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 <ASTROBJ>PG 1336-018</ASTROBJ>. 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 <ASTROBJ>PG 1336-018</ASTROBJ> 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 alpha-formalism) based on the energy equation, and an alternative description (the gamma-formalism) using the angular momentum equation. Results: We find that if the common-envelope evolution is described by the alpha-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<SUB>o</SUB>, and the sdB progenitor initial mass to be below ~2 M<SUB>o</SUB>. However, the results for the gamma-formalism are less restrictive, and a broader sdB mass range (0.3-0.8 M<SUB>o</SUB>) is possible in this case. Future seismic mass determination will give strong constraints on the formation of <ASTROBJ>PG 1336-018</ASTROBJ> and, in particular, on the CE phase.