The EBLM Project XVIII. 3D Obliquities of Five Low-Mass Eclipsing Binaries

Abstract

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 ($\psi$ between 5 and 20$^{\circ}$). Our primary stars straddle the Kraft break at $\sim 6250K$. 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 $5\sigma$ from the expected radius from stellar models.