33 research outputs found
Hydrogen-deficient central stars of planetary nebulae
Central stars of planetary nebulae are low-mass stars on the brink of their final evolution towards white dwarfs. Because of their surface temperature of above 25,000 K their UV radiation ionizes the surrounding material, which was ejected in an earlier phase of their evolution. Such fluorescent circumstellar gas is called a "Planetary Nebula". About one-tenth of the Galactic central stars are hydrogen-deficient. Generally, the surface of these central stars is a mixture of helium, carbon, and oxygen resulting from partial helium burning. Moreover, most of them have a strong stellar wind, similar to massive Pop-I Wolf-Rayet stars, and are in analogy classified as [WC]. The brackets distinguish the special type from the massive WC stars.thesi
The impact of rotation on the line profiles of Wolf-Rayet stars
Massive Wolf-Rayet stars are recognized today to be in a very common, but
short, evolutionary phase of massive stars. While our understanding of
Wolf-Rayet stars has increased dramatically over the past decades, it remains
unclear whether rapid rotators are among them. There are various indications
that rapidly rotating Wolf-Rayet stars should exist. Unfortunately, due to
their expanding atmospheres, rotational velocities of Wolf-Rayet stars are very
difficult to measure. However, recently observed spectra of several Wolf-Rayet
stars reveal peculiarly broad and round emission lines. Could these spectra
imply rapid rotation?
In this work, we model the effects of rotation on the atmospheres of
Wolf-Rayet stars. We further investigate whether the peculiar spectra of five
Wolf-Rayet stars may be explained with the help of stellar rotation, infer
appropriate rotation parameters, and discuss the implications of our results.
We make use of the Potsdam Wolf-Rayet (PoWR) non-LTE model atmosphere code.
Since the observed spectra of Wolf-Rayet stars are mainly formed in their
expanding atmospheres, rotation must be accounted for with a 3D integration
scheme of the formal integral. For this purpose, we assume a rotational
velocity field consisting of an inner co-rotating domain and an outer domain,
where the angular momentum is conserved. We find that rotation can reproduce
the unique spectra analyzed here. However, the inferred rotational velocities
at the stellar surface are large (~200 km/s), and the inferred co-rotation
radii (~10 stellar radii) suggest the existence of very strong photospheric
magnetic fields (~20 kG)
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
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
On the Weak-Wind Problem in Massive Stars: X-ray Spectra Reveal a Massive Hot Wind in \mu\ Columbae
\mu\ Columbae is a prototypical weak-wind O-star for which we have obtained a
high-resolution X-ray spectrum with the Chandra LETG/ACIS-S instrument and a
low resolution spectrum with Suzaku. This allows us, for the first time, to
investigate the role of X-rays on the wind structure in a bona fide weak-wind
system and to determine whether there actually is a massive, hot wind. The
X-ray emission measure indicates that the outflow is an order of magnitude
greater than that derived from UV lines and is commensurate with the nominal
wind-luminosity relationship for O-stars. Therefore, the ``weak-wind
problem''---identified from cool wind UV/optical spectra---is largely resolved
by accounting for the hot wind seen in X-rays. From X-ray line profiles,
Doppler shifts, and relative strengths, we find that this weak-wind star is
typical of other late O dwarfs. The X-ray spectra do not suggest a magnetically
confined plasma---the spectrum is soft and lines are broadened; Suzaku spectra
confirm the lack of emission above 2 keV. Nor do the relative line shifts and
widths suggest any wind decoupling by ions. The He-like triplets indicate that
the bulk of the X-ray emission is formed rather close to the star, within 5
stellar radii. Our results challenge the idea that some OB stars are
``weak-wind'' stars that deviate from the standard wind-luminosity
relationship. The wind is not weak, but it is hot and its bulk is only
detectable in X-rays.Comment: Accepted for publication in ApJ Letter
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
X-rays observations of a super-Chandrasekhar object reveal an ONeMg and a CO white dwarf merger product embedded in a putative SN Iax remnant
The merger of two white dwarfs (WD) is a natural outcome from the evolution
of many binary stars. Recently, a WD merger product, IRAS 00500+6713, was
identified. IRAS 00500+6713 consists of a central star embedded in a circular
nebula. The analysis of the optical spectrum of the central star revealed that
it is hot, hydrogen and helium free, and drives an extremely fast wind with a
record breaking speed. The nebula is visible in infrared and in the [O III]
line images. No nebula spectroscopy was obtained prior to our observations.
Here we report the first deep X-ray imaging spectroscopic observations of IRAS
00500+6713. Both the central star and the nebula are detected in X-rays,
heralding the WD merger products as a new distinct type of strong X-ray
sources. Low-resolution X-ray spectra reveal large neon, magnesium, silicon,
and sulfur enrichment of the central star and the nebula. We conclude that IRAS
00500+6713 resulted from a merger of an ONe and a CO WD, which supports earlier
suggestion for a super-Chandrasekhar mass of this object. X-ray analysis
indicates that the merger was associated with an episode of carbon burning and
possibly accompanied by a SN Iax. In X-rays, we observe the point source
associated with the merger product while the surrounding diffuse nebula is a
supernova remnant. IRAS 00500+6713 will likely terminate its evolution with
another peculiar Type I supernova, where the final core collapse to a neutron
star might be induced by electron captures.Comment: accepted by A&A Letters, 9 pages including appendi
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
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