355 research outputs found
Hot-star wind models with magnetically split line blanketing
Fraction of hot stars posses strong magnetic fields that channel their
radiatively driven outflows. We study the influence of line splitting in the
magnetic field (Zeeman effect) on the wind properties. We use our own global
wind code with radiative transfer in the comoving frame to understand the
influence of the Zeeman splitting on the line force. We show that the Zeeman
splitting has a negligible influence on the line force for magnetic fields that
are weaker than about 100~kG. This means that the wind mass-loss rates and
terminal velocities are not affected by the magnetic line splitting for
magnetic fields as are typically found on the surface of nondegenerate stars.
Neither have we found any strong flux variability that would be due to the
magnetically split line blanketing.Comment: 4 pages, accepted for publication in Astronomy & Astrophysic
Mass loss in main-sequence B stars
We calculate radiatively driven wind models of main-sequence B stars and
provide the wind mass-loss rates and terminal velocities. The main-sequence
mass-loss rate strongly depends on the stellar effective temperature. For the
hottest B stars the mass-loss rate amounts to
, while for the cooler ones
themass-loss rate is lower by more than three orders of magnitude.
Main-sequence B stars with solar abundance and effective temperatures lower
than about (later than spectral type B5) do not have any
homogeneous line-driven wind. We predict the wind mass-loss rates for the solar
chemical composition and for the modified abundance of heavier elements to
study the winds of chemically peculiar stars. The mass-loss rate may either
increase or decrease with increasing abundance, depending on the importance of
the induced emergent flux redistribution. Stars with overabundant silicon may
have homogeneous winds even below the solar abundance wind limit at
. The winds of main-sequence B stars lie below the static
limit, that is, a static atmosphere solution is also possible. This points to
an important problem regarding the initiation of these winds. We discuss the
implications of our models for rotational braking, filling the magnetosphere of
Bp stars, and for chemically peculiar stars.Comment: 10 pages, 7 figures, Astronomy&Astrophysics, in press, discussion
extended and languange corrections include
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
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
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
- …