Mass-loss and diffusion in subdwarf B stars and hot white dwarfs: do weak winds exist?

Abstract

Context. According to previous investigations, the effect of diffusion in the stellar atmospheres and envelopes of hot white dwarfs and subdwarf B (sdB) stars strongly depends on the presence of weak winds with mass-loss ratesṀ < 10 −11 M /yr. Aims. As in most of these stars with luminosities L/L < ∼ 100, no wind signatures have been detected, the mass-loss rates are unknown. In the present paper mass-loss rates are predicted from the original theory of radiatively driven winds. Methods. The method of solution is modified so that the usual parametrization of the line force multipliers is not necessary. This is important especially for very thin winds. In addition we checked whether a one-component description is justified. As a consequence of various simplifications, the mass-loss rates are expected to be overestimated. Results. Results are presented for effective temperatures in the range 25 000 K ≤ T eff ≤ 50 000 K and for various metallicities between solar and Z/Z = 0.01. For (pre-) white dwarfs and sdB stars a stellar mass of M * = 0.5 M is assumed. For fixed values of T eff , M * , and Z, the results predict decreasing mass-loss rates with increasing surface gravity and an increasing dependence of the mass-loss rates on the metallicity. For white dwarfs with log g > 7.0 no wind solution exists even if the metallicity would be solar. Winds with mass-loss rates around 10 −11 to 10 −10 M /yr are predicted for the most luminous sdB stars with surface gravities of log g < ∼ 5.5, if the metallicity is not significantly lower than solar. For lower values ofṀ metals decouple from hydrogen and helium. Conclusions. If weak winds withṀ < ∼ 10 −12 M /yr exist, the metals cannot be coupled to hydrogen and helium. This should lead to additional changes in the surface composition, which have not yet been taken into account in the diffusion calculations with and without mass-loss. A possible scenario is the existence of pure metallic winds with mass-loss rates ofṀ < ∼ 10 −16 M /yr and with hydrostatic hydrogen and helium.

AmFm and lithium gap stars

“…For cooler stars, such as F stars, radiative accelerations are not believed to be able to generate significant mass loss (Abbott 1982). Unglaub (2008) found that radiatively driven winds of sdB stars must be separated (accelerated metals cannot drag H and He) if the mass loss is smaller than 10 −12 M yr −1 (10 times larger than the approximate value obtained in Sect. 4.2.1).…”

Weak wind effects in CNO driven winds of hot first stars

“…On the other hand, if decoupling were to occur at velocities smaller than the escape speed, then the hydrogen and helium components would be unable to leave the star (Porter & Skouza 1999;Krtička & Kubát 2001). For very low metallicities, a purely metallic wind may exist (Babel 1995;Unglaub 2008).…”

“…The study of CNO driven winds is important not only for early stellar generations (cf., Unglaub 2008). The low density winds of present stars are also accelerated mostly by CNO lines because the contribution of other heavier elements is relatively small (e.g., Vink et al 2001).…”

“…Multicomponent effects are connected to the momentum transfer between heavier elements (accelerated by line absorption) and bulk wind material, i.e., hydrogen and helium. In low-density winds, momentum transfer may become inefficient, causing frictional heating or even decoupling of wind components (Castor et al 1976;Springmann & Pauldrach 1992;Krtička & Kubát 2001;Owocki & Puls 2002;Votruba et al 2007;Unglaub 2008).…”

“…Hot star winds are studied mainly by assuming a solar mixture of elements and information about winds of more exotic composition is scarce (Vink & de Koter 2005;Gräfener & Hamann 2008;Unglaub 2008). An interesting mixture of heavier elements, which is uncommon in contemporary Universe, is represented by a pure CNO composition.…”