789 research outputs found
UV Spectroscopy of Metal-Poor Massive Stars in the Small Magellanic Cloud
The Hubble Space Telescope has provided the first clear evidence for weaker
winds of metal-poor massive stars in the Small Magellanic Cloud, confirming
theoretical predictions of the metallicity dependence of mass-loss rates and
wind terminal velocities. For lower luminosity O-type stars however, derived
mass-loss rates are orders of magnitude lower than predicted, and are at
present unexplained.Comment: 4 pages, 3 figures. To appear in 'The Impact of HST on European
Astronomy', Eds., G. De Marchi & F.D. Macchetto, Astrophysics & Space
Science, Springe
Fundamental parameters of Galactic luminous OB stars VI. Temperatures, masses and WLR of Cyg OB2 supergiants
We have analyzed six OB supergiants and one giant covering spectral types
from O3 to B1 in the Galactic OB association Cyg OB2 by means of an updated
version of FASTWIND (SAntolaya-Rey, Puls, Herrero, 1997, A&A 323, 348) that
includes an approximate treatment of metal line blocking and blanketing. This
large coverage in spectral type allows us to derive a new temperature scale for
Galactic O supergiants that is lower than the one obtained by using pure H--He
models, either plane-parallel and hydrostatic or spherical with mass-loss. The
lower temperatures are thus a combined effect of line blanketing and the large
mass-loss rates. In some cases, the newly derived effective temperature is
reduced by up to 8000 K. Changes are larger for earlier stars with large
mass--loss rates. As a consequence, luminosities are modified as well, which
results in a lower number of emerging ionizing photons and reduces the mass
discrepancy. Although there are still significant differences between
spectroscopic and evolutionary masses, we do not find any obvious systematic
pattern of those differences. We derive mass--loss rates and the corresponding
wind momentum--luminosity relation for the analyzed stars. Although consistent
with previous results by Puls et al. (1996, A&A 305, 171) for Galactic stars,
our relation is better defined due to a reduction of errors related to stellar
distances and points to a possible separation between extreme Of stars (Of+,
Of*) and stars with more moderate morphologies. However this finding is only
tentative, as the statistics are still scarce.Comment: accpeted for A&
Nitrogen line spectroscopy of O-stars -- I. Nitrogen III emission line formation revisited
This is the first paper in a series dealing with optical Nitrogen
spectroscopy of O-type stars, aiming at the analysis of Nitrogen abundances.
We implemented a new Nitrogen model atom into the NLTE atmosphere/spectrum
synthesis code FASTWIND, and compare the resulting optical NIII lines at
4634/40/42 A with other predictions, mostly from Mihalas & Hummer (1973, ApJ
179, 827,`MH'), and from the alternative code CMFGEN.
Using similar model atmospheres as MH (not blanketed and wind-free), we are
able to reproduce their results, in particular the triplet emission lines.
According to MH, these should be strongly related to dielectronic recombination
(DR) and the drain by certain two-electron transitions. However, using
realistic, fully line-blanketed atmospheres at solar abundances, the key role
of DR controlling these emission features is superseded -- for O-star
conditions -- by the strength of the stellar wind and metallicity. In the case
of wind-free models, the resulting lower ionizing EUV-fluxes severely suppress
the emission. As the mass-loss rate is increased, pumping through the NIII
resonance line(s) in the presence of a near-photospheric velocity field results
in a net optical triplet line emission. A comparison with results from CMFGEN
is mostly satisfactory, except for the range 30 kK < Teff < 35 kK, where CMFGEN
triggers the triplet emission at lower Teff than FASTWIND. This effect could be
traced down to line overlap effects between the NIII and OIII resonance lines
that so far cannot be simulated by FASTWIND.
Since the efficiency of DR and `two electron drain' strongly depends on the
degree of line-blanketing/-blocking, we predict the emission to become stronger
in a metal-poor environment, though lower wind-strengths and Nitrogen
abundances might counteract this effect. Weak winded stars should display less
triplet emission than stars with `normal' winds.Comment: Accepted by Astronomy & Astrophysics. Main paper: 18 pages, 16
figures; Online-appendix: 6 pages, 14 figure
New predictions for radiation-driven, steady-state mass-loss and wind-momentum from hot, massive stars II. A grid of O-type stars in the Galaxy and the Magellanic Clouds
Reliable predictions of mass-loss rates are important for massive-star
evolution computations. We aim to provide predictions for mass-loss rates and
wind-momentum rates of O-type stars, carefully studying the behaviour of these
winds as functions of stellar parameters like luminosity and metallicity. We
use newly developed steady-state models of radiation-driven winds to compute
the global properties of a grid of O-stars. The self-consistent models are
calculated by means of an iterative solution to the equation of motion using
full NLTE radiative transfer in the co-moving frame to compute the radiative
acceleration. In order to study winds in different galactic environments, the
grid covers main-sequence stars, giants and supergiants in the Galaxy and both
Magellanic Clouds. We find a strong dependence of mass-loss on both luminosity
and metallicity. Mean values across the grid are
and , however we also find a somewhat stronger
dependence on metallicity for lower luminosities. Similarly, the mass
loss-luminosity relation is somewhat steeper for the SMC than for the Galaxy.
In addition, the computed rates are systematically lower (by a factor 2 and
more) than those commonly used in stellar-evolution calculations. Overall, our
results agree well with observations in the Galaxy that account properly for
wind-clumping, with empirical vs. scaling relations, and
with observations of O-dwarfs in the SMC. Our results provide simple fit
relations for mass-loss rates and wind momenta of massive O-stars stars as
functions of luminosity and metallicity, valid in the range \,K. Due to the systematically lower , our new models
suggest that new rates might be needed in evolution simulations of massive
stars.Comment: Accepted for publication in A&A. 16 pages, 13 figure
On the sensitivity of HeI singlet lines to the FeIV model atom in O stars
Recent calculations and analyses of O star spectra have revealed
discrepancies between theory and observations, and between different
theoretical calculations, for the strength of optical HeI singlet
transitions.We investigate the source of these discrepancies. Using a non-LTE
radiative transfer code we have undertaken detailed test calculations for a
range of O star properties. Our principal test model has parameters similar to
those of the O9V star, 10 Lac. We show that the discrepancies arise from
uncertainties in the radiation field in the HeI resonance transition near
584Angs. The radiation field at 584Angs. is influenced by model assumptions,
such as the treatment of line-blanketing and the adopted turbulent velocity,
and by the FeIV atomic data. It isshown that two FeIV transitions near 584Angs
can have a substantial influence on the strength of the HeI singlet
transitions. Because of the difficulty of modeling the HeI singlet lines,
particularly in stars with solar metalicity, the HeI triplet lines should be
preferred in spectral analyses. These lines are much less sensitive to model
assumptions.Comment: 7 pages, 9 figures, accepted for publication in A&
A Medium Resolution Near-Infrared Spectral Atlas of O and Early B Stars
We present intermediate resolution (R ~ 8,000 - 12,000) high signal-to-noise
H- and K-band spectroscopy of a sample of 37 optically visible stars, ranging
in spectral type from O3 to B3 and representing most luminosity classes.
Spectra of this quality can be used to constrain the temperature, luminosity
and general wind properties of OB stars, when used in conjunction with
sophisticated atmospheric model codes. Most important is the need for
moderately high resolutions (R > 5000) and very high signal-to-noise (S/N >
150) spectra for a meaningful profile analysis. When using near-infrared
spectra for a classification system, moderately high signal-to-noise (S/N ~
100) is still required, though the resolution can be relaxed to just a thousand
or two. In the appendix we provide a set of very high quality near-infrared
spectra of Brackett lines in six early-A dwarfs. These can be used to aid in
the modeling and removal of such lines when early-A dwarfs are used for
telluric spectroscopic standards.Comment: 12 pages, 3 tables, 14 figures. AASTex preprint style. To appear in
ApJS, November 2005. All spectra are available by contacting M.M. Hanso
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
- âŠ