SOLES XII. The Aligned Orbit of TOI-2533 b, a Transiting Brown Dwarf Orbiting an F8-type Star

Abstract

Brown dwarfs occupy a middle ground in mass space between gaseous giant planets and ultra-cool dwarf stars, and the characterisation of their orbital orientations may shed light on how these neighbouring objects form. We present an analysis of the Rossiter-McLaughlin (RM) effect across the transit of TOI-2533 $b$, a brown dwarf on a moderately eccentric ($e_b=0.2476\pm 0.0090$) and wide-separation ($a_b/R_{\star }=13.34\pm 0.30$) orbit around an F8-type star, using data from the NEID/WIYN spectrograph in combination with archival photometry and radial velocity observations. Spin-orbit analyses of brown dwarfs are relatively rare, and TOI-2533 stands out as the fifth brown dwarf system with a measured spin-orbit constraint. We derive a sky-projected stellar obliquity of $\lambda =-7\pm 14^{\circ }$ for TOI-2533 $b$, finding that the brown dwarf is consistent with spin-orbit alignment. Our joint model also indicates that TOI-2533 $b$ falls near the lower bound of the hydrogen-burning minimum mass range (M${}_b$ = $74.9\pm 5.3$ M${}_{\mathrm{Jup}}$). Ultimately, we find that TOI-2533 $b$ is consistent with formation from disc fragmentation in a primordially spin-orbit aligned orientation, although we cannot rule out the possibility that the system has been tidally realigned during its lifetime.

13 pages, 3 figures. Original submission 5/24/24; accepted 8/2/24