New mass-loss rates of Magellanic Cloud B supergiants from global wind models

We provide global models of line-driven winds of B supergiants for metallicities corresponding to the Large and Small Magellanic Clouds. The velocity and density structure of the models is determined consistently from hydrodynamical equations with radiative force derived in the comoving frame and level populations computed from kinetic equilibrium equations. We provide a formula expressing the predicted mass-loss rates in terms of stellar luminosity, effective temperature, and metallicity. Predicted wind mass-loss rates decrease with decreasing metallicity as $\dot M\sim Z^{0.60}$ and are proportional to the stellar luminosity. The mass-loss rates increase below the region of the bistability jump at about 20\,kK because of iron recombination. In agreement with previous theoretical and observational studies, we find a smooth change of wind properties in the region of the bistability jump. With decreasing metallicity, the bistability jump becomes weaker and shifts to lower effective temperatures. At lower metallicities above the bistability jump, our predictions provide similar rates to those used in current evolutionary models, but our rates are significantly lower than older predictions below the bistability jump. Our predicted mass-loss rates agree with observational estimates derived from H$\alpha$ line assuming that observations of stellar winds from Galaxy and the Magellanic Clouds are uniformly affected by clumping. The models nicely reproduce the dependence of terminal velocities on temperature derived from ultraviolet spectroscopy.Comment: 8 pages, accepted for publication in Astronomy & Astrophysic

Stellar wind models of central stars of planetary nebulae

Fast line-driven stellar winds play an important role in the evolution of planetary nebulae. We provide global hot star wind models of central stars of planetary nebulae. The models predict wind structure including the mass-loss rates, terminal velocities, and emergent fluxes from basic stellar parameters. We applied our wind code for parameters corresponding to evolutionary stages between the asymptotic giant branch and white dwarf phases. We study the influence of metallicity and wind inhomogeneities (clumping) on the wind properties. Line-driven winds appear very early after the star leaves the asymptotic giant branch (at the latest for T_\rm{eff}\approx10\,kK) and fade away at the white dwarf cooling track (below T_\rm{eff}=105\,kK). Their mass-loss rate mostly scales with the stellar luminosity and, consequently, the mass-loss rate only varies slightly during the transition from the red to the blue part of the Hertzsprung-Russell diagram. There are the following two exceptions to the monotonic behavior: a bistability jump at around $20\,$kK, where the mass-loss rate decreases by a factor of a few (during evolution) due to a change in iron ionization, and an additional maximum at about T_\rm{eff}=40-50\,kK. On the other hand, the terminal velocity increases from about a few hundreds of $\rm{km}\,\rm{s}^{-1}$ to a few thousands of $\rm{km}\,\rm{s}^{-1}$ during the transition as a result of stellar radius decrease. The wind terminal velocity also significantly increases at the bistability jump. Derived wind parameters reasonably agree with observations. The effect of clumping is stronger at the hot side of the bistability jump than at the cool side. Derived fits to wind parameters can be used in evolutionary models and in studies of planetary nebula formation. A predicted bistability jump in mass-loss rates can cause the appearance of an additional shell of planetary nebula.Comment: 14 pages, accepted for publication in Astronomy & Astrophysic

Understanding the rotational variability of K2 targets. HgMn star KIC 250152017 and blue horizontal branch star KIC 249660366

Ultraprecise space photometry enables us to reveal light variability even in stars that were previously deemed constant. A large group of such stars show variations that may be rotationally modulated. This type of light variability is of special interest because it provides precise estimates of rotational rates. We aim to understand the origin of the light variability of K2 targets that show signatures of rotational modulation. We used phase-resolved medium-resolution XSHOOTER spectroscopy to understand the light variability of the stars KIC~250152017 and KIC~249660366, which are possibly rotationally modulated. We determined the atmospheric parameters at individual phases and tested the presence of the rotational modulation in the spectra. KIC 250152017 is a HgMn star, whose light variability is caused by the inhomogeneous surface distribution of manganese and iron. It is only the second HgMn star whose light variability is well understood. KIC 249660366 is a He-weak, high-velocity horizontal branch star with overabundances of silicon and argon. The light variability of this star is likely caused by a reflection effect in this post-common envelope binary.Comment: 8 pages, accepted for publication in Astronomy & Astrophysic

Hot subdwarf wind models with accurate abundances I. Hydrogen dominated stars HD 49798 and BD+18∘ ^\circ\,2647

Hot subdwarfs are helium burning objects in late stages of their evolution. These stars can develop winds driven by light absorption in the lines of heavier elements. The wind strength depends on chemical composition which can significantly vary from star to star. We aim to understand the influence of metallicity on the strength of the winds of the hot hydrogen-rich subdwarfs HD 49798 and BD+18$^\circ\,$2647. We used UV and optical spectra to derive stellar parameters and abundances. For derived stellar parameters, we predicted wind structure (including mass-loss rates and terminal velocities) with our METUJE code. We derived effective temperature $T_\text{eff}=45\,900\,$K and mass $M=1.46\,M_\odot$ for HD 49798 and $T_\text{eff}=73\,000\,$K and $M=0.38\,M_\odot$ for BD+18$^\circ\,$2647. The abundances can be interpreted as a result of interplay between stellar evolution and diffusion. HD 49798 has a strong wind that does not allow for chemical separation and consequently it shows solar chemical composition modified by hydrogen burning. On the other hand, we did not find any wind in BD+18$^\circ\,$2647 and its abundances are therefore most likely affected by radiative diffusion. Accurate abundances do not lead to a significant modification of wind mass-loss rate for HD 49798, because the increase of the contribution of Fe and Ni to the radiative force is compensated by the decrease of the force due to other elements. The resulting wind mass-loss rate $\dot M=2.1\times10^{-9}\,M_\odot\,\text{yr}^{-1}$ predicts an X-ray light curve during the eclipse which closely agrees with observations. On the other hand, the absence of the wind in BD+18$^\circ\,$2647 for accurate abundances is a result of its peculiar chemical composition. Wind models with accurate abundances provide more reliable wind parameters, but the influence of abundances on the wind parameters is limited in many cases.Comment: 11 pages, accepted for publication in Astronomy & Astrophysic