Jiri Krticka

Multicomponent radiatively driven stellar winds - II. Gayley-Owocki heating in multitemperature winds of OB stars

We show that the so-called Gayley-Owocki (Doppler) heating is important for the temperature structure of the wind of main sequence stars cooler than the spectral type O6. The formula for Gayley-Owocki heating is derived directly from the Boltzmann equation as a direct consequence of the dependence of the driving force on the velocity gradient. Since Gayley-Owocki heating deposits heat directly on the absorbing ions, we also investigated the possibility that individual components of the radiatively driven stellar wind have different temperatures. This effect is negligible in the wind of O stars, whereas a significant temperature difference takes place in the winds of main sequence B stars for stars cooler than B2. Typical temperature differences between absorbing ions and other flow components for such stars is of the order 103 K. However, in the case when the passive component falls back onto the star, the absorbing component reaches temperatures of order 106 K, which allows for emission of X-rays. Moreover, we compare our computed terminal velocities with the observed ones. We found quite good agreement between predicted and observed terminal velocities. The systematic difference coming from the using of the so called cooking formula has been removed.

NLTE models of line-driven stellar winds. I. Method of calculation and first results for O stars

New numerical models of line-driven stellar winds of late O stars are presented. Statistical equilibrium (NLTE) equations of the most abundant elements are solved. Properly obtained occupation numbers are used to calculate consistent radiative force and radiative heating terms. Wind density, velocity and temperature are calculated as a solution of model hydrodynamical equations. Contrary to other published models we account for a multicomponent wind nature and do not simplify the calculation of the radiative force (e.g. using force multipliers). We discuss the convergence behaviour of our models. The ability of our models to predict correct values of mass-loss rates and terminal velocities of selected late O stars (mainly giants and supergiants) is demonstrated. The systematic difference between predicted and observed terminal velocities reported in the literature has been removed. Moreover, we found good agreement between the theoretical wind momentum-luminosity relationship and the observed one for Cyg OB2 supergiants.

Comoving frame models of hot star winds I. Test of the Sobolev approximation in the case of pure line transitions

We provide hot star wind models with radiative force calculated using the solution of comoving frame (CMF) radiative transfer equation. The wind models are calculated for the first stars, O stars, and the central stars of planetary nebulae. We show that without line overlaps and with solely thermal line broadening the pure Sobolev approximation provides a reliable estimate of the radiative force even close to the wind sonic point. Consequently, models with the Sobolev line force provide good approximations to solutions obtained with non-Sobolev transfer. Taking line overlaps into account, the radiative force becomes slightly lower, leading to a decrease in the wind mass-loss rate by roughly 40%. Below the sonic point, the CMF line force is significantly lower than the Sobolev one. In the case of pure thermal broadening, this does not influence the mass-loss rate, as the wind mass-loss rate is set in the supersonic part of the wind. However, when additional line broadening is present (e.g., the turbulent one) the region of low CMF line force may extend outwards to the regions where the mass-loss rate is set. This results in a decrease in the wind mass-loss rate. This effect can at least partly explain the low wind mass-loss rates derived from some observational analyses of luminous O stars.

Multicomponent radiatively driven stellar winds - IV. On the helium decoupling in the wind of

We study the possibility of the helium decoupling in the stellar wind of sigma Ori E. To obtain reliable wind parameters for this star we first calculate NLTE wind model and derive wind mass-loss rate and terminal velocity. Using corresponding force multipliers we study the possibility of helium decoupling. We find that helium decoupling is not possible for realistic values of helium charge (calculated from NLTE wind models). Helium decoupling only seems possible for a very low helium charge. The reason for this behavior is the strong coupling between helium and hydrogen. We also find that frictional heating becomes important in the outer parts of the wind of sigma Ori E due to the collisions between some heavier elements and the passive components - hydrogen and helium. For a metallicity ten times lower than the solar one, both hydrogen and helium decouple from the metals and may fall back onto the stellar surface. However, this does not explain the observed chemical peculiarity since both these components decouple together from the absorbing ions. Although we do not include the effects of the magnetic field into our models, we argue that the presence of a magnetic field will likely not significantly modify the derived results because in such case model equations describe the motion parallel to the magnetic field.

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During the evolution of first stars, the CNO elements may emerge on their surfaces due to the mixing processes. Consequently, these stars may have winds driven purely by CNO elements. We study the properties of such stellar winds and discuss their influence on the surrounding environment. For this purpose, we used our own NLTE models and tested which stellar parameters of the first stars at different evolutionary stages result in CNO winds. If such winds are possible, we calculate their hydrodynamic structure and predict their parameters. We show that, while the studied stars do not have any wind driven purely by hydrogen and helium, CNO driven winds exist in more luminous stars. On the other hand, for very hot stars, CNO elements are too ionized to drive a wind. In most cases the derived mass-loss rate is much less than calculated with solar mixture of elements. This is because wind mass-loss rate in present hot stars is dominated by elements heavier than CNO. We conclude that, until a sufficient amount of these elements is created, the influence of line-driven winds is relatively small on the evolution of hot stars (which are not close to the Eddington limit).

Orionis E

We study the stability of isothermal two-component radiatively driven stellar winds to one-dimensional perturbations larger than the Sobolev length, and radiative-acoustic waves in such stellar winds. We perform linear perturbation analysis in comoving fluid-frames of individual components and obtain the dispersion relation in the common fluid frame. For high density winds the difference between velocities of both components is relatively small and the wind is stable for radiative-acoustic waves discovered originally by Abbott, in accordance with the previous studies of the one-component wind. However, for such high density winds we found new types of waves, including a special case of frozen-in wavy patterns. On the other hand, if the velocity difference between wind components is sufficiently large (for low density winds) then the multicomponent stellar wind is unstable even for large-scale perturbations and ion runaway occurs. Thus, isothermal two-component stationary solutions of the radiatively line-driven stellar wind with an abrupt lowering of the velocity gradient are unstable.

CNO-driven winds of hot first stars

We compare the hot-star wind models calculated by 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 better agreement with mass-loss rates derived from observational analysis that takes the clumping into account. (2) As a result of the lowering of mass-loss rates, there is better agreement between the predicted modified wind momentum-luminosity relationship and that derived from observational analysis that takes the clumping into account. (3) Both the lower mass fraction of heavier elements and lower mass-loss rates lead to a decrease in opacity in the X-ray region. This influences the prediction of the X-ray line profile shapes. (4) There is better agreement between predicted PV ionization fractions and those derived from observations.

Multicomponent radiatively driven stellar winds III.Radiative-acoustic waves in a two-component wind

Context. Thermal wind emission in the form of free-free and free-bound emission is known to show up in the infrared and radio continuum of hot and massive stars. For OB supergiants with moderate mass loss rates and a wind velocity distribution with

Hot star wind models with new solar abundances

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