346 research outputs found
A propelling neutron star in the enigmatic Be-star ~Cassiopeia
The enigmatic X-ray emission from the bright optical star,
Cassiopeia, is a long-standing problem. Cas is known to be a binary
system consisting of a Be-type star and a low-mass ()
companion of unknown nature orbiting in the Be-disk plane. Here we apply the
quasi-spherical accretion theory onto a compact magnetized star and show that
if the low-mass companion of Cas is a fast spinning neutron star, the
key observational signatures of Cas are remarkably well reproduced.
Direct accretion onto this fast rotating neutron star is impeded by the
propeller mechanism. In this case, around the neutron star magnetosphere a hot
shell is formed that emits thermal X-rays in qualitative and quantitative
agreement with observed properties of the X-ray emission from Cas. We
suggest that Cas and its analogs constitute a new subclass of Be-type
X-ray binaries hosting rapidly rotating neutron stars formed in supernova
explosions with small kicks. The subsequent evolutionary stage of Cas
and its analogs should be the X Per-type binaries comprising low-luminosity
slowly rotating X-ray pulsars. The model explains the enigmatic X-ray emission
from Cas, and also establishes evolutionary connections between
various types of rotating magnetized neutron stars in Be-binaries.Comment: 6 pages, accepted for publication in MNRA
Neglecting the porosity of hot-star winds can lead to underestimating mass-loss rates
Context: The mass-loss rate is a key parameter of massive stars. Adequate
stellar atmosphere models are required for spectral analyses and mass-loss
determinations. Present models can only account for the inhomogeneity of
stellar winds in the approximation of small-scale structures that are optically
thin. This treatment of ``microclumping'' has led to reducing empirical
mass-loss rates by factors of two and more. Aims: Stellar wind clumps can be
optically thick in spectral lines. We investigate how this ``macroclumping''
impacts on empirical mass-loss rates. Methods: The Potsdam Wolf-Rayet (PoWR)
model atmosphere code is generalized in the ``formal integral'' to account for
clumps that are not necessarily optically thin. Results: Optically thick clumps
reduce the effective opacity. This has a pronounced effect on the emergent
spectrum. Our modeling for the O-type supergiant zeta Puppis reveals that the
optically thin H-alpha line is not affected by wind porosity, but that the PV
resonance doublet becomes significantly weaker when macroclumping is taken into
account. The reported discrepancies between resonance-line and
recombination-line diagnostics can be resolved entirely with the macroclumping
modeling without downward revision of the mass-loss rate. Conclusions:
Mass-loss rates inferred from optically thin emission, such as the H-alpha line
in O stars, are not influenced by macroclumping. The strength of optically
thick lines, however, is reduced because of the porosity effects. Therefore,
neglecting the porosity in stellar wind modeling can lead to underestimating
empirical mass-loss rates.Comment: A&A (in press), see full abstract in the tex
On the Absence of Non-thermal X-ray emission around Runaway O stars
Theoretical models predict that the compressed interstellar medium around
runaway O stars can produce high-energy non-thermal diffuse emission, in
particular, non-thermal X-ray and -ray emission. So far, detection of
non-thermal X-ray emission was claimed for only one runaway star AE Aur. We
present a search for non-thermal diffuse X-ray emission from bow shocks using
archived XMM-Newton observations for a clean sample of 6 well-determined
runaway O stars. We find that none of these objects present diffuse X-ray
emission associated to their bow shocks, similarly to previous X-ray studies
toward Oph and BD433654. We carefully investigated
multi-wavelength observations of AE Aur and could not confirm previous findings
of non-thermal X-rays. We conclude that so far there is no clear evidence of
non-thermal extended emission in bow shocks around runaway O stars.Comment: 6 pages, 2 tables, 3 figures; Accepted to ApJ Letter
High resolution X-ray spectroscopy of bright O type stars
Archival X-ray spectra of the four prominent single, non-magnetic O stars
Zeta Pup, Zeta Ori, Ksi Per and Zeta Oph, obtained in high resolution with
Chandra HETGS/MEG have been studied. The resolved X-ray emission line profiles
provide information about the shocked, hot gas which emits the X-radiation, and
about the bulk of comparably cool stellar wind material which partly absorbs
this radiation. In this paper, we synthesize X-ray line profiles with a model
of a clumpy stellar wind. We find that the geometrical shape of the wind
inhomogeneities is important: better agreement with the observations can be
achieved with radially compressed clumps than with spherical clumps. The
parameters of the model, i.e. chemical abundances, stellar radius, mass-loss
rate and terminal wind velocity, are taken from existing analyses of UV and
optical spectra of the programme stars. On this basis, we also calculate the
continuum-absorption coefficient of the cool-wind material, using the Potsdam
Wolf-Rayet (PoWR) model atmosphere code. The radial location of X-ray emitting
gas is restricted from analysing the fir line ratios of helium-like ions. The
only remaining free parameter of our model is the typical distance between the
clumps; here, we assume that at any point in the wind there is one clump
passing by per one dynamical time-scale of the wind. The total emission in a
model line is scaled to the observation. There is a good agreement between
synthetic and observed line profiles. We conclude that the X-ray emission line
profiles in O stars can be explained by hot plasma embedded in a cool wind
which is highly clumped in the form of radially compressed shell fragments.Comment: a typo corrected, 14 pages, MNRAS, in pres
Detection of magnetic field in the B2 star Oph A with ESO FORS2
Circumstantial evidence suggests that magnetism and enhanced X-ray emission
are likely correlated in early B-type stars: similar fractions of them (
10 %) are strong and hard X-ray sources and possess strong magnetic fields. It
is also known that some B-type stars have spots on their surface. Yet up to now
no X-ray activity associated with spots on early-type stars was detected. In
this Letter we report the detection of a magnetic field on the B2V star
Oph A. Previously, we assessed that the X-ray activity of this star is
associated with a surface spot, herewith we establish its magnetic origin. We
analyzed FORS2 ESO VLT spectra of Oph A taken at two epochs and detected
a longitudinal component of the magnetic field of order of G in one
of the datasets. The detection of the magnetic field only at one epoch can be
explained by stellar rotation which is also invoked to explain observed
periodic X-ray activity. From archival HARPS ESO VLT high resolution spectra we
derived the fundamental stellar parameters of Oph A and further
constrained its age. We conclude that Oph A provides strong evidence for
the presence of active X-ray emitting regions on young magnetized early type
stars.Comment: 4 pages, 1 figure, 2 tables, accepted as a "Letter to the Editor" to
Astronomy & Astrophysic
Phase-dependent X-ray observations of the beta Lyrae system: No eclipse in the soft band
We report on observations of the eclipsing and interacting binary beta Lyrae
from the Suzaku X-ray telescope. This system involves an early B star embedded
in an optically and geometrically thick disk that is siphoning atmospheric
gases from a less massive late B II companion. Motivated by an unpublished
X-ray spectrum from the Einstein X-ray telescope suggesting unusually hard
emission, we obtained time with Suzaku for pointings at three different phases
within a single orbit. From the XIS detectors, the softer X-ray emission
appears typical of an early-type star. What is surprising is the remarkably
unchanging character of this emission, both in luminosity and in spectral
shape, despite the highly asymmetric geometry of the system. We see no eclipse
effect below 10 keV. The constancy of the soft emission is plausibly related to
the wind of the embedded B star and Thomson scattering of X-rays in the system,
although it might be due to extended shock structures arising near the
accretion disk as a result of the unusually high mass-transfer rate. There is
some evidence from the PIN instrument for hard emission in the 10-60 keV range.
Follow-up observations with the RXTE satellite will confirm this preliminary
detection.Comment: to appear in A&A Letter
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