The impact of metallicity on the period-luminosity relation of Mira variables

TL;DR

This study models how metallicity influences the period-luminosity relation of Mira variables, revealing that lower metallicity leads to brighter stars and shifts in pulsation periods, impacting distance measurements.

Contribution

It provides the first detailed theoretical analysis of metallicity effects on Mira variables' period-luminosity relation using evolutionary and hydrodynamic models.

Findings

Lower metallicity increases the slope of the period-luminosity relation.

LMC Mira variables are brighter by 0.2-0.5 mag than galactic ones at the same period.

The fundamental mode pulsation period lower limit shifts from 70 to 120 days with metallicity.

Abstract

Evolution of stars with initial masses $M_\mathrm{ZAMS}=1.1M_\odot$, $1.3M_\odot$, $1.5M_\odot$ and relative mass abundances of metals $Z=0.006$ and 0.02 was computed from the main sequence up to the final AGB stage. Selected models of evolutionary sequences were used as initial conditions for solution of the equations of hydrodynamics describing pulsations of red giants, whereas for each evolutionary sequence of Mira variables pulsating in the fundamental mode we determined the theoretical period-luminosity relation. A change in the metal abundance is shown to substantially affect the period-luminosity relation because of significant growth of the slope with decreasing $Z$. In particular, Mira variables of the LMC ($Z=0.006$) are brighter by 0.2-0.5 mag than galactic Mira variables ($Z=0.02$) with same pulsation periods. The low boundary of fundamental mode pulsations changes from $\Pi\approx 70$ day for $Z=0.02$ to $\Pi\approx 120$ day for $Z=0.006$.