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Paper: Massive Stars in the SMC
Volume: 353, Stellar Evolution at Low Metallicity: Mass Loss, Explosions, Cosmology
Page: 29
Authors: Lennon, D.J.; Evans, C.J.; Trundle, C.
Abstract: In this paper we discuss how the Small Magellanic Cloud is the ideal laboratory in which to study massive stars in a low metallicity environment. We review the observational data for OB stars in the SMC concentrating on those aspects of their spectra which provide information on processes which may strongly influence their evolution, namely mass-loss, rotational mixing and mass-transfer. We illustrate the very weak winds now thought to pertain to late O-dwarfs in the SMC, using HST/STIS observations of the main sequence in the very young cluster NGC346, briefly discussing the quantitative results for these stars, and the difficulties involved in their determination. We show how stars with similar luminosities can have different luminosity classes while stars with similar spectral types and luminosity classes can have significantly different luminosities. These discrepancies can be interprated as evidence for rotational mixing on the main sequence. While the weak winds of the dwarfs present serious difficulties for the determination of wind terminal velocities we show that the supergiants have terminal velocities comparable to OB supergiants in the Milky Way, in reasonable agreement with theory. We also summarize recent work demonstrating that the temperature dependence of wind terminal velocities does not follow the widely adopted step-like approximation, the bistability jump around spectral type B1 does not occur for normal stars. Finally we review surface compositons of OB stars in the SMC finding that 42 out of 45 OB stars with detailed surface abundances are enriched in nitrogen by a factor ∼ 10 or more. While these enhancements are consistent with those produced by models with rotational mixing the rotational velocities of the sample are significantly lower than the values predicted by the models, indicating a possible problem with the evolution of angular momentum in the models or possibly in the efficiency of mixing. In this context we comment on the biases present in the stellar samples discussed in the literature.
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