Oscillatory secular modes: The thermal micropulses

Stars in the narrow mass range of about 2.5 and 3.5 solar masses can develop a thermally unstable He-burning shell during its ignition phase. We study, from the point of view secular stability theory, these so called thermal micropulses and we investigate their properties; the thermal pulses constitute a convenient conceptual laboratory to look thoroughly into the physical properties of a helium-burning shell during the whole thermally pulsing episode. Linear stability analyses were performed on a large number of 3 solar-mass star models at around the end of their core helium-burning and the beginning of the double-shell burning phase. The stellar models were not assumed to be in thermal equilibrium. The thermal mircopulses, and we conjecture all other thermal pulse episodes encountered by shell-burning stars, can be understood as the nonlinear finite-amplitude realization of an oscillatory secular instability that prevails during the whole thermal pulsing episode. Hence, the cyclic nature of the thermal pulses can be traced back to a linear instability concept.