O stars with weak winds: the Galactic case

Abstract

Abstract. We study the stellar and wind properties of a sample of Galactic O dwarfs to track the conditions under which weak winds (i.e. mass loss rates lower than ∼10 −8 M yr −1 ) appear. The sample is composed of low and high luminosity dwarfs including Vz stars and stars known to display qualitatively weak winds. Atmosphere models including non-LTE treatment, spherical expansion and line blanketing are computed with the code CMFGEN (Hillier & Miller 1998, ApJ, 496, 407). Both UV and Hα lines are used to derive wind properties while optical H and He lines give the stellar parameters. We find that the stars of our sample are usually 1 to 4 Myr old. Mass loss rates of all stars are found to be lower than expected from the hydrodynamical predictions of Vink et al. (2001, A&A, 369, 574). For stars with log L L > ∼ 5.2, the reduction is by less than a factor 5 and is mainly due to the inclusion of clumping in the models. For stars with log L L < ∼ 5.2 the reduction can be as high as a factor 100. The inclusion of X-ray emission (possibly due to magnetic mechanisms) in models with low density is crucial to derive accurate mass loss rates from UV lines, while it is found to be unimportant for high density winds. The modified wind momentum -luminosity relation shows a significant change of slope around this transition luminosity. Terminal velocities of low luminosity stars are also found to be low. Both mass loss rates and terminal velocities of low L stars are consistent with a reduced line force parameter α. However, the physical reason for such a reduction is still not clear although the finding of weak winds in Galactic stars excludes the role of a reduced metallicity. There may be a link between an early evolutionary state and a weak wind, but this has to be confirmed by further studies of Vz stars. X-rays, through the change in the ionisation structure they imply, may be at the origin of a reduction of the radiative acceleration, leading to lower mass loss rates. A better understanding of the origin of X-rays is of crucial importance for the study of the physics of weak winds.

Bright OB stars in the Galaxy

“…In order to estimate now the effects of line-blocking/blanketing together with wind effects in the B supergiant domain (as has been done previously for the O-star domain, e.g., Markova et al 2004;Repolust et al 2004;Martins et al 2005), we have combined the different datasets as discussed above into one sample, keeping in mind the encountered problems. Figure 9, left panel, displays the differences between "unblanketed" and "blanketed" effective temperatures for this combined sample, as a function of spectral type.…”

“…While the number of Galactic O and early B stars with reliably determined stellar and wind parameters has progressively increased during the last few years (e.g., Herrero et al 2002;Repolust et al 2004;Garcia & Bianchi 2004;Bouret et al 2005;Martins et al 2005;Crowther et al 2006), mid and late B supergiants (SGs) are currently under-represented in the sample of stars investigated so far. Given the fact that B-SGs represent an important phase in the evolutionary sequence of massive stars, any study aiming to increase our knowledge of these stars would be highly valuable, since it would allow several important issues to be addressed (see below).…”

“…During the last years, the quantitative analyses of spectra in the far-UV/UV and optical domains (e.g., Herrero et al 2002;Bianchi & Garcia 2002;Crowther et al 2002;Bouret et al 2003;Repolust et al 2004;Massey et al 2004;Heap et al 2006) have unambiguously shown that the inclusion of line-blocking and blanketing and wind effects (if present) significantly modifies the temperature scale of O-stars (for a recent calibration at solar metallicity, see Markova et al 2004;Martins et al 2005). Regarding B-SGs, particularly of later subtype, this issue has not been addressed so far, mostly due to lacking T eff estimates.…”

X-ray emission from hydrodynamical simulations in non-LTE wind models

“…To address the "weak wind problem" (Bouret et al 2003;Martins et al 2005), we also included stars that should be subject to it, i.e., their observed wind lines are much weaker than expected (the bottom four stars below the horizontal line in Table 1). …”

“…Note that the parameters derived by Repolust et al (2004), Markova et al (2004), and Martins et al (2005) were obtained using blanketed model atmospheres, i.e., they should be more reliable than the older ones. Stellar masses were obtained using evolutionary tracks (Schaller et al 1992).…”