A seismic approach to test different formation channels of subdwarf B stars

Context. There are many unknowns surrounding the formation of subdwarf B stars, and different formation channels are considered to be possible. Nevertheless, seismic models up to date assume that an sdB star is a post-helium-flash-core surrounded by a thin inert layer of hydrogen. Aims. We examine an alternative formation channel, in which the sdB star originates from a massive (> ∼2 M ) Red Giant with a non-degenerate He-core. Although these sdB stars may evolve through the same region of the log g − Teff diagram as the canonical post-flash sdB stars, their interior structure is rather different. We are interested in how this affects their pulsation modes and whether this can be observed. Methods. Using detailed stellar evolution calculations we construct sdB models from both formation channels. The iron enhancement in the driving region due to diffusion, which causes the excitation of the modes, is approximated by a Gaussian increase. The pulsation modes and frequencies are calculated with a non-adiabatic pulsation code. Results. A detailed comparison of two sdB models from different channels, but with same log g and Teff , shows that the mode excitation is significantly different. The excited frequencies are smaller for the post-flash sdB star than for the post-non-degenerate sdB star. This is a direct result of the differing chemical compositions in the envelope. A more general comparison between two grids of models, tells us that for most post-non-degenerate sdB stars, their excited frequencies cannot be well-matched with the frequencies, excited or non-excited, of a post-flash sdB star. In the rare event that an acceptable seismic match is found, additional information, such as mode identification and log g and Teff determinations, allows us to discriminate between the two formation channels.