1 research outputs found
O stars with weak winds: the Galactic case
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 ~
1e-8 Msol/yr) 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. Both UV and Ha lines are used to
derive wind properties while optical H and He lines give the stellar
parameters. Mass loss rates of all stars are found to be lower than expected
from the hydrodynamical predictions of Vink et al. (2001). For stars with log
L/Lsol > 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/Lsol < 5.2 the
reduction can be as high as a factor 100. The inclusion of X-ray emission in
models with low density is crucial to derive accurate mass loss rates from UV
lines. 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. The physical reason for such weak
winds is still not clear although the finding of weak winds in Galactic stars
excludes the role of a reduced metallicity. 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.Comment: 31 pages, 42 figures. A&A accepted. A version of the paper with full
resolution figures is available at
http://www.mpe.mpg.de/~martins/publications.htm