Aims. Stars in the narrow mass range of about 2.5 and 3.5 M ⊙ 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 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. Methods. Linear stability analyses were performed on a large number of 3 M⊙ star models toward 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. Results. 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.Instituto de Astrofísica de La PlataFacultad de Ciencias Astronómicas y Geofísica
