46 research outputs found
The Wolf-Rayet stars in M31: I. Analysis of the late-type WN stars
Context: Comprehensive studies of Wolf-Rayet stars were performed in the past
for the Galactic and the LMC population. The results revealed significant
differences, but also unexpected similarities between the WR populations of
these different galaxies. Analyzing the WR stars in M31 will extend our
understanding of these objects in different galactic environments. Aims: The
present study aims at the late-type WN stars in M31. The stellar and wind
parameters will tell about the formation of WR stars in other galaxies with
different metallicity and star formation histories. The obtained parameters
will provide constraints to the evolution of massive stars in the environment
of M31. Methods: We used the latest version of the Potsdam Wolf-Rayet model
atmosphere code to analyze the stars via fitting optical spectra and
photometric data. To account for the relatively low temperatures of the late
WN10 and WN11 subtypes, our WN models have been extended into this temperature
regime. Results: Stellar and atmospheric parameters are derived for all known
late-type WN stars in M31 with available spectra. All of these stars still have
hydrogen in their outer envelopes, some of them up to 50% by mass. The stars
are located on the cool side of the zero age main sequence in the
Hertzsprung-Russell diagram, while their luminosities range from to
Lsun. It is remarkable that no star exceeds Lsun. Conclusions: If
formed via single-star evolution, the late-type WN stars in M31 stem from an
initial mass range between 20 and 60 Msun. From the very late-type WN9-11
stars, only one star is located in the S Doradus instability strip. We do not
find any late-type WN stars with the high luminosities known in the Milky Way.Comment: 11+11 pages, 13+18 figures, A&A, in pres
Coupling hydrodynamics with comoving frame radiative transfer: II. Stellar wind stratification in the high-mass X-ray binary Vela X-1
CONTEXT: Vela X-1, a prototypical high mass X-ray binary (HMXB), hosts a
neutron star (NS) in a close orbit around an early-B supergiant donor star.
Accretion of the donor star's wind onto the NS powers its strong X-ray
luminosity. To understand the physics of HMXBs, detailed knowledge about the
donor star winds is required. AIMS: To gain a realistic picture of the donor
star in Vela X-1, we constructed a hydrodynamically consistent atmosphere model
describing the wind stratification while properly reproducing the observed
donor spectrum. To investigate how X-ray illumination affects the stellar wind,
we calculated additional models for different X-ray luminosity regimes.
METHODS: We use the recently updated version of the PoWR code to consistently
solve the hydrodynamic equation together with the statistical equations and the
radiative transfer. RESULTS: The wind flow in Vela X-1 is driven by ions from
various elements with Fe III and S III leading in the outer wind. The
model-predicted mass-loss rate is in line with earlier empirical studies. The
mass-loss rate is almost unaffected by the presence of the accreting NS in the
wind. The terminal wind velocity is confirmed at km/s.
On the other hand, the wind velocity in the inner region where the NS is
located is only km/s, which is not expected on the basis of a
standard -velocity law. In models with an enhanced level of X-rays, the
velocity field in the outer wind can be altered. If the X-ray flux is too high,
the acceleration breaks down because the ionization increases. CONCLUSIONS:
Accounting for radiation hydrodynamics, our Vela X-1 donor atmosphere model
reveals a low wind speed at the NS location, and it provides quantitative
information on wind driving in this important HMXB.Comment: 19 pages, 10 figures, accepted for publication in Astronomy &
Astrophysic
On the consistent treatment of the quasi-hydrostatic layers in hot star atmospheres
Context. Spectroscopic analysis remains the most common method to derive masses of massive stars, the most fundamental stellar parameter. While binary orbits and stellar pulsations can provide much sharper constraints on the stellar mass, these methods are only rarely applicable to massive stars. Unfortunately, spectroscopic masses of massive stars heavily depend on the detailed physics of model atmospheres. Aims. We demonstrate the impact of a consistent treatment of the radiative pressure on inferred gravities and spectroscopic masses of massive stars. Specifically, we investigate the contribution of line and continuum transitions to the photospheric radiative pressure. We further explore the effect of model parameters, e.g., abundances, on the deduced spectroscopic mass. Lastly, we compare our results with the plane-parallel TLUSTY code, commonly used for the analysis of massive stars with photospheric spectra. Methods. We calculate a small set of O-star models with the Potsdam Wolf-Rayet (PoWR) code using different approaches for the quasi-hydrostatic part. These models allow us to quantify the effect of accounting for the radiative pressure consistently. We further use PoWR models to show how the Doppler widths of line profiles and abundances of elements such as iron affect the radiative pressure, and, as a consequence, the derived spectroscopic masses. Results. Our study implies that errors on the order of a factor of two in the inferred spectroscopic mass are to be expected when neglecting the contribution of line and continuum transitions to the radiative acceleration in the photosphere. Usage of implausible microturbulent velocities, or the neglect of important opacity sources such as Fe, may result in errors of approximately 50% in the spectroscopic mass. A comparison with TLUSTY model atmospheres reveals a very good agreement with PoWR at the limit of low mass-loss rates.The first author of this work (A.S.) is supported by the Deutsche Forschungsgemeinschaft (DFG) under grant HA 1455/22. T.S. is grateful for financial support from the Leibniz Graduate School for Quantitative Spectroscopy in Astrophysics, a joint project of the Leibniz Institute for Astrophysics Potsdam (AIP) and the Institute of Physics and Astronomy of the University of Potsdam. A.S. would like to thank the Aspen Center for Physics and the NSF Grant #1066293 for hospitality during the invention and writing of this paper
A new type of X-ray pulsar
X-ray emission from stars much more massive than the Sun was discovered only
35 years ago. Such stars drive fast stellar winds where shocks can develop, and
it is commonly assumed that the X-rays emerge from the shock-heated plasma.
Many massive stars additionally pulsate. However, hitherto it was neither
theoretically predicted nor observed that these pulsations would affect their
X-ray emission. Here we report the discovery of pulsating X-rays from the
massive B-type star Xi1 Canis Majoris. This star is a variable of beta Cephei
type and has a strong magnetic field. Our observations with the XMM-Newton
telescope reveal X-ray pulsations with the same period as the fundamental
stellar pulsation. This discovery challenges our understanding of stellar winds
from massive stars, their X-ray emission, and their magnetism.Comment: manuscript draft. The revised paper is published in Nature
Communication
The donor star of the X-ray pulsar X1908+075
High-mass X-ray binaries (HMXBs) consist of a massive donor star and a
compact object. While several of those systems have been well studied in
X-rays, little is known for most of the donor stars as they are often heavily
obscured in the optical and ultraviolet regime. There is an opportunity to
observe them at infrared wavelengths, however. The goal of this study is to
obtain the stellar and wind parameters of the donor star in the X1908+075
high-mass X-ray binary system with a stellar atmosphere model to check whether
previous studies from X-ray observations and spectral morphology lead to a
sufficient description of the donor star. We obtained H- and K-Band spectra of
X1908+075 and analysed them with the Potsdam Wolf-Rayet (PoWR) model atmosphere
code. For the first time, we calculated a stellar atmosphere model for the
donor star, whose main parameters are: = , kK, and = . The obtained parameters point
towards an early B-type (B0--B3) star, probably in a supergiant phase. Moreover
we determined a more accurate distance to the system of 4.85 0.50 kpc
than the previously reported value.Comment: Accepted for publication in Section 7. Stellar structure and
evolution of Astronomy and Astrophysics. The official date of acceptance is
21/04/201
The Galactic WC and WO stars: The impact of revised distances from Gaia DR2 and their role as massive black hole progenitors
Wolf-Rayet stars of the carbon sequence (WC stars) are an important
cornerstone in the late evolution of massive stars before their core collapse.
As core-helium burning, hydrogen-free objects with huge mass-loss, they are
likely the last observable stage before collapse and thus promising progenitor
candidates for type Ib/c supernovae. Their strong mass-loss furthermore
provides challenges and constraints to the theory of radiatively driven winds.
Thus, the determination of the WC star parameters is of major importance for
several astrophysical fields. With Gaia DR2, for the first time parallaxes for
a large sample of Galactic WC stars are available, removing major uncertainties
inherent to earlier studies. In this work, we re-examine the sample from Sander
et al. (2012) to derive key properties of the Galactic WC population. All
quantities depending on the distance are updated, while the underlying spectral
analyses remain untouched. Contrasting earlier assumptions, our study yields
that WC stars of the same subtype can significantly vary in absolute magnitude.
With Gaia DR2, the picture of the Galactic WC population becomes more complex:
We obtain luminosities ranging from log L = 4.9 to 6.0 with one outlier having
log L = 4.7. This indicates that the WC stars are likely formed from a broader
initial mass range than previously assumed. We obtain mass-loss rates ranging
between log Mdot = -5.1 and -4.1, with Mdot propto L^0.68 and a linear scaling
of the modified wind momentum with luminosity. We discuss the implications for
stellar evolution, including unsolved issues regarding the need of envelope
inflation to address the WR radius problem, and the open questions in regard to
the connection of WR stars with Gamma-ray bursts. WC and WO stars are
progenitors of massive black holes, collapsing either silently or in a
supernova that most-likely has to be preceded by a WO stage.Comment: 19 pages, 13 figures, 6 tables; A&A, v2: version in pres