2025/11/28 by Spejcher, Becca, Martin, David V., Pandina, Jake +17
Physics and Astronomy · Social Sciences · #Stellar, planetary, and galactic studies #Astronomy and Astrophysical Research #Educational Leadership and Practices
paper · doi:10.48550/arxiv.2511.23430
A question that continues to perplex astronomers is the formation of tight stellar binaries. There is too much angular momentum in a collapsing and fragmenting protostellar cloud to form a stellar binary in situ with a separation less than an AU, yet thousands of these short-period binaries have been discovered. One indication of a binary's formation is the angle between the stellar spin and orbital axes -- its obliquity. The classical method for determining projtected stellar obliquity is the Rossiter-McLaughlin effect. This method has been applied to over 100 hot Jupiters, yet only a handful of stellar binaries. Of the binary systems with measured projected obliquities, even fewer have measured 3D obliquities. In this paper, we add five more short-period binary 3D obliquity measurements to the sample that previously consisted of a single system. We present Rossiter-McLaughlin measurements for EBLM J0239-20, EBLM J0941-31, EBLM 1037-25, EBLM 1141-37, and EBLM J2025-45. These systems consist of an M-dwarf eclipsing an F/G type primary. We combined CORALIE and HARPS spectroscopy with TESS photometry of primary and secondary eclipses. We show that even though the sky-projected obliquities seem to be aligned, there is modest but non-zero spin-orbit misalignment (ψ between 5 and 20∘). Our primary stars straddle the Kraft break at ∼ 6250K. Finally, we derive the M-dwarf masses and radii to precisions better than 3%. With the exception of EBLM J0941-31, each system has an inflated radius greater than 5σ from the expected radius from stellar models.