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

Jackson, Richard J.; Jeffries, Robin D.; Randich, S.; Bragaglia, A.; Carraro, G.; Costado, M.T.; Flaccomio, E.; Lanzafame, A.C.; Lardo, C.; Monaco, L.; Morbidelli, L.; Smiljanic, R.; Zaggia, S.

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 2–3s 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.

Citation

Jackson, R. J., Jeffries, R. D., Randich, S., Bragaglia, A., Carraro, G., Costado, M., …Zaggia, S. (2016). The Gaia-ESO Survey: Stellar radii in the young open clusters NGC 2264, NGC 2547, and NGC 2516. Astronomy and Astrophysics Supplement Series, 586, Article A52. https://doi.org/10.1051/0004-6361/201527507

• Journal Article Type: Article
• Acceptance Date: Nov 17, 2015
• Online Publication Date: Jan 26, 2016
• Publication Date: 2016-02
• Publicly Available Date: May 26, 2023
• Journal: Astronomy and Astrophysics
• Print ISSN: 0365-0138
• Publisher: EDP Sciences
• Peer Reviewed: Peer Reviewed
• Volume: 586
• Article Number: A52
• DOI: https://doi.org/10.1051/0004-6361/201527507
• Keywords: stars: rotation, stars: low-mass, stars: activity, open clusters and associations: individual: NGC 2264, open clusters and associations: individual: NGC 2547, open clusters and associations: individual: NGC 2516
• Publisher URL: https://doi.org/10.1051/0004-6361/201527507