }$) every 4.34 days, accompanied by an associated white dwarf TOI-5628B ($M_{\mathrm{WD}}=0.59\pm 0.16M_{\odot }$) at a projected distance of about 2,500 AU. Using TESS, ground-based photometry and SPIRou RVs, we constrain the planet radius to $0.74\pm 0.04R_J$ and mass to $0.09\pm 0.04M_J$, with a $3\ddot{I}ƒ$ upper limit of $0.22M_J$. Building on this system, we further conduct a homogeneous systematic search for co-moving stellar companions with projected semi-major axis between 100 and 10,000 AU around all M dwarfs with confirmed giant planets with periods smaller than 10 days and radii larger than 0.7 $R_J$, as well as a group of field M stars with stellar properties similar to the planet sample, based on the stellar kinematics from Gaia DR3. We measure a stellar multiplicity of $34.2\pm 9.5\%$ for M dwarfs hosting short-period giant planets, which is substantially higher than the fraction of $5.3\pm 3.7\%$ for the field M stars by approximately a factor of 6. Our results suggest that wide-orbit stellar companions tend to promote the formation of short-period giant planets around M stars with masses $0.21\le M_{\ast }\le 0.64M_{\odot }$, and high-eccentricity migration may play an important role in producing such systems.

20 pages, 7 figures, 3 tables, accepted for publication in ApJL