NGTS clusters survey - III. A low-mass eclipsing binary in the Blanco 1 open cluster spanning the fully convective boundary
Triaud, Amaury H. M. J.; Wheatley, Peter J.; Bayliss, Daniel; Moyano, Maximiliano; Dransfield, Georgina; Günther, Maximilian N.; Henderson, Beth A.; Casewell, Sarah L.; Gill, Samuel; Goad, Michael R.; Alves, Douglas R.; Anderson, David R.; Burleigh, Matthew R.; Gillen, Edward; Jenkins, James S.; Vines, Jose I.; Gillon, Michaël; Delrez, Laetitia; Ducrot, Elsa; Jehin, Emmanuël; Queloz, Didier; Hillenbrand, Lynne A.; Smith, Gareth D.; Pedersen, Peter P.; Briegal, Joshua T.; Acton, Jack S.; Tilbrook, Rosanna H.; Sebastian, Daniel; Thompson, Samantha; Murray, Catriona A.
United Kingdom, United States, Chile, Belgium
Abstract
We present the discovery and characterization of an eclipsing binary identified by the Next Generation Transit Survey in the ~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-characterized, 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-characterized, 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 Porb = 1.098 005 24 ± 0.000 000 38 d, and have masses Mpri = 0.3978 ± 0.0033 M⊙ and Msec = 0.2245 ± 0.0018 M⊙, radii Rpri = 0.4037 ± 0.0048 R⊙ and Rsec = 0.2759 ± 0.0055 R⊙, and effective temperatures $T_{\rm pri}=\mbox{$3372\, ^{+44}_{-37}$}$ K and $T_{\rm sec}=\mbox{$3231\, ^{+38}_{-31}$}$ 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-characterized, 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.