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
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.
Key words: stars: AGB and post-AGB / stars: interiors / stars: oscillations
© ESO, 2007