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 * = 0.62 ± 0.02 M ⊙ ), is well aligned with the orbit of its low-mass stellar companion ( M c = 84.8 ± 1.5 M J , P = 1.72 days) with a projected obliquity of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>λ</mml:mi> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:mn>13</mml:mn> <mml:mover> <mml:mrow> <mml:mo>.</mml:mo> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">°</mml:mi> </mml:mrow> </mml:mover> <mml:msubsup> <mml:mrow> <mml:mn>5</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>13.8</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>12.4</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> and a true 3D obliquity of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>ψ</mml:mi> <mml:mo>=</mml:mo> <mml:mn>37</mml:mn> <mml:mover> <mml:mrow> <mml:mo>.</mml:mo> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">°</mml:mi> </mml:mrow> </mml:mover> <mml:msubsup> <mml:mrow> <mml:mn>5</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>13.4</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>10.6</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> . 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 eff ≤ 6250 K); (2) wide-orbit ( a / R * ≥ 10) companions are predominantly aligned; and (3) no significant correlation shows up between obliquity and orbital eccentricity in any of the companion classes. By modeling ∣ λ ∣ with a two-component Gaussian distribution, we find that the low-∣ λ ∣ components of binary stars and brown dwarfs are more concentrated near zero than those of giant planets, while the high-∣ λ ∣ components of brown dwarfs and binaries remain unclear due to the small sample size.