525 research outputs found
The Discordance of Mass-Loss Estimates for Galactic O-Type Stars
We have determined accurate values of the product of the mass-loss rate and
the ion fraction of P^{4+}, Mdot q(P^{4+}), for a sample of 40 Galactic O-type
stars by fitting stellar-wind profiles to observations of the P V resonance
doublet obtained with FUSE, ORFEUS/BEFS, and Copernicus. When P^{4+} is the
dominant ion in the wind, Mdot q(P^{4+}) approximates the mass-loss rate to
within a factor of 2. Theory predicts that P^{4+} is the dominant ion in the
winds of O7-O9.7 stars, though an empirical estimator suggests that the range
from O4-O7 may be more appropriate. However, we find that the mass-loss rates
obtained from P V wind profiles are systematically smaller than those obtained
from fits to Halpha emission profiles or radio free-free emission by median
factors of about 130 (if P^{4+} is dominant between O7 and O9.7) or about 20
(if P^{4+} is dominant between O4 and O7). These discordant measurements can be
reconciled if the winds of O stars in the relevant temperature range are
strongly clumped on small spatial scales. We use a simplified two-component
model to investigate the volume filling factors of the denser regions. This
clumping implies that mass-loss rates determined from "density squared"
diagnostics have been systematically over-estimated by factors of 10 or more,
at least for a subset of O stars. Reductions in the mass-loss rates of this
size have important implications for the evolution of massive stars and
quantitative estimates of the feedback that hot-star winds provide to their
interstellar environments.Comment: 26 pages, 4 figures; accepted for publication in Ap
STIS UV spectroscopy of early B supergiants in M31
We analyze STIS spectra in the 1150-1700 Angstrom wavelength range obtained
for six early B supergiants in the neighboring galaxy M31. Because of their
likely high (nearly solar) abundance, these stars were originally chosen to be
directly comparable to their Galactic counterparts, and represent a much-needed
addition to our current sample of B-type supergiants, in our efforts to study
the dependence of the Wind Momentum-Luminosity Relationship on spectral type
and metallicity. As a first step to determine wind momenta we fit the P-Cygni
profiles of the resonance lines of N V, Si IV and C IV with standard methods,
and derive terminal velocities for all of the STIS targets. From these lines we
also derive ionic stellar wind column densities. Our results are compared with
those obtained previously in Galactic supergiants, and confirm earlier claims
of `normal' wind line intensities and terminal velocities in M31. For half of
the sample we find evidence for an enhanced maximum turbulent velocity when
compared to Galactic counterparts.Comment: 15 pages, 9 figures, 2 tables. Accepted for publication in The
Astrophysical Journa
Spectral Modelling of Star-Forming Regions in the Ultraviolet: Stellar Metallicity Diagnostics for High Redshift Galaxies
The chemical composition of high redshift galaxies is an important property
which gives clues to their past history and future evolution and yet is
difficult to measure with current techniques. In this paper we investigate new
metallicity indicators, based upon the strengths of stellar photospheric
features at rest-frame ultraviolet wavelengths. By combining the evolutionary
spectral synthesis code Starburst99 with the output from the non-LTE model
atmosphere code WM-basic, we have developed a code that can model the
integrated ultraviolet stellar spectra of star-forming regions at metallicities
between 1/20 and twice solar. We use our models to explore a number of spectral
regions that are sensitive to metallicity and clean of other spectral features.
The most promising metallicity indicator is an absorption feature between 1935
A and 2020 A, which arises from the blending of numerous Fe III transitions. We
compare our model spectra to observations of two well studied high redshift
star-forming galaxies, MS1512-cB58 (a Lyman break galaxy at z = 2.7276), and
Q1307-BM1163 (a UV-bright galaxy at z = 1.411). The profiles of the
photospheric absorption features observed in these galaxies are well reproduced
by the models. In addition, the metallicities inferred from their equivalent
widths are in good agreement with previous determinations based on interstellar
absorption and nebular emission lines. Our new technique appears to be a
promising alternative, or complement, to established methods which have only a
limited applicability at high redshifts.Comment: 18 pages, 12 figures, accepted for publication in the Astrophysical
Journa
Star Formation in the Field and Clusters of NGC 5253
We investigate the star formation history of both the bright star clusters
and the diffuse `field star' population in the dwarf starburst galaxy NGC 5253
using STIS longslit ultraviolet spectroscopy. Our slit covers a physical area
of 370 x 1.6 pc and includes 8 apparent clusters and several inter-cluster
regions of diffuse light which we take to be the field. The diffuse light
spectrum lacks the strong O-star wind features which are clearly visible in
spectra of the brightest clusters. This discrepancy provides compelling
evidence that the diffuse light is not reflected light from nearby clusters,
but originates in a UV-bright field star population, and it raises the issue of
whether the star formation process may be operating differently in the field
than in clusters. We compare our spectra to STARBURST99 evolutionary synthesis
models which incorporate a new low metallicity atlas of O-star spectra. We
favor a scenario which accounts for the paucity of O-stars in the field without
requiring the field to have a different IMF than the clusters: stellar clusters
form continuously and then dissolve on ~10 Myr timescales and disperse their
remaining stars into the field. We consider the probable contribution of an
O-star deficient field population to the spatially unresolved spectra of high
redshift galaxies. (Abridged)Comment: 26 pages, 10 figures, accepted for publication in Ap
A Simple Scaling Analysis of X-ray Emission and Absorption in Hot-Star Winds
We present a simple analysis of X-ray emission and absorption for hot-star
winds, designed to explore the natural scalings of the observed X-ray
luminosity with wind and sstellar properties. We show that an exospheric
approximation, in which all of the emission above the optical depth unity
radius escapes the wind, reproduces very well the detailed expression for
radiation transport through a spherically symmetric wind. Using this
approximation we find that the X-ray luminosity scales naturally with the
wind density parameter \Mdot/\vinf, obtaining L_x \sim (\Mdot/\vinf)^2 for
optically thin winds, and L_x \sim (\Mdot/\vinf)^{1+s} for optically thick
winds with an X-ray filling factor that varies in radius as . These
scalings with wind density contrast with the commonly inferred empirical
scalings of X-ray luminosity with bolometric luminosity . The
empirically derived linear scaling of for thick winds can
however be reproduced, through a delicate cancellation of emission and
absorption, if one assumes modest radial fall-off in the X-ray filling factor
( or , depending on details of the secondary
scaling of wind density with luminosity). We also explore the nature of the
X-ray spectral energy distribution in the context of this model, and find that
the spectrum is divided into a soft, optically thick part and a hard, optically
thin part. Finally, we conclude that the energy-dependent emissivity must have
a high-energy cut-off, corresponding to the maximum shock energy, in order to
reproduce the general trends seen in X-ray spectral energy distributions of hot
stars.Comment: 16 pages, 2 figures, requiress aaspp4.sty, accepted by Astrophysical
Journal, to appear in the Aug 10, 1999 issue. Several minor changes have been
made at the suggestion of the referee. We have added an appendix in which we
consider winds with beta-velocity laws, rather than simply constant
velocitie
Ionizing Photon Emission Rates from O- and Early B-type Stars and Clusters
We present new computations of the ionizing spectral energy distributions
(SEDs) and Lyman continuum (Lyc) and HeI continuum photon emission rates, for
hot O-type and early B-type stars. We consider solar metallicity stars, with
effective temperatures ranging from 25,000 to 55,000 K and surface gravities
(cm s^-2) logg ranging from 3 to 4, covering the full range of spectral types
and luminosity classes for hot stars. We use our updated (WM-basic) code to
construct radiation-driven wind atmosphere models for hot stars. Our models
include the coupled effects of hydrodynamics and non-LTE radiative transfer in
spherically outflowing winds, including the detailed effects of metal line
blocking and line blanketing on the radiative transfer and energy balance. We
incorporate our hot-star models into our population synthesis code (STARS), and
we compute the time-dependent SEDs and resulting Lyc and HeI emission rates for
evolving star clusters. We present results for continuous and impulsive star
formation for a range of assumed stellar initial mass functions.Comment: 23 pages, 7 figures. To appear in the Astrophysical Journal. For grid
of star models see ftp://wise3.tau.ac.il/pub/star
Mass-loss rates of Very Massive Stars
We discuss the basic physics of hot-star winds and we provide mass-loss rates
for (very) massive stars. Whilst the emphasis is on theoretical concepts and
line-force modelling, we also discuss the current state of observations and
empirical modelling, and address the issue of wind clumping.Comment: 36 pages, 15 figures, Book Chapter in "Very Massive Stars in the
Local Universe", Springer, Ed. Jorick S. Vin
3-D radiative transfer in clumped hot star winds I. Influence of clumping on the resonance line formation
The true mass-loss rates from massive stars are important for many branches
of astrophysics. For the correct modeling of the resonance lines, which are
among the key diagnostics of stellar mass-loss, the stellar wind clumping
turned out to be very important. In order to incorporate clumping into
radiative transfer calculation, 3-D models are required. Various properties of
the clumps may have strong impact on the resonance line formation and,
therefore, on the determination of empirical mass-loss rates. We incorporate
the 3-D nature of the stellar wind clumping into radiative transfer
calculations and investigate how different model parameters influence the
resonance line formation. We develop a full 3-D Monte Carlo radiative transfer
code for inhomogeneous expanding stellar winds. The number density of clumps
follows the mass conservation. For the first time, realistic 3-D models that
describe the dense as well as the tenuous wind components are used to model the
formation of resonance lines in a clumped stellar wind. At the same time,
non-monotonic velocity fields are accounted for. The 3-D density and velocity
wind inhomogeneities show very strong impact on the resonance line formation.
The different parameters describing the clumping and the velocity field results
in different line strengths and profiles. We present a set of representative
models for various sets of model parameters and investigate how the resonance
lines are affected. Our 3-D models show that the line opacity is reduced for
larger clump separation and for more shallow velocity gradients within the
clumps. Our new model demonstrates that to obtain empirically correct mass-loss
rates from the UV resonance lines, the wind clumping and its 3-D nature must be
taken into account.Comment: Astronomy and Astrophysics, accepted for publicatio
Wind Accretion and State Transitions in Cygnus X-1
We present the results of a spectroscopic monitoring program (from 1998 to
2002) of the H-alpha emission strength in HDE 226868, the optical counterpart
of the black hole binary, Cyg X-1. The H-alpha profiles consist of (1) a P
Cygni component associated with the wind of the supergiant, (2) emission
components that attain high velocity at the conjunctions and that probably form
in enhanced outflows both towards and away from the black hole, and (3) an
emission component that moves in anti-phase with the supergiant's motion. We
argue that the third component forms in accreted gas near the black hole, and
the radial velocity curve of the emission is consistent with a mass ratio of
M_X / M_opt = 0.36 +/- 0.05. We find that there is a general anti-correlation
between the H-alpha emission strength and X-ray flux in the sense that when the
H-alpha emission is strong (W_\lambda < -0.5 Angstroms) the X-ray flux is
weaker and the spectrum harder. On the other hand, there is no correlation
between H-alpha emission strength and X-ray flux when H-alpha is weak. During
the low/hard X-ray state, the strong wind is fast and the accretion rate is
relatively low, while in the high/soft state the weaker, highly ionized wind
attains only a moderate velocity and the accretion rate increases. We argue
that the X-ray transitions from the normal low/hard to the rare high/soft state
are triggered by episodes of decreased mass loss rate in the supergiant donor
star.Comment: 45 pages, 16 figures, ApJ, in pres
Hydrodynamic model atmospheres for WR stars: Self-consistent modeling of a WC star wind
We present the first non-LTE atmosphere models for WR stars that incorporate
a self-consistent solution of the hydrodynamic equations. The models account
for iron-group line-blanketing and clumping, and compute the hydrodynamic
structure of a radiatively driven wind consistently with the non-LTE radiation
transport in the co-moving frame. We construct a self-consistent wind model
that reproduces all observed properties of an early-type WC star (WC5). We find
that the WR-type mass-loss is initiated at high optical depth by the so-called
`Hot Iron Bump' opacities (Fe IX-XVI). The acceleration of the outer wind
regions is performed by iron-group ions of lower excitation in combination with
C and O. Consequently, the wind structure shows two acceleration regions, one
close to the hydrostatic wind base in the optically thick part of the
atmosphere, and another farther out in the wind. In addition to the radiative
acceleration, the `Iron Bump' opacities are responsible for an intense heating
of deep atmospheric layers. We find that the observed narrow OVI-emissions in
the optical spectra of WC stars originate from this region. By their dependence
on the clumping factor we gain important information about the location where
the density inhomogeneities in WR-winds start to develop.Comment: accepted by A&
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