Modeling He-rich subdwarfs through the hot-flasher Scenario

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.Comment: 13 pages, including 8 figures and 6 tables. Accepted for publication in A&A. Replaced to match the final version, including a note added in proof regarding PG 1544+48

Modeling He-rich subdwarfs through the hot-flasher scenario

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.Instituto de Astrofísica de La PlataFacultad de Ciencias Astronómicas y Geofísica

Discovery of photospheric CaX emission lines in the far-UV spectrum of the hottest known white dwarf (KPD0005+5106)

For the first time, we have identified photospheric emission lines in the far-UV spectrum of a white dwarf. They were discovered in the Far Ultraviolet Spectroscopic Explorer spectrum of the hot (Teff~200,000 K) DO white dwarf KPD0005+5106 and they stem from extremely highly ionized calcium (CaX 1137, 1159 Ang). Their photospheric origin is confirmed by non-LTE line-formation calculations. This is the highest ionisation stage of any element ever observed in a stellar photosphere. Calcium has never been detected before in any hot white dwarf or central star of planetary nebula. The calcium abundance determination for KPD0005+5106 (1-10 times solar) is difficult, because the line strengths are rather sensitive to current uncertainties in the knowledge of effective temperature and surface gravity. We discuss the possibility that the calcium abundance is much lower than expected from diffusion/levitation equilibrium theory. The same emission lines are exhibited by the [WCE]-type central star NGC2371. Another CaX line pair (1461, 1504 Ang) is probably present in a Hubble Space Telescope spectrum of the PG1159-type central star NGC246.Comment: Accepted for publication in A&A Letter

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

The effect of diffusion on the chemical composition of subdwarf B (sdB) stars and of hot white dwarfs strongly depends on the presence of weak winds. In the paper, for stars with half a solar mass, for various effective temperatures between 25000 K and 50000 K, and for various metallicities between solar and 1/100 solar, the mass loss rates are predicted as a function of the surface gravity. With assumptions similar to the original theory of radiatively driven winds, the method of solution of the momentum equation has been modified, because the usual parametrization of the line force multiplier leads to complications in the case of weak winds. According to the results, weak winds with mass-loss rates of the order E-11 solar masses per year may exist only for the most luminous sdB stars. For the more compact ones, the decoupling of the metals from the bulk matter (hydrogen and helium) is expected in the wind region, because the momentum exchange via Coulomb collisions is not effective enough. Thus multicomponent effects are of great importance. For the case of white dwarfs no wind solution exists at all, if multicomponent effects are neglected.Comment: 19 pages, 13 figure

NLTE wind models of hot subdwarf stars

We calculate NLTE models of stellar winds of hot compact stars (central stars of planetary nebulae and subdwarf stars). The studied range of subdwarf parameters is selected to cover a large part of these stars. The models predict the wind hydrodynamical structure and provide mass-loss rates for different abundances. Our models show that CNO elements are important drivers of subdwarf winds, especially for low-luminosity stars. We study the effect of X-rays and instabilities on these winds. Due to the line-driven wind instability, a significant part of the wind could be very hot.Comment: 7 pages, to appear in Astrophysics and Space Science. The final publication will be available at springerlink.com

Chandra grating spectroscopy of three hot white dwarfs

High-resolution soft X-ray spectroscopic observations of single hot white dwarfs are scarce. With the Chandra Low-Energy Transmission Grating, we have observed two white dwarfs, one is of spectral type DA (LB 1919) and the other is a non-DA of spectral type PG1159 (PG 1520+525). The spectra of both stars are analyzed, together with an archival Chandra spectrum of another DA white dwarf (GD 246). The soft X-ray spectra of the two DA white dwarfs are investigated in order to study the effect of gravitational settling and radiative levitation of metals in their photospheres. LB 1919 is of interest because it has a significantly lower metallicity than DAs with otherwise similar atmospheric parameters. GD 246 is the only white dwarf known that shows identifiable individual iron lines in the soft X-ray range. For the PG1159 star, a precise effective temperature determination is performed in order to confine the position of the blue edge of the GW Vir instability region in the HRD. (abridged)Comment: A&A, in pres

Testing the forward modeling approach in asteroseismology. I. Seismic solutions for the hot B subdwarf Balloon 090100001 with and without a priori mode identification

Context: Balloon 090100001, the brightest of the known pulsating hot B subdwarfs, exhibits simultaneoulsy both short- and long-period pulsation modes, and shows relatively large amplitudes for its dominant modes. For these reasons, it has been studied extensively over the past few years, including a successful experiment carried out at the Canada-France-Hawaii Telescope to pin down or constrain the value of the degree index ℓ of several pulsation modes through multicolor photometry. Aims: The primary goal of this paper is to take advantage of such partial mode identification to test the robustness of our standard approach to the asteroseismology of pulsating subdwarf B stars. The latter is based on the forward approach whereby a model that best matches the observed periods is searched for in parameter space with no a priori assumption about mode identification. When successful, this method leads to the determination of the global structural parameters of the pulsator. As a bonus, it also leads, after the fact, to complete mode identification. For the first time, with the availability of partial mode identification for Balloon 090100001, we are able to evaluate the sensitivity of the inferred seismic model to possible uncertainty in mode identification. Methods: We carry out a number of exercises based on the double optimization technique that we developed within the framework of the forward modeling approach in asteroseismology. We use the set of ten periods corresponding to the independent pulsation modes for which values of ℓ have been either formally identified or constrained through multicolor photometry in Balloon 090100001. These exercises differ in that they assume different a priori mode identification. Results: Our primary result is that the asteroseismic solution stands very robust, whether or not external constraints on the values of the degree ℓ are used. Although this may come as a small surprise, the test proves to be conclusive, and small differences in mode identification among the ten modes do not affect in any significant way, at the typical accuracy presently achieved, the final emergent seismic model. This is due to the structure of the p-mode pulsation spectra in sdB stars. In all cases, the inferred structural parameters of Balloon 090100001 remain practically unchanged. They correspond, and this constitutes our second important result, to a star beyond the TAEHB with T_eff = 28 000 ± 1 200 K, log g = 5.383 ± 0.004, M⋆/Msun = 0.432 ± 0.015, and log{M_env/M⋆} = -4.89 ± 0.14. Other structural parameters are also derived.Peer reviewe