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arXiv · 2509.21517

The EBLM project XVI. Moderate spin-orbit misalignment of the low mass eclipsing binary EBLM J0021-16

Becca Spejcher·David V. Martin·Jake Pandina·Andy Zhang·Max Ammons·Wata Thubthong·Amaury Triaud·Ritika Sethi·Noah Vowell·Adrian Barker·Pierre Maxted·Alison Duck·Shelby Summers·François Bouchy·Monika Lendl·Maxime Marmier·Malte Tewes·Stéphane Udry
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Abstract

Thousands of tight ($<1$ AU) main sequence binaries have been discovered, but it is uncertain how they formed. There is likely too much angular momentum in a collapsing, fragmenting protostellar cloud to form such binaries in situ, suggesting some post processing. One probe of a binary's dynamical history is the angle between the stellar spin and orbital axes -- its obliquity. The classical method for determining stellar obliquity is the Rossiter-McLaughlin effect. It has been applied to over 100 hot Jupiters, but less than a dozen stellar binaries. In this paper, we present the Rossiter-McLaughlin measurement of EBLM J0021-16, a $0.19M_\odot$ M-dwarf eclipsing a $1.05M_\odot$ G-dwarf on a 5.97 day, almost-circular orbit. We combine CORALIE spectroscopy with TESS photometry and a measured primary star rotation period of 7.04 days, according to star spot modulation. We show that the orbital axis is misaligned with the primary star's spin axis, with a true 3D obliquity of $\psi=28.9\pm2.1^{\circ}$. EBLM J0021-16, being neither spin-orbit aligned nor synchronized, yet with an almost circular orbit, is a curious case for tidal evolution in tight binaries. It becomes one of a handful of eclipsing binaries with true obliquity measurements. Finally, we derive the M-dwarf's mass and radius to a fractional precision better than 1\%. The radius of the M-dwarf is inflated by 6\% ($7.4\sigma$) with respect to stellar models, consistent with many other M-dwarfs in the literature.

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Becca Spejcher, David V. Martin, Jake Pandina, Andy Zhang, Max Ammons, Wata Thubthong, Amaury Triaud, Ritika Sethi, Noah Vowell, Adrian Barker, Pierre Maxted, Alison Duck, Shelby Summers, François Bouchy, Monika Lendl, Maxime Marmier, Malte Tewes, Stéphane Udry. 2025-09-25. The EBLM project XVI. Moderate spin-orbit misalignment of the low mass eclipsing binary EBLM J0021-16. https://arxiv.org/abs/2509.21517

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