The EBLM project III. A Saturn-size low-mass star at the hydrogen-burning limit

TL;DR

This paper reports the discovery and detailed characterization of a low-mass star near the hydrogen-burning limit in an eclipsing binary system, providing valuable data for stellar models.

Contribution

The study presents the first precise measurement of a Saturn-sized low-mass star at the hydrogen-burning limit using combined photometric and spectroscopic data.

Findings

The companion has a mass of approximately 85 Jupiter masses.

The companion's radius is comparable to Saturn's, around 0.84 Jupiter radii.

The object is among the densest non-remnant objects known.

Abstract

We report the discovery of an eclipsing binary system with mass-ratio q $\sim$ 0.07. After identifying a periodic photometric signal received by WASP, we obtained CORALIE spectroscopic radial velocities and follow-up light curves with the Euler and TRAPPIST telescopes. From a joint fit of these data we determine that EBLM J0555-57 consists of a sun-like primary star that is eclipsed by a low-mass companion, on a weakly eccentric 7.8-day orbit. Using a mass estimate for the primary star derived from stellar models, we determine a companion mass of $85 \pm 4 M_{\rm Jup}$ ($0.081M_{\odot}$) and a radius of $0.84^{+0.14}_{-0.04} R_{\rm Jup}$ ($0.084 R_{\odot}$) that is comparable to that of Saturn. EBLM J0555-57Ab has a surface gravity $\log g_\mathrm{2} = 5.50^{+0.03}_{-0.13}$ and is one of the densest non-stellar-remnant objects currently known. These measurements are consistent with models of low-mass stars.