Oscillatory secular modes: the thermal micropulses

¹ Wetterchrüzstr. 8c, 4410 Liestal, Switzerland e-mail: alfred@gautschy.ch
² Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque S/N (1900) La Plata and Instituto de Astrofisica La Plata (IALP), Argentina

Received: 31 May 2006
Accepted: 6 June 2007

Abstract

Aims.Stars in the narrow mass range of about 2.5 and 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 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.