Models of Mira variables of the Large Magellanic Cloud

Abstract

Consistent stellar evolution and nonlinear radial stellar pulsation calculations were carried out for models of asymptotic giant branch stars with initial masses $1.5M_\odot\le M_\mathrm{ZAMS}\le 3M_\odot$ and initial metal abundance $Z=0.006$. All the models are shown to be either the fundamental mode or the first overtone pulsators. The lower limit of the first overtone period increases with increasing mass of the Mira model from $\Pi_{1,\min}\approx 80$ days for $M=1.3M_\odot$ to $\Pi_{1,\min}\approx 120$ days for $M=2.6M_\odot$. The upper limit of the first overtone period and lower limit of the fundamental mode period depend on the stellar structure during mode switching and range from $\Pi_{1,\max}=130$, $\Pi_{0,\min}=190$ days for $M=0.96M_\odot$ to $\Pi_{1,\max}=210$, $\Pi_{0,\min}=430$ days for $M=2.2M_\odot$. The slope of the theoretical period--luminosity relation of Mira variables perceptibly increases with decreasing $Z$. Fourier spectra of the kinetic energy of twelve hydrodynamic models show a split of the fundamental mode maximum into several equidistant components. Frequency intervals between split components fall within the range $0.03 \le \Delta\nu/\nu_0 \le 0.1$. The superposition of radial oscillations with the fundamental mode splitting leads to the long-term amplitude variations with the cycle length from 10 to 30 times longer than the fundamental mode period. A more thorough analysis of hydrodynamic models is required for understanding the origin of the principal pulsation mode splitting.