555 research outputs found
The Physical Properties and Effective Temperature Scale of O-type Stars as a Function of Metallicity. III. More Results from the Magellanic Clouds
In order to better determine the physical properties of hot, massive stars as
a function of metallicity, we obtained very high SNR optical spectra of 26 O
and early B stars in the Magellanic Clouds. These allow accurate modeling even
in cases where the He I 4471 line has an equivalent width of only a few tens of
mA. The spectra were modeled with FASTWIND, with good fits obtained for 18
stars; the remainder show signatures of being binaries. We include stars in
common to recent studies to investigate possible systematic differences. The
"automatic" FASTWIND modeling method of Mokiem and collaborators produced
temperatures 1100 K hotter on the average, presumably due to the different
emphasis given to various temperature-sensitive lines. More significant,
however, is that the automatic method always produced some "best" answer, even
for stars we identify as composite (binaries). The temperatures found by the
TLUSTY/CMFGEN modeling of Bouret, Heap, and collaborators yielded temperatures
1000 K cooler than ours, on average. Significant outliers were due either to
real differences in the data (some of the Bouret/Heap data were contaminated by
moonlight continua) or the fact we could detect the HeI line needed to better
constrain the temperature. Our new data agrees well with the effective
temperature scale we presented previously. We confirm that the "Of"
emission-lines do not track luminosity classes in the exact same manner as in
Milky Way stars. We revisit the the issue of the "mass discrepancy", finding
that some of the stars in our sample do have spectroscopic masses that are
significantly smaller than those derived from stellar evolutionary models. We
do not find that the size of the mass discrepancy is simply related to either
effective temperature or surface gravity.Comment: ApJ, in pres
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The IACOB project. V. Spectroscopic parameters of the O-type stars in the modern grid of standards for spectral classification
The IACOB and OWN surveys are two ambitious complementary observational
projects which have made available a large multi-epoch spectroscopic database
of optical high resolution spectra of Galactic massive O-type stars. As a first
step in the study of the full sample of (more than 350) O stars surveyed by the
IACOB/OWN projects, we have performed the quantitative spectroscopic analysis
of a subsample of 128 stars included in the modern grid of O-type standards for
spectral classification. We use semi-automatized tools to determine the set of
spectroscopic parameters that can be obtained from the optical spectrum of
O-type stars. We also benefit from the multi-epoch character of the surveys to
perform a spectroscopic variability study of the sample, accounting for
spectroscopic binarity and variability of the main wind diagnostic lines. We
provide a general overview of the stellar and wind parameters of this reference
sample, and updated recipes for the SpT\,--\,Teff/log g calibrations for
Galactic O-type stars. We evaluate our semi-automatized analysis strategy with
40 stars from the literature, and find a good agreement. The agreement
between the synthetic spectra associated with fastwind best fitting models and
the observed spectra is good for most targets, but 46 stars present a
particular behavior of the wind diagnostic lines that cannot be reproduced by
our grid of spherically symmetric unclumped models. These are potential targets
of interest for more detailed investigations of clumpy winds and/or the
existence of additional circumstellar components. Last, our variability study
has led to the detection of signatures of spectroscopic binarity in 27\% of the
stars and small amplitude radial velocity variations in the photospheric lines
of another 30\%. Additionally, 31\% of the investigated stars show variability
in the wind diagnostic lines.Comment: 20 pages, 18 figures, accepted for publication in Astronomy &
Astrophysic
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
Dynamics of Line-Driven Winds from Disks in Cataclysmic Variables. II. Mass Loss Rates and Velocity Laws
We analyze the dynamics of 2D stationary line-driven winds from accretion
disks in cataclysmic variables (CVs), by generalizing the Castor, Abbott and
Klein theory. In paper 1, we have solved the wind Euler equation, derived its
two eigenvalues, and addressed the solution topology and wind geometry. Here,
we focus on mass loss and velocity laws. We find that disk winds, even in
luminous novalike variables, have low optical depth, even in the strongest
driving lines. This suggests that thick-to-thin transitions in these lines
occur. For disks with a realistic radial temperature, the mass loss is
dominated by gas emanating from the inner decade in r. The total mass loss rate
associated with a luminosity 10 Lsun is 10^{-12} Msun/yr, or 10^{-4} of the
mass accretion rate. This is one order of magnitude below the lower limit
obtained from P Cygni lines, when the ionizing flux shortwards of the Lyman
edge is supressed. The difficulties with such small mass loss rates in CVs are
principal, and confirm our previous work. We conjecture that this issue may be
resolved by detailed nonLTE calculations of the line force within the context
of CV disk winds, and/or better accounting for the disk energy distribution and
wind ionization structure. We find that the wind velocity profile is well
approximated by the empirical law used in kinematical modeling. The
acceleration length scale is given by the footpoint radius of the wind
streamline in the disk. This suggests an upper limit of 10 Rwd to the
acceleration scale, which is smaller by factors of a few as compared to values
derived from line fitting.Comment: 14 pages, 3 Postscript figures, also from
http://www.pa.uky.edu/~shlosman/publ.html. Astrophysical Journal, submitte
Broad Line Emission in Low-Metallicity Blue Compact Dwarf Galaxies: Evidence for Stellar Wind, Supernova and Possible AGN Activity
We present spectra of a large sample of low-metallicity blue compact dwarf
galaxies which exhibit broad components in their strong emission lines, mainly
in Hbeta, [O III]4959, 5007 and Halpha. Twenty-three spectra have been obtained
with the MMT, 14 of which show broad emission. The remaining 21 spectra with
broad emission have been selected from the Data Release 5 of the Sloan Digital
Sky Survey. The most plausible origin of broad line emission is the evolution
of massive stars and their interaction with the circumstellar and interstellar
medium. The broad emission with the lowest H luminosities (10^36 -
10^39 erg/s) is likely produced in circumstellar envelopes around hot Ofp/WN9
and/or LBV stars. The broad emission with the highest Halpha luminosities
(10^40 - 10^42 erg/s) probably arises from type IIp or type IIn supernovae
(SNe). It can also come from active galactic nuclei (AGN) containing
intermediate-mass black holes, although we find no strong evidence for hard
non-thermal radiation in our sample galaxies. The oxygen abundance in the host
galaxies with SN candidates is low and varies in the range 12 + log O/H = 7.36
- 8.31. However, type IIn SN / AGN candidates are found only in galaxies with
12 + log O/H < 7.99. Spectroscopic monitoring of these type IIn SN / AGN
candidates over a time scale of several years is necessary to distinguish
between the two possibilities.Comment: 50 pages, 6 figures. Accepted for publication in the Astrophysical
Journa
NLTE analysis of spectra: OBA stars
Methods of calculation of NLTE model atmosphere are discussed. The NLTE trace
element procedure is compared with the full NLTE model atmosphere calculation.
Differences between LTE and NLTE atmosphere modeling are evaluated. The ways of
model atom construction are discussed. Finally, modelling of expanding
atmospheres of hot stars with winds is briefly reviewed.Comment: in Determination of Atmospheric Parameters of B-, A-, F- and G-Type
Stars, E. Niemczura et al. eds., Springer, in pres
Towards an understanding of the Of?p star HD 191612: optical spectroscopy
We present extensive optical spectroscopy of the early-type magnetic star HD
191612 (O6.5f?pe-O8fp). The Balmer and HeI lines show strongly variable
emission which is highly reproducible on a well-determined 538-d period. Metal
lines and HeII absorptions (including many selective emission lines but
excluding He II 4686A emission) are essentially constant in line strength, but
are variable in velocity, establishing a double-lined binary orbit with P(orb)
= 1542d, e=0.45. We conduct a model-atmosphere analysis of the primary, and
find that the system is consistent with a O8: giant with a B1: main-sequence
secondary. Since the periodic 538-d changes are unrelated to orbital motion,
rotational modulation of a magnetically constrained plasma is strongly favoured
as the most likely underlying `clock'. An upper limit on the equatorial
rotation is consistent with this hypothesis, but is too weak to provide a
strong constraint.Comment: Accepted for MNRA
Runaway Massive Binaries and Cluster Ejection Scenarios
The production of runaway massive binaries offers key insights into the
evolution of close binary stars and open clusters. The stars HD 14633 and HD
15137 are rare examples of such runaway systems, and in this work we
investigate the mechanism by which they were ejected from their parent open
cluster, NGC 654. We discuss observational characteristics that can be used to
distinguish supernova ejected systems from those ejected by dynamical
interactions, and we present the results of a new radio pulsar search of these
systems as well as estimates of their predicted X-ray flux assuming that each
binary contains a compact object. Since neither pulsars nor X-ray emission are
observed in these systems, we cannot conclude that these binaries contain
compact companions. We also consider whether they may have been ejected by
dynamical interactions in the dense environment where they formed, and our
simulations of four-body interactions suggest that a dynamical origin is
possible but unlikely. We recommend further X-ray observations that will
conclusively identify whether HD 14633 or HD 15137 contain neutron stars.Comment: Accepted to ApJ, 11 page
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