An evolutionary study of the pulsating subdwarf B eclipsing binary PG 1336-018 (NY Virginis)

Abstract

Context. The formation of subdwarf B (sdB) stars is not well understood within the current framework of stellar single and binary evolution. Aims. In this study, we focus on the formation and evolution of the pulsating sdB star in the very short-period eclipsing binary PG 1336−018. We aim at refining the formation scenario of this unique system, so that it can be confronted with observations. Methods. We probe the stellar structure of the progenitors of sdB stars in short-period binaries using detailed stellar evolution calculations. Applying this to PG 1336−018 we reconstruct the common-envelope phase during which the sdB star was formed. The results are interpreted in terms of the standard common-envelope formalism (the α-formalism) based on the energy equation, and an alternative description (the γ-formalism) using the angular momentum equation. Results. We find that if the common-envelope evolution is described by the α-formalism, the sdB progenitor most likely experienced a helium flash. We then expect the sdB mass to be between 0.39 and 0.48 M , and the sdB progenitor initial mass to be below ∼2 M . However, the results for the γ-formalism are less restrictive, and a broader sdB mass range (0.3-0.8 M ) is possible in this case. Future seismic mass determination will give strong constraints on the formation of PG 1336−018 and, in particular, on the CE phase.Key words. subdwarfs -stars: evolution -binaries: close -binaries: eclipsing -methods: numerical IntroductionSubdwarf B (sdB) stars are the dominant population of faint blue objects at high galactic latitudes (Green et al. 1986), and are found in both the disk and halo. They are also ubiquitous in giant elliptical galaxies, where they are believed to be the main source of the ultraviolet excess (Brown et al. 1997). In the Hertzsprung-Russell diagram they lie on the blue extension of the Horizontal Branch, and are therefore also known as Extreme Horizontal Branch (EHB) stars. It is generally thought that they are low mass (0.5 M ) core-helium burning stars with extremely thin hydrogen envelopes (<0.02 M ) (Heber 1986;Saffer et al. 1994). Their envelopes are too thin to sustain hydrogen burning, hence they will evolve directly to the white dwarf cooling track after core-helium exhaustion, without going through the Asymptotic Giant Branch and Planetary Nebulae phases.It is not clearly understood how the sdB progenitor manages to loose almost its entire hydrogen-envelope, but nevertheless starts core-helium fusion. Both single star evolution with enhanced mass loss on the Red Giant Branch (RGB) (D'Cruz et al. 1996), and binary evolution models (Mengel et al. 1976) have been proposed as formation channels. Extensive surveys show that a large fraction of sdB stars are in binaries (e.g. Allard et al. 1994;Morales-Rueda et al. 2006). This motivated Han et al. (2002Han et al. ( , 2003 to perform a detailed investigation of the main binary evolution channels that can produce an sdB star. They found that an sdB star can be formed after one or two commonenve...

Interpreting the line-profile variations of subdwarf B pulsators: the case of PG 1336-018 (NY Virginis)

“…The aim of that work was to determine the orbital solution and mass of the components of the eclipsing binary PG 1336−018 (m B = 13.6) by combining high-speed multicolor VLT/ultracam 1 lightcurves (about two full orbits) and the RV curve obtained from high-resolution time-series of about 400 VLT/uves spectra. We found three solutions of equal statistical significance due to the large parameter space and correlations between parameters, two of which are consistent with standard binary evolution models (Hu et al 2007). One of the favored solutions of Paper I with the mass of the sdB primary of 0.466 M was recently found by Charpinet et al (2008) who arrive at the same mass by independent asteroseismic modeling through frequency matching of the observed oscillation frequencies found by Kilkenny et al (2003).…”

Impact of helium diffusion and helium-flash-induced carbon production on gravity-mode pulsations in subdwarf B stars

“…Asteroseismology provides an alternative method, since it allows a detailed study of the interior of non-radially pulsating stars. The consistency of both approaches has been achieved for PG 1336−018 (see Vučković et al 2007;Hu et al 2007;Charpinet et al 2008).…”

Gravitational settling in pulsating subdwarf B stars and their progenitors

“…We distinguish two types of sdB stars: the canonical post-flash sdB star and the newly proposed post-non-degenerate sdB star (Han et al 2002;Hu et al 2007Hu et al , 2008 1 . The latter type originates from an intermediate mass progenitor (2 M M ZAMS 4 M ) that ignited He quiescently, whereas the progenitor of the canonical sdB star is a low mass star (M ZAMS 2 M ).…”

“…To obtain sdB models with the same core mass for the series with diffusion, we had to remove the envelope before He-ignition. Still, the core mass is large enough so that He is ignited after mass-loss, see Hu et al (2007).…”