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

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 $\dot M\sim Z^{0.59}$. The predicted mass-loss rates are lower than mass-loss rates derived from H$\alpha$ diagnostics and can be reconciled with observational results assuming clumping factor $C_\text{c}=9$. 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 $T_\text{eff}\leq40\,000\,$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

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

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

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$\alpha$ 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 $1.6$. On the other hand, our predictions are by factor of $4.7$ lower than pure H$\alpha$ 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

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 $Z/Z_\odot\gtrsim0.1$ 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 $Z/Z_\odot\lesssim0.1$ 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 $Z/Z_\odot\lesssim0.1$ 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

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