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

Gareth D. Smith, Edward Gillen, Didier Queloz, Lynne A. Hillenbrand, Jack S. Acton, Douglas R. Alves, David R. Anderson, Daniel Bayliss, Joshua T. Briegal, Matthew R. Burleigh, Sarah L. Casewell, Laetitia Delrez, Georgina Dransfield, Elsa Ducrot, Samuel Gill, Michaël Gillon, Michael R. Goad, Maximilian N. Günther, Beth A. Henderson, James S. JenkinsEmmanuël Jehin, Maximiliano Moyano, Catriona A. Murray, Peter P. Pedersen, Daniel Sebastian, Samantha Thompson, Rosanna H. Tilbrook, Amaury H. M. J. Triaud, Jose I. Vines, Peter J. Wheatley

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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 Tpri=3372+44−37 K and Tsec=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.
Original languageEnglish
Pages (from-to)5991–6011
Number of pages21
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
Early online date7 Sept 2021
Publication statusPublished - Nov 2021


  • binaries: eclipsing
  • binaries: spectroscopic
  • open clusters and associations: individual: Blanco 1
  • stars: evolution
  • stars: fundamental parameters
  • stars: low mass


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