34 research outputs found
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
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
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
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
Wind inhibition by X-ray irradiation in HMXBs: the influence of clumping and the final X-ray luminosity
In wind-powered X-ray binaries, the radiatively driven stellar wind from the
primary may be inhibited by the X-ray irradiation. This creates the feedback
that limits the X-ray luminosity of the compact secondary. Wind inhibition
might be weakened by the effect of small-scale wind inhomogeneities (clumping)
possibly affecting the limiting X-ray luminosity. We study the influence of
X-ray irradiation on the stellar wind for different radial distributions of
clumping. We calculate hot star wind models with external irradiation and
clumping using our global wind code. The models are calculated for different
parameters of the binary. We determine the parameters for which the X-ray wind
ionization leads to a decrease of the radiative force. This causes a decrease
of the wind velocity and even of the mass-loss rate in the case of extreme
X-ray irradiation. Clumping weakens the effect of X-ray irradiation because it
favours recombination and leads to an increase of the wind mass-loss rate. The
best match between the models and observed properties of high-mass X-ray
binaries (HMXB) is derived with radially variable clumping. We describe the
influence of X-ray irradiation on the terminal velocity and on the mass-loss
rate in a parametric way. The X-ray luminosities predicted within the Bondi
theory agree nicely with observations when accounting for X-ray irradiation.
The ionizing feedback regulates the accretion onto the compact companion
resulting in a relatively stable X-ray source. The wind-powered accretion model
can account for large luminosities in HMXBs only when introducing the ionizing
feedback. There are two possible states following from the dependence of X-ray
luminosity on the wind terminal velocity and mass-loss rate. One state has low
X-ray luminosity and a nearly undisturbed wind, and the second state has high
X-ray luminosity and exhibits a strong influence of X-rays on the flow.Comment: 15 pages, accepted for publication in Astronomy & Astrophysic