The EBLM Project IV. Spectroscopic orbits of over 100 eclipsing M dwarfs masquerading as transiting hot-Jupiters

TL;DR

This study presents precise spectroscopic orbits for over 100 eclipsing M dwarfs initially mistaken for hot-Jupiters, providing valuable data for stellar and exoplanet research, including a significant increase in known low-mass star systems.

Contribution

It offers a large, detailed spectroscopic dataset of M-dwarf binaries, improving the characterization of low-mass stars and refining the understanding of the brown dwarf desert and mass spectrum.

Findings

Discovered 34 systems with secondary mass below 0.2 M_sun.

Detected eccentricities as small as 0.001.

Extended the mass spectrum resolution for low-mass companions.

Abstract

We present 2271 radial velocity measurements taken on 118 single-line binary stars, taken over eight years with the CORALIE spectrograph. The binaries consist of F/G/K primaries and M-dwarf secondaries. They were initially discovered photometrically by the WASP planet survey, as their shallow eclipses mimic a hot-Jupiter transit. The observations we present permit a precise characterisation of the binary orbital elements and mass function. With modelling of the primary star this mass function is converted to a mass of the secondary star. In the future, this spectroscopic work will be combined with precise photometric eclipses to draw an empirical mass/radius relation for the bottom of the mass sequence. This has applications in both stellar astrophysics and the growing number of exoplanet surveys around M-dwarfs. In particular, we have discovered 34 systems with a secondary mass below $0.2 M_\odot$, and so we will ultimately double the known number of very low-mass stars with well characterised mass and radii. We are able to detect eccentricities as small as 0.001 and orbital periods to sub-second precision. Our sample can revisit some earlier work on the tidal evolution of close binaries, extending it to low mass ratios. We find some binaries that are eccentric at orbital periods < 3 days, while our longest circular orbit has a period of 10.4 days. By collating the EBLM binaries with published WASP planets and brown dwarfs, we derive a mass spectrum with twice the resolution of previous work. We compare the WASP/EBLM sample of tightly-bound orbits with work in the literature on more distant companions up to 10 AU. We note that the brown dwarf desert appears wider, as it carves into the planetary domain for our short-period orbits. This would mean that a significantly reduced abundance of planets begins at $\sim 3M_{\rm Jup}$, well before the Deuterium-burning limit. [abridged]