The evolutionary properties and peculiar thermal pulses of metal-deficient low-mass stars

We investigate the evolutionary behavior of low-mass star models with very low original metal content (log Z = -10, -6, -5) and Y = 0.23. The computations have been extended from the main sequence up to the double shell burning phase. Theoretical isochrones on the H-R diagram are presented for a range of ages spanning 7-15 x 10(9) yr. Attention has been paid to understand whether, and to what extent, stellar populations in the quoted metallicity range can produce currently observable RR Lyrae variables, the result being that, apart from an intrinsic scarcity of existing stars with such a composition, evolutionary properties are such that a vanishingly small number of RR Lyrae variables is expected to exist. However, if existing, metal-deficient RR Lyrae stars would present pulsational properties not easily distinguishable from those of standard Population II variables. Double shell burning structures are presented with discussion of the dependence of selected evolutionary features on the original metal content and, in particular, the occurrence of unusually strong He shell flashes following the first reignition of He shell burning. The H shell burning along the He intershell accretion phase and the following He shell burning reignition of the M = 0.8 M., log Z = -10 model are discussed in detail. It is found that, even at the first episode of He shell reignition, the burning grows in so strong a flash that a convective shell develops at once, becoming large enough to interact with the H shell, so that a large amount of fresh protons is suddenly injected into the high-temperature He-burning region. After the third episode of hydrogen ingestion, it has not been possible to follow in detail the development of the instability since a time-dependent treatment of the convection would be required. It is however estimated that during the flash a maximum luminosity of L(He) = 2.5 x 10(8) L. would have been attained by He burning alone, in contrast with a maximum of L(He) = 7.5 x 10(4) L. attained at the first pulse of an equivalent structure with a normal metallicity. Whatever the further evolution of such an episode (a partial or total envelope ejection not being excluded), it remains the fact that a large amount of hydrogen is burned out at a very high temperature and in a very short time. Such an He shell flash could be regarded as the low-energy (intrisically nonexplosive) counterpart of the He detonation occurring in a more massive, more degenerate He shell, which induces a sub-Chandrasekhar explosion of the underlying white dwarf, suggesting that, in this last occurrence too, the hydrogen possibly surrounding the He layer could be similarly burned out at the moment of the He ignition. Thus, the present results may help to understand the lack of the hydrogen signature in the spectra of Type Ia supernovae, if such supernovae are mainly produced by sub-Chandrasekhar He detonations.