Pulsations along the Asymptotic Giant Branch (AGB): a global view

Abstract

In three preceding articles, we studied the relation between the light curves of Mira variables and their stellar properties. The analysis was restricted to a sample of stars offering high density and quality of observations in the visible, allowing for the definition of simple parameterizations of the light curves. Clear relations were revealed between the evolution of the stars along the AGB and parameters such as the width of the light pulse or the deviation of its ascending branch from a sine wave. However, the restriction to Mira variables was found to be a major drawback preventing a complete picture to be drawn. The present article overcomes this deficiency by reviewing a large sample of semi-regular and irregular variables. It gives evidence for two families of stars having different progenies. The first family enters the AGB already pulsating with amplitudes at the scale of one unit of magnitude. It keeps cooling down and expanding, while pulsating with increasing amplitudes and periods and decreasing width of the light pulse; its progeny is dominated by some of the Mira variables studied in the three preceding articles. The second family enters the AGB before displaying significant pulsations; some of its progeny may leave the AGB without entering the Thermally Pulsing Branch and experiencing strong TDU events; others may instead become Mira variables studied in the three preceding articles and pulsating with a broader light pulse. The possible transition of both families from oxygen-rich to carbon-rich is studied and described. Comments on the interpretation of these results in the context of current state-of-the-art knowledge are presented. While the stars of the first family tend to have larger initial masses than the stars of the second family, details of the properties of the convective envelope seem also to play a role in defining which families a star belongs to.