846 research outputs found
The winds of the hot massive first stars
We study dynamical aspects of circumstellar environment around massive
zero-metallicity first stars. For this purpose we apply our NLTE wind models.
We show that the hydrogen-helium stellar wind from stationary massive first
generation (Population III) stars (driven either by the line (bound-bound) or
continuum (bound-free and free-free) transitions) is unlikely. The possibility
of expulsion of chemically homogeneous wind and the role of minor isotopes are
also discussed. Finally, we estimate the importance of hydrogen and helium
lines for shutting off the initial accretion onto first stars and its influence
on initial mass function of first stars.Comment: 15 pages, accepted for publication in Astronomy & Astrophysic
X-ray emission from hydrodynamical simulations in non-LTE wind models
Hot stars are sources of X-ray emission originating in their winds. Although
hydrodynamical simulations that are able to predict this X-ray emission are
available, the inclusion of X-rays in stationary wind models is usually based
on simplifying approximations. To improve this, we use results from
time-dependent hydrodynamical simulations of the line-driven wind instability
(seeded by the base perturbation) to derive the analytical approximation of
X-ray emission in the stellar wind. We use this approximation in our non-LTE
wind models and find that an improved inclusion of X-rays leads to a better
agreement between model ionization fractions and those derived from servations.
Furthermore, the slope of the L_x-L relation is in better agreement with
observations, however the X-ray luminosity is underestimated by a factor of
three. We propose a possible solution for this discrepancy.Comment: 9 pages, accepted for publication in Astronomy and Astrophysic
New mass-loss rates of Magellanic Cloud B supergiants from global wind models
We provide global models of line-driven winds of B supergiants for
metallicities corresponding to the Large and Small Magellanic Clouds. The
velocity and density structure of the models is determined consistently from
hydrodynamical equations with radiative force derived in the comoving frame and
level populations computed from kinetic equilibrium equations. We provide a
formula expressing the predicted mass-loss rates in terms of stellar
luminosity, effective temperature, and metallicity. Predicted wind mass-loss
rates decrease with decreasing metallicity as and are
proportional to the stellar luminosity. The mass-loss rates increase below the
region of the bistability jump at about 20\,kK because of iron recombination.
In agreement with previous theoretical and observational studies, we find a
smooth change of wind properties in the region of the bistability jump. With
decreasing metallicity, the bistability jump becomes weaker and shifts to lower
effective temperatures. At lower metallicities above the bistability jump, our
predictions provide similar rates to those used in current evolutionary models,
but our rates are significantly lower than older predictions below the
bistability jump. Our predicted mass-loss rates agree with observational
estimates derived from H line assuming that observations of stellar
winds from Galaxy and the Magellanic Clouds are uniformly affected by clumping.
The models nicely reproduce the dependence of terminal velocities on
temperature derived from ultraviolet spectroscopy.Comment: 8 pages, accepted for publication in Astronomy & Astrophysic
Stellar wind models of central stars of planetary nebulae
Fast line-driven stellar winds play an important role in the evolution of
planetary nebulae. We provide global hot star wind models of central stars of
planetary nebulae. The models predict wind structure including the mass-loss
rates, terminal velocities, and emergent fluxes from basic stellar parameters.
We applied our wind code for parameters corresponding to evolutionary stages
between the asymptotic giant branch and white dwarf phases. We study the
influence of metallicity and wind inhomogeneities (clumping) on the wind
properties. Line-driven winds appear very early after the star leaves the
asymptotic giant branch (at the latest for T_\rm{eff}\approx10\,kK) and fade
away at the white dwarf cooling track (below T_\rm{eff}=105\,kK). Their
mass-loss rate mostly scales with the stellar luminosity and, consequently, the
mass-loss rate only varies slightly during the transition from the red to the
blue part of the Hertzsprung-Russell diagram. There are the following two
exceptions to the monotonic behavior: a bistability jump at around kK,
where the mass-loss rate decreases by a factor of a few (during evolution) due
to a change in iron ionization, and an additional maximum at about
T_\rm{eff}=40-50\,kK. On the other hand, the terminal velocity increases from
about a few hundreds of to a few thousands of
during the transition as a result of stellar radius
decrease. The wind terminal velocity also significantly increases at the
bistability jump. Derived wind parameters reasonably agree with observations.
The effect of clumping is stronger at the hot side of the bistability jump than
at the cool side. Derived fits to wind parameters can be used in evolutionary
models and in studies of planetary nebula formation. A predicted bistability
jump in mass-loss rates can cause the appearance of an additional shell of
planetary nebula.Comment: 14 pages, accepted for publication in Astronomy & Astrophysic
Limited Role for C. pneumoniae, CMV and HSV-1 in Cerebral Large and Small Vessel Atherosclerosis
Aims: To explore whether Chlamydia pneumoniae, Cytomegalovirus and Herpes Simplex Virus type 1 could be detected in large and small cerebral arteries, as well as in an area of brain parenchyma where white matter lesions (leukoaraiosis) can be found, in patients with clinically unmanifested cerebrovascular atherosclerosis. Methods and results( Arterial specimens from the basilar artery and middle cerebral artery, and brain samples from the basal ganglia and periventricular white matter were obtained. Neuropathological changes were assessed in Haematoxylin-Eosin stained sections. Polymerase chain reaction (PCR) was performed on paraffin embedded sections. Subsequently, we performed immunohistochemical staining on samples, which were found positive in PCR. We failed to detect C. pneumoniae, CMV, or HSV-1, in any of the cerebral large vessels. In the brain tissue, we found only one case positive for CMV, and one for C. pneumoniae. Conclusions (our findings suggest a limited role for C. pneumoniae, CMV and HSV-1 in cerebral large and small vessel atherosclerosis
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