151 research outputs found
Stellar mass-loss near the Eddington limit. Tracing the sub-photospheric layers of classical Wolf-Rayet stars
Towards the end of their evolution hot massive stars develop strong stellar
winds and appear as emission line stars, such as WR stars or LBVs. The
quantitative description of the mass loss in these important pre-SN phases is
hampered by unknowns such as clumping and porosity due to an in-homogeneous
wind structure, and by an incomplete theoretical understanding of optically
thick stellar winds. In this work we investigate the conditions in deep
atmospheric layers of WR stars to find out whether these comply with the theory
of optically thick winds, and whether we find indications of clumping in these
layers. We use a new semi-empirical method to determine sonic-point optical
depths, densities, and temperatures for a large sample of WR stars of the
carbon (WC) and oxygen (WO) sequence. Based on an artificial model sequence we
investigate the reliability of our method and its sensitivity to uncertainties
in stellar parameters. We find that the WR stars in our sample obey an
approximate relation with P_rad/P_gas~80 at the sonic point. This 'wind
condition' is ubiquitous for radiatively driven, optically thick winds, and
sets constraints on possible wind/envelope solutions affecting radii, mass-loss
rates, and clumping properties. Our results suggest that the presence of an
optically thick wind may force many stars near the Eddington limit to develop
clumped, radially extended sub-surface zones. The clumping in these zones is
most likely sustained by the non-linear strange-mode instability, and may be
the origin of the observed wind clumping. The properties of typical late-type
WC stars comply with this model. Solutions without sub-surface clumping and
inflation are also possible but demand for compact stars with comparatively low
mass-loss rates. These objects may resemble the small group of WO stars with
their exceptionally hot stellar temperatures and highly ionized winds.Comment: accepted by A&
Narrow He II emission in star-forming galaxies at low metallicity. Stellar wind emission from a population of Very Massive Stars
In a recent study star-forming galaxies with HeII emission between redshifts
2 and 4.6 have been found to occur in two modes, distinguished by the width of
their HeII emission lines. Broad HeII emission has been attributed to stellar
emission from a population of evolved Wolf-Rayet (WR) stars while narrow HeII
emission has been attributed to nebular emission excited by a population of
very hot PopIII stars formed in pockets of pristine gas at moderate redshifts.
In this work we propose an alternative scenario for the origin of the narrow
HeII emission, namely very massive stars (VMS) at low metallicity (Z) which
form strong but slow WR-type stellar winds due to their proximity to the
Eddington limit. We estimate the expected HeII line fluxes and equivalent
widths based on wind models for VMS and population synthesis models, and
compare the results with recent observations of star-forming galaxies at
moderate redshifts. The observed HeII line strengths and equivalent widths are
in line with what is expected for a population of VMS in one or more young
super-clusters located within these galaxies. In our scenario the two observed
modes of HeII emission originate from massive stellar populations in distinct
evolutionary stages at low Z. If this interpretation is correct there is no
need to postulate the existence of PopIII stars at moderate redshifts to
explain the observed narrow HeII emission. An interesting possibility is the
existence of self-enriched VMS with similar WR-type spectra at extremely low Z.
Stellar HeII emission from such very early generations of VMS may be detectable
in future studies of star-forming galaxies at high redshifts with the James
Webb Space Telescope. The fact that the HeII emission of VMS is largely
neglected in current population synthesis models will generally affect the
interpretation of the integrated spectra of young stellar populations.Comment: 4 pages, 1 figure, A&A letters (accepted
Revised element abundances for WC-type central stars
According to previous spectral analyses of Wolf-Rayet type central stars,
late [WC] subtypes show systematically higher carbon-to-helium abundance ratios
than early [WC] subtypes. If this were true, it would rule out that these stars
form an evolutionary sequence. However, due to the different parameter domains
and diagnostic lines, one might suspect systematic errors being the source of
this discrepancy. In an ongoing project we are therefore checking the [WC]
analyses by means of the last generation of non-LTE models for expanding
stellar atmospheres which account for line-blanketing and wind clumping. So
far, the abundance discrepancy is not resolved. Further element abundances (H,
N, Fe) are determined and compared with evolutionary predictions.Comment: 4 pages, 5 figures, in conference proceedings of "Planetary Nebulae
in our Galaxy and Beyond" IAU Symposion 234, 2006, editors: Michael J.
Barlow, Roberto H. M\'ende
Stellar envelope inflation near the Eddington limit. Implications for the radii of Wolf-Rayet stars and luminous blue variables
(shortened) It has been proposed that the envelopes of luminous stars may be
subject to substantial radius inflation. The inflation effect has been
discussed in relation to the radius problem of WR stars, but has yet failed to
explain the large observed radii of Galactic WR stars. We wish to obtain a
physical perspective of the inflation effect, and study the consequences for
the radii of WR stars, and LBVs. For WR stars the observed radii are up to an
order of magnitude larger than predicted by theory, whilst S Doradus-type LBVs
are subject to humongous radius variations, which remain as yet ill-explained.
We use a dual approach to investigate the envelope inflation, based on
numerical models for stars near the Eddington limit, and a new analytic
formalism to describe the effect. An additional new aspect is that we take the
effect of density inhomogeneities (clumping) within the outer stellar envelopes
into account. Due to the effect of clumping we are able to bring the observed
WR radii in agreement with theory. Based on our new formalism, we find that the
radial inflation is a function of a dimensionless parameter W, which largely
depends on the topology of the Fe-opacity peak, i.e., on material properties.
For W>1, we discover an instability limit, for which the stellar envelope
becomes gravitationally unbound, i.e. there no longer exists a static solution.
Within this framework we are also able to explain the S Doradus-type
instabilities for LBVs like AG Car, with a possible triggering due to changes
in stellar rotation. The stellar effective temperatures in the upper HR diagram
are potentially strongly affected by the inflation effect. This may have
particularly strong effects on the evolved massive LBV and WR stars just prior
to their final collapse, as the progenitors of SNe Ibc, SNe II, and long GRBs.Comment: 15 pages, 11 Figures, recommended for publication in A&
The Eddington factor as the key to understand the winds of the most massive stars. Evidence for a Γ-dependence of Wolf-Rayet type mass loss
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