Models of long-period variables of the globular cluster 47 Tuc

TL;DR

This study models long-period variable stars in 47 Tuc, revealing their pulsation periods, luminosities, and the reasons for irregular oscillations, with implications for stellar evolution understanding.

Contribution

It provides detailed stellar evolution and pulsation models for 47 Tuc's variables, clarifying their period-luminosity relations and instability causes, which were previously not well understood.

Findings

Pulsation periods range from 5 to 70 days during eAGB and TP-AGB stages.

Hydrodynamic models align with observed period-luminosity relations.

Low-mass Mira variables exhibit irregular oscillations and instability.

Abstract

Stellar evolution computations were carried out for stars with a main sequence mass $M_\mathrm{ZAMS}=0.86M_\odot$ and initial metal abundance $Z=0.003$ and $Z=0.004$. Selected models of evolutionary sequences were used for calculation of radial pulsations in the RGB, eAGB and TP-AGB evolutionary stages. Not all pulsating red giants of the globular cluster 47 Tuc are shown to belong to the Mira variables because the lower limit of pulsation periods at the TP-AGB stage is $\approx 70$ day, whereas during the eAGB evolutionary stage the periods of radial oscillations range from $\approx 5$ to $\approx 40$ day. Periods and luminosities of hydrodynamic models of eAGB and TP--AGB pulsating stars locate along the common period-luminosity relation. Small masses of Mira variables in the globular cluster 47 Tuc ($0.54M_\odot\le M\le 0.70M_\odot$) is the main reason for irregular large-amplitude oscillations and the dynamical instability of outer stellar layers at pulsation periods $\Pi > 200$ day.