Bibcode
Mokiem, M. R.; de Koter, A.; Evans, C. J.; Puls, J.; Smartt, S. J.; Crowther, P. A.; Herrero, A.; Langer, N.; Lennon, D. J.; Najarro, F.; Villamariz, M. R.; Vink, J. S.
Bibliographical reference
Astronomy and Astrophysics, Volume 465, Issue 3, April III 2007, pp.1003-1019
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4
2007
Journal
Citations
146
Refereed citations
109
Description
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 49{-}54 kK (compared to
45{-}46 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
Z ˜ 1/2 Z&sun; 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. 5.0 < log
L/L&sun;< 6.1.
Appendix A is only available in electronic form at http://www.aanda.org