2,635 research outputs found
Radiative transfer in stellar atmospheres
This review presents basic equations for the solution of the NLTE radiative
transfer problem for trace elements and methods for its solution are
summarized. The importance of frequency coupling in radiative transfer in
stellar atmospheres is emphasized.Comment: presented at the workshop held in Nice, France, 30.7.-4.8.2007, to
appear in Non-LTE Line Formation for Trace Elements in Stellar Atmospheres,
R. Monier et al. eds., EAS Publ. Se
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
Hot star wind models with new solar abundances
We compare the hot star wind models calculated assuming older solar abundance
determination with models calculated using the recently published values
derived from 3D hydrodynamical model atmospheres. We show that the use of new
abundances with lower metallicity improves the agreement between wind
observation and theory in several aspects: (1) The predicted wind mass-loss
rates are lower by a factor of 0.76. This leads to a better agreement with
mass-loss rate determinations derived from observations with account of
clumping. (2) As a result of the lowering of mass-loss rates, there is a better
agreement between predicted modified wind momentum-luminosity relationship and
that derived from observations with account of clumping. (3) Both the lower
mass fraction of heavier elements and lower mass-loss rates lead to a decrease
of the opacity in the X-ray region. This has influence on the prediction of the
X-ray line profile shapes. (4) There is a better agreement between predicted PV
ionization fractions and those derived from observations.Comment: 4 pages, accepted for publication in Astronomy & Astrophysics Letter
Global hot-star wind models for stars from Magellanic Clouds
We provide mass-loss rate predictions for O stars from Large and Small
Magellanic Clouds. We calculate global (unified, hydrodynamic) model
atmospheres of main sequence, giant, and supergiant stars for chemical
composition corresponding to Magellanic Clouds. The models solve radiative
transfer equation in comoving frame, kinetic equilibrium equations (also known
as NLTE equations), and hydrodynamical equations from (quasi-)hydrostatic
atmosphere to expanding stellar wind. The models allow us to predict wind
density, velocity, and temperature (consequently also the terminal wind
velocity and the mass-loss rate) just from basic global stellar parameters. As
a result of their lower metallicity, the line radiative driving is weaker
leading to lower wind mass-loss rates with respect to the Galactic stars. We
provide a formula that fits the mass-loss rate predicted by our models as a
function of stellar luminosity and metallicity. On average, the mass-loss rate
scales with metallicity as . The predicted mass-loss
rates are lower than mass-loss rates derived from H diagnostics and can
be reconciled with observational results assuming clumping factor
. On the other hand, the predicted mass-loss rates either agree
or are slightly higher than the mass-loss rates derived from ultraviolet wind
line profiles. The calculated \ion{P}{v} ionization fractions also agree with
values derived from observations for LMC stars with
K. Taken together, our theoretical predictions
provide reasonable models with consistent mass-loss rate determination, which
can be used for quantitative study of stars from Magellanic Clouds.Comment: accepted for publication in A&A, 12 pages, 8 figure
Effect of rotational mixing and metallicity on the hot star wind mass-loss rates
Hot star wind mass-loss rates depend on the abundance of individual elements.
This dependence is usually accounted for assuming scaled solar chemical
composition. However, this approach may not be justified in evolved rotating
stars. The rotational mixing brings CNO-processed material to the stellar
surface, increasing the abundance of nitrogen at the expense of carbon and
oxygen, which potentially influences the mass-loss rates. We study the
influence of the modified chemical composition resulting from the rotational
mixing on the wind parameters, particularly the wind mass-loss rates. We use
our NLTE wind code to predict the wind structure and compare the calculated
wind mass-loss rate for the case of scaled solar chemical composition and the
composition affected by the CNO cycle. We show that for a higher mass-fraction
of heavier elements the change of chemical composition
from the scaled solar to the CNO-processed scaled solar composition does not
significantly affect the wind mass-loss rates. The missing line force caused by
carbon and oxygen is compensated for by nitrogen line force. However, for a
very low-mass fraction of heavier elements the
rotational mixing significantly affects the wind mass-loss rates. Moreover, the
decrease of the mass-loss rate with metallicity is stronger at such low
metallicities. We study the relevance of the wind momentum-luminosity
relationship for different metallicities and show that for a metallicity
the relationship displays a large scatter, which
depreciates the use of this relationship at the lowest metallicities.Comment: 7 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
CMF models of hot star winds II. Reduction of O star wind mass-loss rates in global models
We calculate global (unified) wind models of main-sequence, giant, and
supergiant O stars from our Galaxy. The models are calculated by solving
hydrodynamic, kinetic equilibrium (also known as NLTE) and comoving-frame (CMF)
radiative transfer equations from the (nearly) hydrostatic photosphere to the
supersonic wind. For given stellar parameters, our models predict the
photosphere and wind structure and in particular the wind mass-loss rates
without any free parameters. Our predicted mass-loss rates are by a factor of
2--5 lower than the commonly used predictions. A possible cause of the
difference is abandoning of the Sobolev approximation for the calculation of
the radiative force, because our models agree with predictions of CMF NLTE
radiative transfer codes. Our predicted mass-loss rates agree nicely with the
mass-loss rates derived from observed near-infrared and X-ray line profiles and
are slightly lower than mass-loss rates derived from combined UV and H
diagnostics. The empirical mass-loss rate estimates corrected for clumping may
therefore be reconciled with theoretical predictions in such a way that the
average ratio between individual mass-loss rate estimates is not higher than
about . On the other hand, our predictions are by factor of
lower than pure H mass-loss rate estimates and can be reconciled with
these values only assuming a microclumping factor of at least eight.Comment: 13 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
Radiative return via electron pair production: Monte Carlo simulation of the process e+ e- --> pi+ pi- e+ e-
Contributions from the reaction e+e- --> pi+pi- e+e- to the pion form factor
measurement via radiative return method are discussed basing on the results of
a Monte Carlo generator (EKHARA). The generator contains contributions from the
initial and final state emission of a e+e- pair from e+e- --> pi+pi- production
diagrams and the pi+pi- pair production from space-like and time-like Bhabha
diagrams. A detailed study is performed for the Phi- factory energy. Tests of
the generation procedure are also presented
- …