Oscillatory secular modes: The thermal micropulses

TL;DR

This paper investigates the secular stability of helium-burning shells in intermediate-mass stars, revealing that thermal micropulses are driven by an oscillatory secular instability, providing a linear stability perspective on their cyclic behavior.

Contribution

It offers a linear stability analysis of thermal micropulses in 3 solar-mass stars, linking their cyclic nature to an underlying oscillatory secular instability.

Findings

Thermal micropulses are driven by an oscillatory secular instability.

The cyclic nature of thermal pulses can be explained by linear instability theory.

Models show these instabilities occur during the transition from core to shell burning.

Abstract

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.