EBLM XVII – tidal synchronization and circularization in tight stellar binaries

ABSTRACT Tidal interactions in close stellar binaries are central to their orbital and rotational evolution, making observational tests of theoretical predictions essential for our understanding of the evolution of these, as well as close exoplanetary systems. Such tests require precise measurements of the orbital eccentricity and stellar rotation. The EBLM (Eclipsing Binary Low Mass) survey delivers a homogeneous sample of eclipsing binaries, composed of F/G/K primaries and M-dwarf (or low-mass K-dwarf) secondaries. We analyse 68 unequal mass binaries ($0.1 \le q \le 0.6$, where q is the mass ratio), with measurable primary star rotation rates from TESS, and over a decade of radial velocity observations. This sample probes the critical regime where tidal effects are expected to transition between being efficient and inefficient. We find that $\sim$75 per cent of our sample has circularized, with eccentric systems confined to $P_{\rm orb} \gtrsim 3$ d, with modest eccentricities ($e \lt 0.25$). Roughly $\sim$78 per cent of our sample is synchronized, with nearly all binaries within a 3-d orbital period residing in a well-defined ‘synchronization zone’. Beyond this, a minority of asynchronous systems persist, which cannot be easily explained by our application of current tidal mechanisms or by differential rotation.