733 research outputs found
Atmospheres and Winds of PN Central Stars
The progress over the last years in modelling the atmospheres and winds of PN
central stars is reviewed. We discuss the effect of the inclusion of the
blanketing by millions of metal lines in NLTE on the diagnostics of
photospheric and stellar wind lines, which can be used to determine stellar
parameters such as effective temperature, gravity, radius, mass loss rate and
distance. We also refer to recent work on the winds of massive O-type stars,
which indicates that their winds are possibly inhomogeneous and clumped. We
investigate implications from this work on the spectral diagnostics of PN
central stars and introduce a method to determine wind clumping factors from
the relative strengths of Halpha and HeII 4686. Based on new results we discuss
the wind properties of CSPN.Comment: 8 pages, 12 figures; Proceedings, IAU Symposium No. 234, 2006,
"Planetary Nebulae in our Galaxy and Beyond", M.J. Barlow and R.H. Mendez,
ed
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
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
An Analysis of STIS HST UV Spectra of M33 early B Supergiants
We present terminal velocities of M33 B-supergiants, obtained from STIS HST
spectra as part of our programme to investigate the Wind Momentum -- Luminosity
Relationship (WLR) in the Local Group. Terminal velocities are derived from
their NV, CIV, and SiIV resonance lines in UV spectra. Comparing with IUE
spectra of Galactic B-supergiants we found evidence of low metallicity in three
of our objects. %%The terminal velocities are found to be consistent with the
%%values found for Galactic stars, except for B-133. For this The terminal
velocities are consistent with the corresponding values of Galactic stars,
except for B-133. For this star we find a very large vterm and a red SiIV
component deeper than the blue one, that might be an indication of binarity.
The average ratio between terminal and turbulent wind velocities is 0.25, well
above the value found for Galactic stars.Comment: 9 pages, 6 figure
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