LONG-TERM NONLINEAR THERMAL EFFECTS IN THE PULSATION OF MIRA VARIABLES

Dynamical simulations of long-period variables calculated for greater than 500 yr, i.e., longer than the typical thermal timescale for the s tellar envelope, reveal clear and significant changes in the internal energy (entropy) structure of the pulsating envelope. These changes ca use the envelope to switch from a nearby steady pulsation in the first harmonic overtone to stable pulsation in the fundamental mode. Howeve r, as a result of the changes in the envelope's structure, the period of this fundamental mode is significantly shorter than that predicted for the initial, static envelope by linear mode analysis. Our major co nclusion, derived from these simulations, is that the pulsation featur e of LPVs, including their periods, cannot be predicted by the common linear analysis but only by use of full, nonlinear hydrodynamic models calculated for longer time intervals than has been the common practic e.