Speaker
Description
We construct a Period-Luminosity (Period-$M_{\rm bol}$) diagram for Mira variables in the Milky Way (MW) and compare it with those in the Large and Small Magellanic Clouds (LMC and SMC) in order to investigate the evolution of asymptotic giant branch (AGB) stars. The sample of MW Miras is taken from the General Catalogue of Variable Stars. Distances are estimated using the 3.4$\mu$m Period-Luminosity relation derived from LMC Miras and calibrated with VLBI parallaxes of MW Miras. The C-rich/O-rich classification is performed using a two-color diagram based on mid- and far-infrared photometry.
In the Period-Luminosity diagram, O-rich and C-rich Miras show different distributions. MW O-rich Miras evolve from the Entrance Line, determined by the transition from the first-overtone mode to the fundamental mode, to the Terminal Line, beyond which the stars become optically invisible. O-rich Miras occupy the region between these lines rather evenly. These lines are determined rather sharply, independently of metallicity. In the LMC, the distribution of lower-mass O-rich Miras overlaps that of MW Miras, although the corresponding mass range is significantly different.
Stars above the occupation region of lower-mass O-rich stars should become carbon stars through the Third Dredge-Up. In actuality, however, carbon stars are observed at lower luminosities than expected. This is due to extinction caused by the circumstellar dust envelope, and the stars reduce their luminosities through backwarming. This indicates that they do not form as low-luminosity objects but become fainter after becoming carbon stars. This is the solution to the carbon star mystery.
The present work reveals that, in studying the evolution of AGB stars, we should properly take into account the effects of dust in the circumstellar envelope.