3,426 research outputs found
The metallicity dependence of WR winds
Wolf-Rayet (WR) stars are the most advanced stage in the evolution of the
most massive stars. The strong feedback provided by these objects and their
subsequent supernova (SN) explosions are decisive for a variety of
astrophysical topics such as the cosmic matter cycle. Consequently,
understanding the properties of WR stars and their evolution is indispensable.
A crucial but still not well known quantity determining the evolution of WR
stars is their mass-loss rate. Since the mass loss is predicted to increase
with metallicity, the feedback provided by these objects and their spectral
appearance are expected to be a function of the metal content of their host
galaxy. This has severe implications for the role of massive stars in general
and the exploration of low metallicity environments in particular. Hitherto,
the metallicity dependence of WR star winds was not well studied. In this
contribution, we review the results from our comprehensive spectral analyses of
WR stars in environments of different metallicities, ranging from slightly
super-solar to SMC-like metallicities. Based on these studies, we derived
empirical relations for the dependence of the WN mass-loss rates on the
metallicity and iron abundance, respectively.Comment: 5 pages, 4 figures, to be published in the Proceedings of the IAU
Symposium No. 329 "The lives and death-throes of massive stars
Simultaneous brachial diplegia and rotational vertigo due to combined spinal anterior and vertebrobasilar embolism
Wolf-Rayet stars in the Small Magellanic Cloud: I. Analysis of the single WN stars
Wolf-Rayet (WR) stars have a severe impact on their environments owing to
their strong ionizing radiation fields and powerful stellar winds. Since these
winds are considered to be driven by radiation pressure, it is theoretically
expected that the degree of the wind mass-loss depends on the initial
metallicity of WR stars. Following our comprehensive studies of WR stars in the
Milky Way, M31, and the LMC, we derive stellar parameters and mass-loss rates
for all seven putatively single WN stars known in the SMC. Based on these data,
we discuss the impact of a low-metallicity environment on the mass loss and
evolution of WR stars. The quantitative analysis of the WN stars is performed
with the Potsdam Wolf-Rayet (PoWR) model atmosphere code. The physical
properties of our program stars are obtained from fitting synthetic spectra to
multi-band observations. In all SMC WN stars, a considerable surface hydrogen
abundance is detectable. The majority of these objects have stellar
temperatures exceeding 75 kK, while their luminosities range from 10^5.5 to
10^6.1 Lsun. The WN stars in the SMC exhibit on average lower mass-loss rates
and weaker winds than their counterparts in the Milky Way, M31, and the LMC. By
comparing the mass-loss rates derived for WN stars in different Local Group
galaxies, we conclude that a clear dependence of the wind mass-loss on the
initial metallicity is evident, supporting the current paradigm that WR winds
are driven by radiation. A metallicity effect on the evolution of massive stars
is obvious from the HRD positions of the SMC WN stars at high temperatures and
high luminosities. Standard evolution tracks are not able to reproduce these
parameters and the observed surface hydrogen abundances. Homogeneous evolution
might provide a better explanation for their evolutionary past.Comment: 18+12 pages; 22+8 figures; accepted for publication in A&
The rapid evolution of the exciting star of the Stingray Nebula
SAO244567, the exciting star of the Stingray nebula, is rapidly evolving.
Previous analyses suggested that it has heated up from an effective temperature
of about 21kK in 1971 to over 50kK in the 1990s. Canonical post-asymptotic
giant branch evolution suggests a relatively high mass while previous analyses
indicate a low-mass star. Fitting line profiles from static and expanding
non-LTE model atmospheres to the observed UV and optical spectra, taken during
1988-2013, allowed us to study the temporal change of effective temperature,
surface gravity, mass-loss rate, and terminal wind velocity. In addition, we
determined the chemical composition of the atmosphere. We find that the central
star has steadily increased its effective temperature from 38kK in 1988 to a
peak value of 60kK in 2002. During the same time, the star was contracting, as
concluded from an increase in surface gravity from log g = 4.8 to 6.0 and a
drop in luminosity. Simultaneously, the mass-loss rate declined from log
(dM/dt/Msun/yr)=-9.0 to -11.6 and the terminal wind velocity increased from
1800km/s to 2800km/s. Since around 2002, the star stopped heating and has
cooled down again to 55kK by 2006. It has a largely solar surface composition
with the exception of slightly subsolar carbon, phosphorus, and sulfur. By
comparison with stellar-evolution calculations, we confirm that SAO244567 must
be a low-mass star (M < 0.55 Msun). However, the slow evolution of the
respective stellar evolutionary models is in strong contrast to the observed
fast evolution and the young planetary nebula with a kinematical age of only
about 1000 years. We speculate that the star could be a late He-shell flash
object. Alternatively, it could be the outcome of close-binary evolution. Then
SAO244567 would be a low-mass (0.354 Msun) helium prewhite dwarf after the
common-envelope phase, during which the planetary nebula was ejected.Comment: 16 pages, 13 figures, accepted for publication in A&
Defect complexes formed with Ag atoms in CDTE, ZnTe, and ZnSe
Using the radioactive acceptor Ag for perturbed --angular correlation (PAC) spectroscopy for the first time, defect complexes formed with Ag are investigated in the II-VI semiconductors CdTe, ZnTe and ZnSe. The donors In, Br and the Te-vacancy were found to passivate Ag acceptors in CdTe via pair formation, which was also observed in In-doped ZnTe. In undoped or Sb-doped CdTe and in undoped ZnSe, the PAC experiments indicate the compensation of Ag acceptors by the formation of double broken bond centres, which are characterised by an electric field gradient with an asymmetry parameter close to h = 1. Additionally, a very large electric field gradient was observed in CdTe, which is possibly connected with residual impurities
The Metallicity of the Redshift 4.16 Quasar BR2248-1242
We estimate the metallicity in the broad emission-line region of the redshift
z=4.16 quasar, BR2248-1242, by comparing line ratios involving nitrogen to
theoretical predictions. BR2248-1242 has unusually narrow emission lines with
large equivalent widths, thus providing a rare opportunity to measure several
line-ratio abundance diagnostics. The combined diagnostics indicate a
metallicity of ~2 times solar. This result suggests that an episode of vigorous
star formation occurred near BR2248-1242 prior to the observed z=4.16 epoch.
The time available for this enrichment episode is only ~1.5 Gyr at z=4.16 (for
H_{0}=65 km s^-1 Mpc^-1, Omega_{m}=0.3 and Omega_Lambda ~< 1). This evidence
for high metallicities and rapid star formation is consistent with the expected
early-epoch evolution of dense galactic nuclei.Comment: 8 pages, 3 figures. Prepared in AAStex. Submitted to the
Astrophysical Journal Revised version: added 1 referenc
Stellar population of the superbubble N206 in the LMC I. Analysis of the Of-type stars
Massive stars are the key agents of feedback. Consequently, quantitative
analysis of massive stars are required to understand how the feedback of these
objects shapes/ creates the large scale structures of the ISM. The giant HII
region N206 in the Large Magellanic Cloud contains an OB association that
powers a X-ray superbubble, serving as an ideal laboratory in this context. We
obtained optical spectra with the muti-object spectrograph FLAMES at the
ESO-VLT. When possible, the optical spectroscopy was complemented by UV spectra
from the HST, IUE, and FUSE archives. Detailed spectral classifications are
presented for our sample Of-type stars. For the quantitative spectroscopic
analysis we use the Potsdam Wolf-Rayet (PoWR) model atmosphere code. The
physical parameters and nitrogen abundances of our sample stars are determined
by fitting synthetic spectra to the observations. The stellar and wind
parameters of nine Of-type stars are used to construct wind momentum,luminosity
relationship. We find that our sample follows a relation close to the
theoretical prediction, assuming clumped winds. The most massive star in the
N206 association is an Of supergiant which has a very high mass-loss rate. Two
objects in our sample reveal composite spectra, showing that the Of primaries
have companions of late O subtype. All stars in our sample have an evolutionary
age less than 4 million years, with the O2-type star being the youngest. All
these stars show a systematic discrepancy between evolutionary and
spectroscopic masses. All stars in our sample are nitrogen enriched. Nitrogen
enrichment shows a clear correlation with increasing projected rotational
velocities. The mechanical energy input from the Of stars alone is comparable
to the energy stored in the N206 superbubble as measured from the observed
X-ray and H alpha emission.Comment: Accepted for the pubblication in Astronomy & Astrophysic
Observational properties of massive black hole binary progenitors
The first directly detected gravitational waves (GW 150914) were emitted by
two coalescing black holes (BHs) with masses of ~36Msun and ~29Msun. Several
scenarios have been proposed to put this detection into an astrophysical
context. The evolution of an isolated massive binary system is among commonly
considered models. Various groups have performed detailed binary-evolution
calculations that lead to BH merger events. However, the question remains open
as to whether binary systems with the predicted properties really exist. The
aim of this paper is to help observers to close this gap by providing spectral
characteristics of massive binary BH progenitors during a phase where at least
one of the companions is still non-degenerate. Stellar evolution models predict
fundamental stellar parameters. Using these as input for our stellar atmosphere
code (PoWR), we compute a set of models for selected evolutionary stages of
massive merging BH progenitors at different metallicities. The synthetic
spectra obtained from our atmosphere calculations reveal that progenitors of
massive BH merger events start their lives as O2-3V stars that evolve to
early-type blue supergiants before they undergo core-collapse during the
Wolf-Rayet phase. When the primary has collapsed, the remaining system will
appear as a wind-fed high-mass X-ray binary. We provide feedback parameters,
broad band magnitudes, and spectral templates that should help to identify such
binaries in the future. Comparisons of empirically determined mass-loss rates
with those assumed by evolution calculations reveal significant differences.
The consideration of the empirical mass-loss rates in evolution calculations
will possibly entail a shift of the maximum in the predicted binary-BH merger
rate to higher metallicities, that is, more candidates should be expected in
our cosmic neighborhood than previously assumed.Comment: 64 pages, 30 figures, accepted for publication in Astronomy &
Astrophysics, v2: typos correcte
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