The VLT-FLAMES Tarantula Survey

XXX. Red stragglers in the clusters Hodge 301 and SL 639^⋆

¹ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
e-mail: britavskiy@iac.es
² Dpto. Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
³ ESA, European Space Astronomy Centre, Apdo. de Correos 78, 28691 Villanueva de la Cañda, Madrid, Spain
⁴ UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
⁵ Argelander-Institut fü Astronomie der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁶ Lennard-Jones Laboratories, Keele University, Staffordshire ST5 5BG, UK
⁷ School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
⁸ Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁹ Zentrum für Astronomie der Universität Heidelberg, Astronomisches Rechen-Institut, Mönchhofstr. 12-14, 69120 Heidelberg, Germany
¹⁰ Heidelberger Institut für Theoretische Studien, Schloss-Wolfsbrunnenweg 35, 69118 Heidelberg, Germany
¹¹ Departamento de Física Teórica e Experimental, Universidade Federal do Rio Grande do Norte, CP 1641, Natal, RN 59072-970, Brazil
¹² Institute of astrophysics, KU Leuven, Celestijnlaan 200D, 3001 Leuven, Belgium
¹³ Anton Pannenkoek Institute for Astronomy, University of Amsterdam, 1090 GE Amsterdam, The Netherlands

Received: 2 November 2018
Accepted: 26 February 2019

Abstract

Aims. We estimate physical parameters for the late-type massive stars observed as part of the VLT-FLAMES Tarantula Survey (VFTS) in the 30 Doradus region of the Large Magellanic Cloud (LMC).

Methods. The observational sample comprises 20 candidate red supergiants (RSGs) which are the reddest ((B − V) > 1 mag) and brightest (V < 16 mag) objects in the VFTS. We use optical and near-infrared (near-IR) photometry to estimate their temperatures and luminosities, and introduce the luminosity–age diagram to estimate their ages.

Results. We derive physical parameters for our targets, including temperatures from a new calibration of (J − K_s)₀ colour for luminous cool stars in the LMC, luminosities from their J-band magnitudes (thence radii), and ages from comparisons with current evolutionary models. We show that interstellar extinction is a significant factor for our targets, highlighting the need to take it into account in the analysis of the physical parameters of RSGs. We find that some of the candidate RSGs could be massive AGB stars. The apparent ages of the RSGs in the Hodge 301 and SL 639 clusters show a significant spread (12–24 Myr). We also apply our approach to the RSG population of the relatively nearby NGC 2100 cluster, finding a similarly large spread.

Conclusions. We argue that the effects of mass transfer in binaries may lead to more massive and luminous RSGs (which we call “red stragglers”) than expected from single-star evolution, and that the true cluster ages correspond to the upper limit of the estimated RSG ages. In this way, the RSGs can serve as a new and potentially reliable age tracer in young star clusters. The corresponding analysis yields ages of 24₋₃⁺⁵ Myr for Hodge 301, 22₋₅⁺⁶ Myr for SL 639, and 23₋₂⁺⁴ Myr for NGC 2100.