NGTS clusters survey -- III: A low-mass eclipsing binary in the Blanco 1 open cluster spanning the fully convective boundary

Abstract

We present the discovery and characterisation of an eclipsing binary identified by the Next Generation Transit Survey in the $\sim$115 Myr old Blanco 1 open cluster. NGTS J0002-29 comprises three M dwarfs: a short-period binary and a companion in a wider orbit. This system is the first well-characterised, low-mass eclipsing binary in Blanco 1. With a low mass ratio, a tertiary companion and binary components that straddle the fully convective boundary, it is an important benchmark system, and one of only two well-characterised, low-mass eclipsing binaries at this age. We simultaneously model light curves from NGTS, TESS, SPECULOOS and SAAO, radial velocities from VLT/UVES and Keck/HIRES, and the system's spectral energy distribution. We find that the binary components travel on circular orbits around their common centre of mass in $P_{\mathrm{orb}}=1.09800524\pm 0.00000038$ days, and have masses $M_{\mathrm{pri}}=0.3978\pm 0.0033$ M${}_{\odot }$ and $M_{\sec }=0.2245\pm 0.0018$ M${}_{\odot }$, radii $R_{\mathrm{pri}}=0.4037\pm 0.0048$ R${}_{\odot }$ and $R_{\sec }=0.2759\pm 0.0055$ R${}_{\odot }$, and effective temperatures $T_{\mathrm{pri}}=3372{}_{-37}^{+44}$ K and $T_{\sec }=3231{}_{-31}^{+38}$ K. We compare these properties to the predictions of seven stellar evolution models, which typically imply an inflated primary. The system joins a list of 19 well-characterised, low-mass, sub-Gyr, stellar-mass eclipsing binaries, which constitute some of the strongest observational tests of stellar evolution theory at low masses and young ages.

21 pages, 9 figures. Accepted for publication in MNRAS