Secular perion changes and fundamental parameters of long--period Cepheids

TL;DR

This study uses hydrodynamic models to analyze long-period Cepheids, deriving their fundamental parameters and comparing theoretical predictions with observational data to improve understanding of their evolution and physical properties.

Contribution

It provides new hydrodynamic simulations of long-period Cepheids during core helium burning, linking pulsation periods and period changes to fundamental stellar parameters.

Findings

Theoretical radius estimates agree with observations within 3-10%.

Derived fundamental parameters for seven long-period Cepheids.

Established a link between period change rates and stellar evolution.

Abstract

Hydrodynamic computations of nonlinear Cepheid pulsation models with periods from 20 to 100 day on the evolutionary stage of core helium burning were carried out. Equations of radiation hydrodynamics and time--dependent convection were solved with initial conditions obtained from selected models of evolutionary sequences of population I stars with initial masses from $8M_\odot$ to $12.5M_\odot$. For each crossing of the instability strip the pulsation period $\Pi$ and the rate of period change $\dot\Pi$ were derived as a function of evolutionary time. Comparing results of our computations with observational estimates of $\Pi$ and $\dot\Pi$ we determined fundamental parameters (the age, the mass, the luminosity and the radius) of seven long--period Cepheids. Theoretical estimates of the stellar radius are shown to agree with radius measurements by the Baade--Wesselink technique within 3\% for RS~Pup and GY~Sge whereas for SV~Vul the disagreement between theory and observations does not exceed 10\%.