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. Th e computations have been extended from the main sequence up to the dou ble 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 populati ons in the quoted metallicity range can produce currently observable R R Lyrae variables, the result being that, apart from an intrinsic scar city of existing stars with such a composition, evolutionary propertie s are such that a vanishingly small number of RR Lyrae variables is ex pected to exist. However, if existing, metal-deficient RR Lyrae stars would present pulsational properties not easily distinguishable from t hose of standard Population II variables. Double shell burning structu res are presented with discussion of the dependence of selected evolut ionary features on the original metal content and, in particular, the occurrence of unusually strong He shell flashes following the first re ignition of He shell burning. The H shell burning along the He intersh ell accretion phase and the following He shell burning reignition of t he 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 gr ows in so strong a flash that a convective shell develops at once, bec oming large enough to interact with the H shell, so that a large amoun t of fresh protons is suddenly injected into the high-temperature He-b urning region. After the third episode of hydrogen ingestion, it has n ot been possible to follow in detail the development of the instabilit y since a time-dependent treatment of the convection would be required . It is however estimated that during the flash a maximum luminosity o f 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. What ever the further evolution of such an episode (a partial or total enve lope ejection not being excluded), it remains the fact that a large am ount of hydrogen is burned out at a very high temperature and in a ver y short time. Such an He shell flash could be regarded as the low-ener gy (intrisically nonexplosive) counterpart of the He detonation occurr ing in a more massive, more degenerate He shell, which induces a sub-C handrasekhar 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 hydr ogen signature in the spectra of Type Ia supernovae, if such supernova e are mainly produced by sub-Chandrasekhar He detonations.