The Gaia-ESO Survey: Stellar radii in the young open clusters NGC 2264, NGC 2547, and NGC 2516^⋆,⋆⋆

¹ Astrophysics Group, Keele University, Keele, Staffordshire ST5 5BG, UK
e-mail: r.j.jackson@keele.ac.uk
² INAF–Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy
³ INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁴ European Southern Observatory, Alonso de Cordova 3107 Vitacura, Santiago de Chile, Chile
⁵ Instituto de Astrofísica de Andalucía-CSIC, Apdo. 3004, 18080 Granada, Spain
⁶ INAF–Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
⁷ Dipartimento di Fisica e Astronomia, Sezione AstrofiFsica, Università di Catania, via S. Sofia 78, 95123 Catania, Italy
⁸ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
⁹ Departamento de Ciencias Físicas, Universidad Andrés Bello, República 220, 837-0134 Santiago, Chile
¹⁰ Department for Astrophysics, Nicolaus Copernicus Astronomical Center, ul. Rabiańska 8, 87-100 Toruń, Poland
¹¹ INAF–Padova Observatory, Vicolo dell’Osservatorio 5, 35122 Padova, Italy

Received: 6 October 2015
Accepted: 17 November 2015

Abstract

Context. Rapidly rotating, low-mass members of eclipsing binary systems have measured radii that are significantly larger than predicted by standard evolutionary models. It has been proposed that magnetic activity is responsible for this radius inflation.

Aims. By estimating the radii of low-mass stars in three young clusters (NGC 2264, NGC 2547, NGC 2516, with ages of ~5, ~35 and ~140 Myr respectively), we aim to establish whether similar radius inflation is seen in single, magnetically active stars.

Methods. We use radial velocities from the Gaia-ESO Survey (GES) and published photometry to establish cluster membership and then combine GES measurements of projected equatorial velocities with published rotation periods to estimate the average radii for groups of fast-rotating cluster members as a function of their luminosity and age. The average radii are compared with the predictions of both standard evolutionary models and variants that include magnetic inhibition of convection and starspots.

Results. At a given luminosity, the stellar radii in NGC 2516 and NGC 2547 are larger than predicted by standard evolutionary models at the ages of these clusters. The discrepancy is least pronounced and not significant (≃10 per cent) in zero age main sequence stars with radiative cores, but more significant in lower-mass, fully convective pre main-sequence cluster members, reaching ≃30 ± 10 per cent. The uncertain age and distance of NGC 2264 preclude a reliable determination of any discrepancy for its members.

Conclusions. The median radii we have estimated for low-mass fully convective stars in the older clusters are inconsistent (at the 2–3σ level) with non-magnetic evolutionary models and more consistent with models that incorporate the effects of magnetic fields or dark starspots. The available models suggest this requires either surface magnetic fields exceeding 2.5 kG, spots that block about 30 per cent of the photospheric flux, or a more moderate combination of both.