Oscillatory secular modes: the thermal micropulses
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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
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