Thermal Stability of Rotating Low‐Mass Subgiants and Red Giants

Abstract

A linear analysis of the thermal stability of rotating low-mass stars evolving from the zero-age main sequence (ZAMS) to the red giant branch (RGB) tip has been carried out. Two stellar models are considered: one for 1.2 M☉ with solar metallicity (Z = 0.0188), and the other for 0.8 M☉ with Z = 0.0005. An instability in a thermonuclear burning shell on the RGB in the first of these cases could potentially be related to the phenomenon of Li-rich red giants, while a hydrogen shell instability in the second model may help to explain the observed chemical abundance anomalies in globular cluster red giants. A range of surface rotational velocities in MS stars has been considered, and we have assumed that, beginning at the ZAMS, the stars (including their convective envelopes) rotate differentially with depth. This assumption, which leads to significantly higher centrifugal accelerations in the H-burning shell than with the case of solid-body rotation on the MS, is expected to favour the development of thermal instabilities. However, all of our models for rotating low-mass stars—even those that had much higher rotation rates on the MS than those observed for F-, G-, and K-type dwarfs—were found to be thermally stable throughout their evolution from the ZAMS to the RGB tip. Only when helium burning was ignited to end the first ascent of the giant branch did we find a thermal instability (the helium flash).