The VLT-FLAMES survey of massive stars: mass loss and rotation of early-type stars in the SMC

Abstract

We have studied the optical spectra of a sample of 31 O-and early B-type stars in the Small Magellanic Cloud, 21 of which are associated with the young massive cluster NGC 346. Stellar parameters are determined using an automated fitting method (Mokiem et al. 2005, A&A, 441, 711), which combines the stellar atmosphere code fastwind (Puls et al. 2005, A&A, 435, 669) with the genetic algorithm based optimisation routine pikaia (Charbonneau 1995, ApJS, 101, 309). Comparison with predictions of stellar evolution that account for stellar rotation does not result in a unique age, though most stars are best represented by an age of 1-3 Myr. The automated method allows for a detailed determination of the projected rotational velocities. The present day v r sin i distribution of the 21 dwarf stars in our sample is consistent with an underlying rotational velocity (v r ) distribution that can be characterised by a mean velocity of about 160−190 km s −1 and an effective half width of 100−150 km s −1 . The v r distribution must include a small percentage of slowly rotating stars. If predictions of the time evolution of the equatorial velocity for massive stars within the environment of the SMC are correct (Maeder & Meynet 2001, A&A, 373, 555), the young age of the cluster implies that this underlying distribution is representative for the initial rotational velocity distribution. The location in the Hertzsprung-Russell diagram of the stars showing helium enrichment is in qualitative agreement with evolutionary tracks accounting for rotation, but not for those ignoring v r . The mass loss rates of the SMC objects having luminosities of log L /L 5.4 are in excellent agreement with predictions by Vink et al. (2001, A&A, 369, 574). However, for lower luminosity stars the winds are too weak to determineṀ accurately from the optical spectrum. Three targets were classifiedas Vz stars, two of which are located close to the theoretical zero-age main sequence. Three lower luminosity targets that were not classified as Vz stars are also found to lie near the ZAMS. We argue that this is related to a temperature effect inhibiting cooler from displaying the spectral features required for the Vz luminosity class.

Critical study of the distribution of rotational velocities of Be stars

“…Nevertheless, depending on the statistical problem, the suited simplified functional form Π(x) is probably different. Moreover, this leads us to conclude that it is not enough to claim that Gaussian or Maxwellian distributions are consistent with the observed distributions simply because the first two moments (average and dispersion) coincide, as the literature sometimes concludes (e.g., Mokiem et al 2006;Dufton et al 2006b;Hunter et al 2008).…”

Binary star progenitors of long gamma-ray bursts

“…The following hypothesis therefore seems conceivable: the IRF of single O stars in the SMC ends at about 210 km s −1 -too early to allow quasi-chemically homogeneous evolution and collapsar formation. However, massive close binary evolution enhances the IRF to what we may call the apparent IRF as measured by Mokiem et al (2006), which leads to the redshift dependent GRB rate as worked out by Yoon et al (2006). According to the binary population synthesis model of Podsiadlowski et al (1992), about 10% of all massive binaries might lead to a Case A merger or early Case B mass transfer, which is sufficient to populate the rapidly rotating part of the IRF of Mokiem et al In that context, the rapidly rotating O star in the sample of Mokiem et al (2006) which does not appear as runaway star could either have an undetected high proper motion, or it could be the result of a Case A merger -where no runaway is produced.…”

“…However, that data of Mokiem et al (2006) reveals another interesting feature: two of the the three mentioned stars are runaway stars, with radial velocities deviating by 30...70 km s −1 from the average cluster radial velocity. While dealing with low number statistics, this information opens another possibility: that the most rapidly rotating young O stars in the SMC are products of binary evolution.…”

“…However, it is to be said that single stars which rotate initially with 90% of their break-up velocity might not form in nature (cf. Mokiem et al 2006, and see below). Thus, perhaps the main benefit of the massive close binaries is just to produce very rapidly rotating O stars.…”

“…The best constraint so far on the distribution of initial rotational velocities (IRF) comes from the recent study of young O stars in the SMC, mostly from the cluster NGC 346 (Mokiem et al 2006). According to Yoon et al (2006), the three most rapid rotators from the sample of 21 O stars would qualify for the quasi-chemically homogeneous evolution scenario, and remarkably, all three stars are found to be helium-enhanced.…”