We present 1D numerical simulations aimed at studying the hot-flasher scenario for the formation of He-rich subdwarf stars. Sequences were calculated for a wide range of metallicities and physical assumptions, such as the stellar mass at the moment of the helium core flash. This allows us to study the two previously proposed flavors of the hot-flasher scenario ("deep" and "shallow" mixing cases) and to identify a third transition type. Our sequences are calculated by solving simultaneously the mixing and burning equations within a diffusive convection picture, and in the context of standard mixing length theory. We are able to follow chemical evolution during deep-mixing events in which hydrogen is burned violently, and therefore able to present a homogeneous set of abundances for different metallicities and varieties of hot-flashers. We extend the scope of our work by analyzing the effects of non-standard assumptions, such as the effect of chemical gradients, extra-mixing at convective boundaries, possible reduction in convective velocities, or the interplay between difussion and mass loss. Particular emphasis is placed on the predicted surface properties of the models. We find that the hot-flasher scenario is a viable explanation for the formation and surface properties of He-sdO stars. Our results also show that, during the early He-core burning stage, element diffusion may produce the transformation of (post hot-flasher) He-rich atmospheres into He-deficient ones. If this is so, then we find that He-sdO stars should be the progenitors of some of the hottest sdB stars.

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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.

Abstract.We investigate the influence of diffusion and mass loss on the chemical composition in subdwarf B stars in the range 25 000 K ≤ T eff ≤ 35 000 K, 5.5 ≤ log g ≤ 6.0. Within the outer hydrogen-rich envelope characterized by mass <10 −2 M * for the elements H, He, C, N and O the equations of continuity, the momentum equations and the equation of radiative transfer are solved simultaneously. For various mass loss rates the time evolution of the chemical composition is predicted within time scales of ≈10 8 yr, which correspond to the typical lifetimes of the sdB's near the Extended Horizontal Branch. According to the results weak winds withṀ ≈ 10 −13 M /yr may explain the typical helium deficiencies by more than one order of magnitude in the atmospheres of these stars. Winds with 10 −14 M /yr <Ṁ < 10 −13 M /yr may lead to strong deficiencies as well as to enrichments of the CNO elements. The composition in the outer envelope changes in time scales similar to the typical lifetimes of the sdB's. From estimates of the radiative acceleration in the wind region, which make the existence of weak winds plausible, and from a comparison with the case of hot white dwarfs we suggest that the abundance anomalies observed in sdB stars are related to the combined effects of diffusion and mass loss.

We have analysed a sample of 23 hot DAs to better understand the source of the circumstellar features reported in previous work. Unambiguous detections of circumstellar material are again made at eight stars. The velocities of the circumstellar material at three of the white dwarfs are coincident with the radial velocities of interstellar medium (ISM) along the sight‐line to the stars, suggesting that the objects may be ionizing the ISM in their locality. In three further cases, the circumstellar velocities are close to the ISM velocities, indicating that these objects are ionizing either the ISM or evaporated planetesimals/material in a circumstellar disc. The circumstellar velocity at WD 1614−084 lies far from the ISM velocities, indicating the ionization of either an undetected ISM component or circumstellar material. The material seen at WD 0232+035 can be attributed to the photoionization of material lost from its M dwarf companion. The measured column densities of the circumstellar material lie within the ionized ISM column density ranges predicted to exist in hot DA Strömgren spheres.