Evolution and pulsation period change in the Large Magellanic Cloud Cepheids

Abstract

Theoretical estimates of the pulsation period change rates in LMC Cepheids are obtained from consistent calculation of stellar evolution and nonlinear stellar pulsation for stars with initial chemical composition X = 0.7, Z = 0.008, initial masses 5M ⊙ ≤ M ZAMS ≤ 9M ⊙ and pulsation periods ranged from 2.2 to 29 day. The Cepheid hydrodynamical models correspond to the evolutionary stage of thermonuclear core helium burning. During evolution across the instability strip in the HR diagram the pulsation period Π of Cepheids is the quadratic function of the evolution time for the both fundamental mode and first overtone. Cepheids with initial masses M ZAMS ≥ 7M ⊙ pulsate in the fundamental mode and the period change rate \(\dot \Pi\) varies nearly by a factor of two for both crossings of the instability strip. In the period-period change rate diagram the values of Π and \(\dot \Pi\) concentrate within the strips, their slope and half-width depending on both the direction of the movement in the HR-diagram and the pulsation mode. For oscillations in the fundamental mode the half-widths of the strip are \(\delta log\dot \Pi = 0.35\) and \(\delta log\dot \Pi = 0.2\) for the first and the secon crossings of the instability strip, respectively. Results of computations are comparedwith observations of nearly 700 LMC Cepheids. Within existing observational uncertainties of \(\dot \Pi\) the theoretical dependences of the period change rate on the pulsation period are in a good agreement with observations.