Evolution of asymptotic giant branch stars
As stars evolve through the AGB phase, they cool down and at the same time grow in brightness by burning their nuclear fuel faster and faster. The stars can cool down to such an extent that dust begins to condensate in the outer convective envelope. The dust formation together with large pulsations drives a superwind off the surface of these stars. AGB stars lose mass at such an extremely high rate that the whole outer envelope of hydrogen, enriched with heavy metals, is quickly lifted off. What is left in the centre is a white dwarf star. The EU-funded project POSTAGBINGALAXIES (Evolved stars: Clues to the chemical evolution of galaxies) aimed to offer insights into the nature of this excessive mass loss. Project scientists gathered observations of the circumstellar envelope of AGB and post-AGB stars. The Toruń catalogue of galactic post-AGB and related objects offered a starting point for accumulating information about their mass-loss history and the chemistry inside envelopes after the AGB phase. The existing Toruń catalogue was extended and new catalogues for post-AGB stars in the Large and Small Magellanic Clouds were created to facilitate the study of the AGB phase in galaxies with different metallicities. In addition, a sample of cool R Coronae Borealis stars was analysed using spectroscopic methods to better understand the later stages of stellar evolution. Through observations of dicarbon and carbon monoxide molecules, project scientists determined the chemical and physical conditions in the circumstellar shell of post-AGB stars. These conditions were then used for the modelling of other molecular abundances, such as lithium, under the assumption of local thermal-dynamic equilibrium. The computed models of observed changes in molecular lines showed that the existence of AGB stars' superwind cannot be explained by the stellar effective temperature. Instead, project results argue that the envelope structure – in particular, the velocity profiles – should be among the parameters that determine the mass loss rate.
Keywords
Asymptotic giant branch, stars, white dwarf stars, galaxies, circumstellar, Magellanic Clouds
