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 ∼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 Porb = 1.09800524 ± 0.00000038 days, 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-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.

Fundamental Properties of Late-Type Stars in Eclipsing Binaries

“…The case of NGTS 0002-29 is also remarkable. The system is a member of the Blanco 1 open cluster [37] with and estimated age of 90 − 150 Ma. Comparing with the BHAC15 models at this age range, radius discrepancies are reduced to ∼5.5% and ∼3.2% for components A and B, respectively.…”

The EBLM project – IX. Five fully convective M-dwarfs, precisely measured with CHEOPS and TESS light curves

“…Similarly to SW21, we compiled a comparison sample of precisely measured low mass stars from literature, classified in single stars, double lined binaries, and single lined binaries. (Carter et al 2011;Nefs et al 2013;Gillen et al 2017;Parsons et al 2018;Smith et al 2021;Swayne et al 2021). We compared the radii with both the MIST and the Exeter/Lyon (Baraffe et al 2015) models for solar metallicity.…”