The VLT-FLAMES survey of massive stars: wind properties and evolution of hot massive stars in the Large Magellanic Cloud^*

¹ Astronomical Institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
² UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
³ Universitäts-Sternwarte München, Scheinerstr., 1, 81679 München, Germany
⁴ The Department of Pure and Applied Physics, The Queen's University of Belfast, Belfast BT7 1NN, Northern Ireland, UK
⁵ Department of Physics and Astronomy, University of Sheffield, Hicks Building, Hounsfield Rd, Shefffield S3 7RH, UK
⁶ Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
⁷ Departamento de Astrofísica, Universidad de La Laguna, Avda. Astrofísico Francisco Sánchez, s/n, 38071 La Laguna, Spain
⁸ Astronomical Institute, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands
⁹ The Isaac Newton Group of Telescopes, Apartado de Correos 321, 38700, Santa Cruz de La Palma, Canary Islands, Spain
¹⁰ Instituto de Estructura de la Materia, Consejo Superior de Investigaciones Científicas, CSIC, Serrano 121, 28006 Madrid, Spain
¹¹ Grantecan S.A., 38200 La Laguna, Tenerife, Spain
¹² Astrophysics Group, Lennard-Jones Laboratories, Keele University, Staffordshire ST55BG, UK

Received: 2 October 2006
Accepted: 24 January 2007

Abstract

We have studied the optical spectra of a sample of 28 O- and early B-type stars in the Large Magellanic Cloud, 22 of which are associated with the young star forming region N11. Our observations sample the central associations of LH9 and LH10, and the surrounding regions. Stellar parameters are determined using an automated fitting method (Mokiem et al. 2005), which combines the stellar atmosphere code fastwind (Puls et al. 2005) with the genetic algorithm based optimisation routine pikaia (Charbonneau 1995). We derive an age of 7.0 ± 1.0 and 3.0 ± 1.0 Myr for LH9 and LH10, respectively. The age difference and relative distance of the associations are consistent with a sequential star formation scenario in which stellar activity in LH9 triggered the formation of LH10. Our sample contains four stars of spectral type O2. From helium and hydrogen line fitting we find the hottest three of these stars to be ~ kK (compared to ~ kK for O3 stars). Detailed determination of the helium mass fraction reveals that the masses of helium enriched dwarfs and giants derived in our spectroscopic analysis are systematically lower than those implied by non-rotating evolutionary tracks. We interpret this as evidence for efficient rotationally enhanced mixing leading to the surfacing of primary helium and to an increase of the stellar luminosity. This result is consistent with findings for SMC stars by Mokiem et al. (2006). For bright giants and supergiants no such mass discrepancy is found; these stars therefore appear to follow tracks of modestly or non-rotating objects. The set of programme stars was sufficiently large to establish the mass loss rates of OB stars in this environment sufficiently accurate to allow for a quantitative comparison with similar objects in the Galaxy and the SMC. The mass loss properties are found to be intermediate to massive stars in the Galaxy and SMC. Comparing the derived modified wind momenta D_mom as a function of luminosity with predictions for LMC metallicities by Vink et al. (2001) yields good agreement in the entire luminosity range that was investigated, i.e. .