155 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
Investigating the lack of main-sequence companions to massive Be stars
About 20% of all B-type stars are classical Be stars. The Be phenomenon is
strongly correlated with rapid rotation, the origin of which remains unclear.
It may be rooted in single- or binary-star evolution. In the framework of the
binary channel, the initially more massive star transfers mass and angular
momentum to the original secondary, which becomes a Be star. The system then
evolves into a Be binary with a post-main-sequence companion, which may later
be disrupted in a supernova event. Hence, if the binary channel dominates the
formation of Be stars, one may expect a strong lack of close Be binaries with
main sequence (MS) companions. Through an extensive, star-by-star review of the
literature of a magnitude-limited sample of Galactic early-type Be stars, we
investigate whether Be binaries with MS companions are known to exist. Our
sample is constructed from the BeSS database and cross-matched with all
available literature on the individual stars. Out of an initial list of 505 Be
stars, we compile a final sample of 287 Galactic Be stars earlier than B1.5
with V<=12 mag. Out of those, 13 objects were reported as Be binaries with
known post-MS companions and 11 as binaries with unknown, uncertain or debated
companions. We find no confirmed reports of Be binaries with MS companions. For
the remaining 263 targets, no significant reports of multiplicity exist in the
literature, implying that they are either Be binaries with faint companions, or
truly single. The clear lack of reported MS companions to Be stars, which
stands in contrast to the high number of detected B+B MS binaries, strongly
supports the hypothesis that early-type Be stars are binary interaction
products that spun up after mass and angular momentum transfer from a companion
star. Taken at face value, our results may suggest that a large majority of the
early-type Be stars have formed through binary mass-transfer.Comment: 15 pages (incl. appendix), 6 figures, 3 tables, accepted for
publication in A&
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&
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
Low-metallicity massive single stars with rotation. II. Predicting spectra and spectral classes of chemically-homogeneously evolving stars
Context. Metal-poor massive stars are supposed to be progenitors of certain
supernovae, gamma-ray bursts and compact object mergers, potentially
contributing to the early epochs of the Universe with their strong ionizing
radiation. However, they remain mainly theoretical as individual spectroscopic
observations of such objects have rarely been carried out below the metallicity
of the SMC.
Aims. This work aims at exploring what our state-of-the-art theories of
stellar evolution combined with those of stellar atmospheres predict about a
certain type of metal-poor (0.02 Z) hot massive stars, the chemically
homogeneously evolving ones, called TWUIN stars.
Methods. Synthetic spectra corresponding to a broad range in masses (20-130
M) and covering several evolutionary phases from the zero-age
main-sequence up to the core helium-burning stage were computed.
Results. We find that TWUIN stars show almost no emission lines during most
of their {core hydrogen-burning} lifetimes. Most metal lines are completely
absent, including nitrogen. During their core helium-burning stage, lines
switch to emission and even some metal lines (oxygen and carbon, but still
almost no nitrogen) show up. Mass loss and clumping play a significant role in
line-formation in later evolutionary phases, particularly during core
helium-burning. Most of our spectra are classified as an early O type giant or
supergiant, and we find Wolf-Rayet stars of type WO in the core helium-burning
phase.
Conclusions. An extremely hot, early O type star observed in a
low-metallicity galaxy could be the outcome of chemically homogeneous evolution
and therefore the progenitor of a long-duration gamma-ray burst or a type
Ic supernova. TWUIN stars may play an important role in reionizing the Universe
due to their being hot without showing prominent emission lines during the
majority of their lifetimes.Comment: Accepted by Astronomy and Astrophysics. In Pres
The role of stellar expansion on the formation of gravitational wave sources
Massive stars are the progenitors of black holes and neutron stars, the
mergers of which can be detected with gravitational waves (GW). The expansion
of massive stars is one of the key factors affecting their evolution in close
binary systems, but it remains subject to large uncertainties in stellar
astrophysics. For population studies and predictions of GW sources, the stellar
expansion is often simulated with the analytic formulae from Hurley et al.
(2000). These formulae need to be extrapolated for stars beyond 50 solar masses
and are often considered outdated. In this work we present five different
prescriptions developed from 1D stellar models to constrain the maximum
expansion of massive stars. We adopt these prescriptions to investigate how
stellar expansion affects mass transfer interactions and in turn the formation
of GW sources. We show that limiting radial expansion with updated 1D stellar
models, when compared to the use of Hurley et al. (2000) radial expansion
formulae, does not significantly affect GW source properties (rates and
masses). This is because most mass transfer events leading to GW sources are
initialised before the donor star reaches its maximum expansion. The only
significant difference was found for the mass distribution of massive binary
black hole mergers (total mass > 50 solar masses) formed from stars that may
evolve beyond the Humphreys-Davidson limit, whose radial expansion is the most
uncertain. We conclude that understanding the expansion of massive stars and
the origin of the Humphrey-Davidson limit is a key factor for the study of GW
sources.Comment: Accepted for publication in MNRA
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
A combined HST and XMM-Newton campaign for the magnetic O9.7 V star HD 54879: towards constraining the weak-wind problem of massive stars
Context: HD 54879 (O9.7 V) is one of a dozen O-stars for which an organized
atmospheric magnetic field has been detected. To gain insights into the
interplay between atmospheres, winds, and magnetic fields of massive stars, we
acquired UV and X-ray data of HD 54879 using the Hubble Space Telescope and the
XMM-Newton satellite. In addition, 35 optical amateur spectra were secured to
study the variability of HD 54879. A multiwavelength (X-ray to optical)
spectral analysis is performed using the Potsdam Wolf-Rayet (PoWR) model
atmosphere code and the xspec software.
Results: The photospheric parameters are typical for an O9.7 V star. The
microturbulent, macroturbulent, and projected rotational velocities are lower
than previously suggested (<4 km/s). An initial mass of 16 and an
age of 5 Myr are inferred from evolutionary tracks. We derive a mean X-ray
emitting temperature of [K] and an X-ray luminosity of
[erg/s]. Short- and long-scale variability is seen in
the H-alpha line, but only a very long period of yr could be
estimated. Assessing the circumstellar density of HD 54879 using UV spectra, we
can roughly estimate the mass-loss rate HD 54879 would have in the absence of a
magnetic field as . The
magnetic field traps the stellar wind up to the Alfv\'en radius >
, implying that its true mass-loss rate is . Hence, density enhancements around magnetic stars
can be exploited to estimate mass-loss rates of non-magnetic stars of similar
spectral types, essential for resolving the weak wind problem.
Conclusions: Our study confirms that strongly magnetized stars lose little or
no mass, and supplies important constraints on the weak-wind problem of massive
main sequence stars.Comment: Accepted for publication in A&A on Aug. 9, 2017, 12 + 1 pages, 15
figures. Paper replaced due to typos and missing acknowledgment
Probing Wolf-Rayet Winds: Chandra/HETG X-Ray Spectra of WR 6
With a deep Chandra/HETGS exposure of WR 6, we have resolved emission lines
whose profiles show that the X-rays originate from a uniformly expanding
spherical wind of high X-ray-continuum optical depth. The presence of strong
helium-like forbidden lines places the source of X-ray emission at tens to
hundreds of stellar radii from the photosphere. Variability was present in
X-rays and simultaneous optical photometry, but neither were correlated with
the known period of the system or with each other. An enhanced abundance of
sodium revealed nuclear processed material, a quantity related to the
evolutionary state of the star. The characterization of the extent and nature
of the hot plasma in WR 6 will help to pave the way to a more fundamental
theoretical understanding of the winds and evolution of massive stars.Comment: Accepted by the Astrophysical Journa
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