An Aligned Very-Low-Mass Star Orbiting an M dwarf and Obliquity Patterns Across Giant Planets, Brown Dwarfs, and Binary Stars

Abstract

Stellar obliquity serves as a key diagnostic for tracing the dynamical evolution of bound systems-from giant planets and brown dwarfs to stellar binaries-revealing whether these diverse populations share analogous histories. Here, we report the first obliquity measurement for a double M dwarf system, determined via the Rossiter-McLaughlin effect. The spin axis of the primary star, TOI-5375 ($M_{\ast }=0.62\pm 0.02M_{\odot }$), is well aligned with the orbit of its low-mass stellar companion ($M_c=84.8\pm 1.5M_J$, $\mathrm{P}=1.72\mathrm{days}$) with a projected obliquity of $\hat{I}»=-13.5_{-13.8}^{+12.4}{}^{\circ }$ and a true 3D obliquity of $\ddot{I}ˆ=37.5_{-13.4}^{+10.6}{}^{\circ }$. The result indicates that the system either formed with a primordially aligned configuration or has undergone tidal realignment. We further investigate obliquity patterns across giant planets, brown dwarfs and binary stars. It turns out that a few obliquity trends observed in giant planets also tentatively exhibit in the latter two higher-mass populations: 1) well-aligned orbits are preferentially found around cooler host stars ($T_{\mathrm{eff}}\le 6250K$); 2) wide-orbit ($a/R_{\ast }\ge 10$) companions are predominantly aligned; 3) no significant correlation shows up between obliquity and orbital eccentricity in any of the companion classes. By modeling $|\hat{I}»|$ with a two-component Gaussian distribution, we find that the low-$|\hat{I}»|$ components of binary stars and brown dwarfs are more concentrated near zero than giant planets while the high-$|\hat{I}»|$ components of brown dwarfs and binaries remain unclear due to the small sample size.

14 pages, 5 figures, 2 tables, accepted for publication in ApJL