477 research outputs found
An explanation for the curious mass loss history of massive stars: from OB stars, through Luminous Blue Variables to Wolf-Rayet stars
The stellar winds of massive stars show large changes in mass-loss rates and
terminal velocities during their evolution from O-star through the Luminous
Blue Variable phase to the Wolf-Rayet phase. The luminosity remains
approximately unchanged during these phases. These large changes in wind
properties are explained in the context of the radiation driven wind theory, of
which we consider four different models. They are due to the evolutionary
changes in radius, gravity and surface composition and to the change from
optically thin (in continuum) line driven winds to optically thick radiation
driven winds.Comment: Accepted for publication in Astronomy and Astrophysics (Letter to the
Editor
Radiation-driven winds of hot luminous stars XVII. Parameters of selected central stars of PN from consistent optical and UV spectral analysis and the universality of the mass-luminosity relation
Context: The commonly accepted mass-luminosity relation of central stars of
planetary nebulae (CSPNs) might not be universally valid. While earlier optical
analyses could not derive masses and luminosities independently (instead taking
them from theoretical evolutionary models) hydrodynamically consistent
modelling of the stellar winds allows using fits to the UV spectra to
consistently determine also stellar radii, masses, and luminosities without
assuming a mass-luminosity relation. Recent application to a sample of CSPNs
raised questions regarding the validity of the theoretical mass-luminosity
relation of CSPNs.
Aims: The results of the earlier UV analysis are reassessed by means of a
simultaneous comparison of observed optical and UV spectra with corresponding
synthetic spectra.
Methods: Using published stellar parameters (a) from a consistent UV analysis
and (b) from fits to optical H and He lines, we calculate simultaneous optical
and UV spectra with our model atmosphere code, which has been improved by
implementing Stark broadening for H and He lines.
Results: Spectra computed with the parameter sets from the UV analysis yield
good agreement to the observations, but spectra computed with the stellar
parameters from the published optical analysis and using corresponding
consistent wind parameters show large discrepancies to both the observed
optical and UV spectra. The published optical analyses give good fits to the
observed spectrum only because the wind parameters assumed in these analyses
are inconsistent with their stellar parameters. By enforcing consistency
between stellar and wind parameters, stellar parameters are obtained which
disagree with the core-mass-luminosity relation for the objects analyzed. This
disagreement is also evident from a completely different approach: an
investigation of the dynamical wind parameters.Comment: 22 pages, 18 fugre
Synthetic Spectra for Type Ia Supernovae at Early Epochs
We present the current status of our construction of synthetic spectra for
type Ia supernovae. These properly take into account the effects of NLTE and an
adequate representation of line blocking and blanketing. The models are based
on a sophisticated atomic database. We show that the synthetic spectrum
reproduces the observed spectrum of 'normal' SN-Ia near maximum light from the
UV to the near-IR. However, further improvements are necessary before truly
quantitative analyses of observed SN-Ia spectra can be performed. In
particular, the inner boundary condition has to be fundamentally modified. This
is due to the dominance of electron scattering over true absorption processes
coupled with the flat density structure in these objectsComment: To appear in "Proceedings of the IAU Colloquium 192 - Supernovae (10
Years of SN1993J)", eds. J.M. Marcaide and K.W. Weile
Model atmospheres for type Ia supernovae: Basic steps towards realistic synthetic spectra
Type Ia supernovae are an important tool for studying the expansion history
of the universe. Advancing our yet incomplete understanding of the explosion
scenario requires detailed and realistic numerical models in order to interpret
and analyze the growing amount of observational data. Here we present first
results of our new NLTE model calculations for the expanding atmospheres of
type Ia supernovae that employ a detailed and consistent treatment of all
important NLTE effects as well as line blocking and blanketing. The comparison
of the synthetic spectra resulting from these models with observed data shows
that the employed methods represent an important step towards a more realistic
description of the atmospheres of supernovae Ia.Comment: 4 pages, 1 figure, to appear in: Proceedings of the 11th Workshop on
Nuclear Astrophysics, Ringberg Castle, Germany, 200
The structure of line-driven winds
Following procedures pioneered by Castor, Abbott & Klein (1975, [CAK]),
spherically-symmetric supersonic winds for O stars are computed for matching to
plane-parallel moving reversing layers (RL's) from Paper I (Lucy 2007). In
contrast to a CAK wind, each of these solutions is singularity-free, thus
allowing its mass-loss rate to be fixed by the regularity condition at the
sonic point within the RL. Moreover, information propagation in these winds by
radiative-acoustic waves is everywhere outwardly-directed, justifying the
implicit assumption in Paper I that transonic flows are unaffected by
inwardly-directed wave motions.Comment: Accepted by A&A; 7 pages, 1 table, 4 figure
The Effect of Magnetic Field Tilt and Divergence on the Mass Flux and Flow Speed in a Line-Driven Stellar Wind
We carry out an extended analytic study of how the tilt and
faster-than-radial expansion from a magnetic field affect the mass flux and
flow speed of a line-driven stellar wind. A key motivation is to reconcile
results of numerical MHD simulations with previous analyses that had predicted
non-spherical expansion would lead to a strong speed enhancement. By including
finite-disk correction effects, a dynamically more consistent form for the
non-spherical expansion, and a moderate value of the line-driving power index
, we infer more modest speed enhancements that are in good quantitative
agreement with MHD simulations, and also are more consistent with observational
results. Our analysis also explains simulation results that show the
latitudinal variation of the surface mass flux scales with the square of the
cosine of the local tilt angle between the magnetic field and the radial
direction. Finally, we present a perturbation analysis of the effects of a
finite gas pressure on the wind mass loss rate and flow speed in both spherical
and magnetic wind models, showing that these scale with the ratio of the sound
speed to surface escape speed, , and are typically 10-20% compared
to an idealized, zero-gas-pressure model.Comment: Accepted for publication in ApJ, for the full version of the paper go
to: http://www.bartol.udel.edu/~owocki/preprints/btiltdiv-mdotvinf.pd
Chlorine and Sulfur in Nearby Planetary Nebulae and H II Regions
We derive the chlorine abundances in a sample of nearby planetary nebulae
(PNe) and H II regions that have some of the best available spectra. We use a
nearly homogeneous procedure to derive the abundance in each object and find
that the Cl/H abundance ratio shows similar values in H II regions and PNe.
This supports our previous interpretation that the underabundance we found for
oxygen in the H II regions is due to the depletion of their oxygen atoms into
organic refractory dust components. For other elements, the bias introduced by
ionization correction factors in their derived abundances can be very
important, as we illustrate here for sulfur using photoionization models. Even
for low-ionization PNe, the derived sulfur abundances can be lower than the
real ones by up to 0.3 dex, and the differences found with the abundances
derived for H II regions that have similar S/H can reach 0.4 dex.Comment: 2 pages, 1 figure, proceedings of the IAU Symposium No. 283,
Planetary Nebulae: an Eye to the Futur
Numerical Models for the Diffuse Ionized Gas in Galaxies. II. Three-dimensional radiative transfer in inhomogeneous interstellar structures as a tool for analyzing the diffuse ionized gas
Aims: We systematically explore a plausible subset of the parameter space
involving effective temperatures and metallicities of the ionizing stellar
sources, the effects of the hardening of their radiation by surrounding leaky
HII regions with different escape fractions, as well as different scenarios for
the clumpiness of the DIG, and compute the resulting line strength ratios for a
number of diagnostic optical emission lines.
Methods: For the ionizing fluxes we compute a grid of stellar spectral energy
distributions (SEDs) from detailed, fully non-LTE model atmospheres that
include the effects of stellar winds and line blocking and blanketing. To
calculate the ionization and temperature structure in the HII regions and the
diffuse ionized gas we use spherically symmetric photoionization models as well
as state-of-the-art three-dimensional (3D) non-LTE radiative transfer
simulations, considering hydrogen, helium, and the most abundant metals.
Results: We provide quantitative predictions of how the line ratios from HII
regions and the DIG vary as a function of metallicity, stellar effective
temperature, and escape fraction from the HII region. The range of predicted
line ratios reinforces the hypothesis that the DIG is ionized by (filtered)
radiation from hot stars; however, comparison of observed and predicted line
ratios indicates that the DIG is typically ionized with a softer SED than
predicted by the chosen stellar population synthesis model. Even small changes
in simulation parameters like the clumping factor can lead to considerable
variation in the ionized volume. Both for a more homogeneous gas and a very
inhomogeneous gas containing both dense clumps and channels with low gas
density, the ionized region in the dilute gas above the galactic plane can
cease to be radiation-bounded, allowing the ionizing radiation to leak into the
intergalactic medium.Comment: 21 pages, 9 figures, accepted by A&
Radiation-driven winds of hot luminous stars. XVI. Expanding atmospheres of massive and very massive stars and the evolution of dense stellar clusters
Context: Starbursts, and particularly their high-mass stars, play an
essential role in the evolution of galaxies. The winds of massive stars not
only significantly influence their surroundings, but the mass loss also
profoundly affects the evolution of the stars themselves. In addition to the
evolution of each star, the evolution of the dense cores of massive starburst
clusters is affected by N-body interactions, and the formation of very massive
stars via mergers may be decisive for the evolution of the cluster.
Aims: To introduce an advanced diagnostic method of O-type stellar
atmospheres with winds, including an assessment of the accuracy of the
determinations of abundances, stellar and wind parameters.
Methods: We combine consistent models of expanding atmospheres with detailed
stellar evolutionary calculations of massive and very massive single stars with
regard to the evolution of dense stellar clusters. Accurate predictions of the
mass loss rates of very massive stars requires a highly consistent treatment of
the statistical equilibrium and the hydrodynamic and radiative processes in the
expanding atmospheres.
Results: We present computed mass loss rates, terminal wind velocities, and
spectral energy distributions of massive and very massive stars of different
metallicities, calculated from atmospheric models with an improved level of
consistency.
Conclusions: Stellar evolutionary calculations using our computed mass loss
rates show that low-metallicity very massive stars lose only a very small
amount of their mass, making it unlikely that very massive population III stars
cause a significant helium enrichment of the interstellar medium.
Solar-metallicity stars have higher mass-loss rates, but these are not so high
to exclude very massive stars formed by mergers in dense clusters from ending
their life massive enough to form intermediate-mass black holes.Comment: Accepted by A&
The Importance of XUV Radiation as a Solution to the P V Mass Loss Rate Discrepancy in O-Stars
A controversy has developed regarding the stellar wind mass loss rates in
O-stars. The current consensus is that these winds may be clumped which implies
that all previously derived mass loss rates using density-squared diagnostics
are overestimated by a factor of ~ 2. However, arguments based on FUSE
observations of the P V resonance line doublet suggest that these rates should
be smaller by another order of magnitude, provided that P V is the dominant
phosphorous ion among these stars. Although a large mass loss rate reduction
would have a range of undesirable consequences, it does provide a
straightforward explanation of the unexpected symmetric and un-shifted X-ray
emission line profiles observed in high energy resolution spectra. But
acceptance of such a large reduction then leads to a contradiction with an
important observed X-ray property: the correlation between He-like ion source
radii and their equivalent X-ray continuum optical depth unity radii. Here we
examine the phosphorous ionization balance since the P V fractional abundance,
q(P V), is fundamental to understanding the magnitude of this mass loss
reduction. We find that strong "XUV" emission lines in the He II Lyman
continuum can significantly reduce q(P V). Furthermore, owing to the unique
energy distribution of these XUV lines, there is a negligible impact on the S V
fractional abundance (a key component in the FUSE mass loss argument). We
conclude that large reductions in O-star mass loss rates are not required, and
the X-ray optical depth unity relation remains valid.Comment: Accepted for publication in ApJ Letters, 15 pages, 5 color figure
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