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

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

Visual and ultraviolet flux variability of the bright CP star θ\theta Aur

Chemically peculiar stars of the upper part of the main sequence show periodical variability in line intensities and continua, modulated by the stellar rotation, which is attributed to the existence of chemical spots on the surface of these stars. The flux variability is caused by the changing redistribution rate of the radiative flux predominantly from the short-wavelength part of the spectra to the long-wavelength part, which is a result of abundance anomalies. We study the nature of the multi-spectral variability of one of the brightest chemically peculiar stars, $\theta$ Aur. We predict the flux variability of $\theta$ Aur from the emerging intensities calculated for individual surface elements of the star taking into account horizontal variation of chemical composition derived from Doppler abundance maps. The simulated optical variability in the Str\"omgren photometric system and the ultraviolet flux variability agree well with observations. The IUE flux distribution is reproduced in great detail by our models. The resonance lines of magnesium and possibly also some lines of silicon are relatively weak in the ultraviolet domain, which indicates non-negligible vertical abundance gradients in the atmosphere. We also derive a new period of the star, $P=3.618\,664(10)$ d, from all available photometric and magnetic measurements and show that the observed rotational period is constant over decades. The ultraviolet and visual variability of $\theta$ Aur is mostly caused by silicon bound-free absorption and chromium and iron line absorption. These elements redistribute the flux mainly from the far-ultraviolet region to the near-ultraviolet and optical regions in the surface abundance spots. The light variability is modulated by the stellar rotation. The ultraviolet domain is key for understanding the properties of chemically peculiar stars. (abridged)Comment: 12 pages, accepted for publication in Astronomy & Astrophysic

X-ray emission from hydrodynamical simulations in non-LTE wind models

Hot stars are sources of X-ray emission originating in their winds. Although hydrodynamical simulations that are able to predict this X-ray emission are available, the inclusion of X-rays in stationary wind models is usually based on simplifying approximations. To improve this, we use results from time-dependent hydrodynamical simulations of the line-driven wind instability (seeded by the base perturbation) to derive the analytical approximation of X-ray emission in the stellar wind. We use this approximation in our non-LTE wind models and find that an improved inclusion of X-rays leads to a better agreement between model ionization fractions and those derived from servations. Furthermore, the slope of the L_x-L relation is in better agreement with observations, however the X-ray luminosity is underestimated by a factor of three. We propose a possible solution for this discrepancy.Comment: 9 pages, accepted for publication in Astronomy and Astrophysic

Weak wind effects in CNO driven winds of hot first stars

During the evolution of rotating first stars, which initially consisted of only hydrogen and helium, CNO elements may emerge to their surface. These stars may therefore have winds that are driven only by CNO elements. We study weak wind effects (Gayley-Owocki heating and multicomponent effects) in stellar winds of first generation stars driven purely by CNO elements. We apply our NLTE multicomponent models and hydrodynamical simulations. The multicomponent effects (frictional heating and decoupling) are important particularly for low metallicity winds, but they influence mass loss rate only if they cause decoupling for velocities lower than the escape velocity. The multicomponent effects also modify the feedback from first stars. As a result of the decoupling of radiatively accelerated metals from hydrogen and helium, the first low-energy cosmic ray particles are generated. We study the interaction of these particles with the interstellar medium concluding that these particles easily penetrate the interstellar medium of a given minihalo. We discuss the charging of the first stars by means of their winds. Gayley-Owocki heating, frictional heating, and the decoupling of wind components occur in the winds of evolved low-metallicity stars and the solar metallicity main-sequence stars.Comment: 10 pages, accepted for publication in Astronomy & Astrophysic

Light variations due to the line-driven wind instability and wind blanketing in O stars

A small fraction of the radiative flux emitted by hot stars is absorbed by their winds and redistributed towards longer wavelengths. This effect, which leads also to the heating of the stellar photosphere, is termed wind blanketing. For stars with variable winds, the effect of wind blanketing may lead to the photometric variability. We have studied the consequences of line driven wind instability and wind blanketing for the light variability of O stars. We combined the results of wind hydrodynamic simulations and of global wind models to predict the light variability of hot stars due to the wind blanketing and instability. The wind instability causes stochastic light variability with amplitude of the order of tens of millimagnitudes and a typical timescale of the order of hours for spatially coherent wind structure. The amplitude is of the order of millimagnitudes when assuming that the wind consists of large number of independent concentric cones. The variability with such amplitude is observable using present space borne photometers. We show that the simulated light curve is similar to the light curves of O stars obtained using BRITE and CoRoT satellites.Comment: 7 pages, accepted for publication in Astronomy & Astrophysic