Stellar Evolution Along the Asymptotic Giant Branch as Revealed by the Shape of Miras’ Visual Light Curves

Abstract A new analysis of a sample of visual light curves of Mira variables is presented. The curves cover the past four decades and are selected from the AAVSO database as including a large number of high-density observations. The aim is to offer a more quantitative and more systematic picture than available from earlier studies. The results corroborate earlier descriptions and reveal new correlations between the shapes of the light curves and the evolution of the star along the asymptotic giant branch (AGB). A family of nearly sinusoidal curves associated with M spectral types and displaying no sign of having experienced third-dredge-up events is identified with confidence. A detailed study of its properties is presented and used to suggest possible interpretations. All other curves are distinct from this family and start departing from it by displaying a broader luminosity minimum, progressively taking the form of a hump climbing the ascending branch. Possible relations between such progression and the evolution of the star along the AGB are discussed. New correlations between parameters defining the shape of the light curve and the state of the star are revealed; while the average trend is established with confidence, deviations from it cause a significant scatter of the parameters of the curves. Comments aimed at shedding light on the underlying physics are presented together with speculative interpretations, in the hope that they could encourage and inspire new studies, in particular based on models of the inner star dynamics.