A Small Brown Dwarf in an Aligned Orbit around a Young, Fully-Convective M Star

TL;DR

This study measures the orbital alignment of a brown dwarf around a fully-convective M dwarf star, revealing a well-aligned system and providing new insights into brown dwarf formation and dynamical history.

Contribution

First obliquity measurement of a brown dwarf orbiting an M dwarf, expanding understanding of brown dwarf system alignments.

Findings

System has a projected obliquity of ~5 degrees.

Star's rotation is nearly edge-on with an inclination of ~90 degrees.

Brown dwarf's radius suggests an unusual formation history.

Abstract

A star's spin-orbit angle can give us insight into a system's formation and dynamical history. In this paper, we use MAROON-X observations of the Rossiter-McLaughlin (RM) effect to measure the projected obliquity of the LP 261-75 (also known as TOI-1779) system, focusing on the fully-convective M dwarf LP 261-75A and the transiting brown dwarf LP 261-75C. This is the first obliquity constraint of a brown dwarf orbiting an M dwarf and the seventh obliquity constraint of a brown dwarf overall. We measure a projected obliquity of $5^{+11}_{-10}$ degrees and a true obliquity of $14^{+8}_{-7}$ degrees for the system, meaning that the system is well-aligned and that the star is rotating very nearly edge-on, with an inclination of $90^o\,\pm\,11^o$. The system thus follows along with the trends observed in transiting brown dwarfs around hotter stars, which typically have low obliquities. The tendency for brown dwarfs to be aligned may point to some enhanced obliquity damping in brown dwarf systems, but there is also a possibility that the LP 261-75 system was simply formed aligned. In addition, we note that the brown dwarf's radius ($R_C\,=\,0.9$ R$_J$) is not consistent with the youth of the system or radius trends observed in other brown dwarfs, indicating that LP 261-75C may have an unusual formation history.