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

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 ($M_c=84.8\pm 1.5M_J$, $\mathrm{P}=1.72\mathrm{d}\mathrm{a}\mathrm{y}\mathrm{s}$) with a projected obliquity of $\lambda =-13.5_{-13.8}^{+12.4}{}^{\circ }$ and a true 3D obliquity of $\psi =37.5_{-13.4}^{+10.6}{}^{\circ }$. 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 ($T_{\mathrm{e}\mathrm{f}\mathrm{f}}\le 6250K$); 2) wide-orbit ($a\mathrm{/}{R}_{\ast }\ge 10$) companions are predominantly aligned; 3) no significant correlation shows up between obliquity and orbital eccentricity in any of the companion classes. By modeling $\mathrm{\mid }\lambda \mathrm{\mid }$ with a two-component Gaussian distribution, we find that the low-$\mathrm{\mid }\lambda \mathrm{\mid }$ components of binary stars and brown dwarfs are more concentrated near zero than giant planets while the high-$\mathrm{\mid }\lambda \mathrm{\mid }$ components of brown dwarfs and binaries remain unclear due to the small sample size.