Probing the driving region in hot subdwarf stars through nonadiabatic asteroseismology: the principle of the method

Context. The k-mechanism that drives pulsations in hot subdwarf stars is closely linked to the action of diffusive processes, including radiative levitation, which modulate the local contents of heavy elements in the stellar envelope. Iron, in particular, is important for its dominant contribution to the Z-bump feature in the envelope opacity, although other iron-peak elements, such as nickel for instance, may also play a significant role. Aims. Our main goal is to evaluate the potential of nonadiabatic asteroseismology for studying diffusive phenomena in these stars. In this exploratory work, we consider iron as a test case to establish the principle of the method. Methods. From model experiments, we explore the behavior of the pulsation engine under assumed local iron enrichments in the Z-bump layers, and we show how this may be related, through the period range of unstable modes, to some observed properties, i.e., the range of periods effectively detected, characterizing hot pulsating subdwarf stars. This connects nonadiabatic physics with observables. Results. We demonstrate that the strength of the pulsation engine is predominently controled by the amount of heavy metals (iron in our experiments) present at the Z-bump location, the chemical composition in other parts of the star being irrelevant to the process. We also show that this property can be used to probe directly the amount of metals present in this particular region, irrespective of the physical process involved to produce such abundances. In particular, we show that limits on the abundances needed for the onset of pulsations can be estimated. These can even be derived for individual stars based on their observed pulsation period range, as illustrated with two well-studied pulsating sdB stars: PG 1325 + 101 and Feige 48. Conclusions. We conclude by emphasizing the strong potential of nonadiabatic asteroseismology for hot subdwarf stars, which may hold the key for better understanding diffusive and competing mixing processes in stellar envelopes.