Evolution of cepheids with pulsationally driven mass loss

Abstract

We have run models of intermediate mass stars (5, 6, 7, and 8 M_□ with Y=0.28, Z=0.02) with pulsationally driven mass loss occurring in the Cepheid instability strip. We used the new ¹²C(α,γ)¹⁶O rates of Caughlan et. al. (1985). The enhanced rate extends the tip of the blue loop, allowing the 5 and 6 Mp models to re-enter the Cepheid strip, unlike the models calculated using the old rates (Becker, 1981). We attempted to see if mass loss during the Cepheid stage could redden the tip of the blue loop sufficiently to place it inside the instability strip, thereby "trapping" the star, and allowing it to lose mass for a period of time significantly longer than the normal crossing time. Our results show that this mechanism does in fact work for a 7 MD star with mass loss rates as low as 5×10-7 MO yr-1. Observations of P-Cygni profiles in Cepheids indicate that this rate is not unreasonable. This behavior acts to reduce the discrepancy between the evolutionary and pulsation-derived masses for Cepheids. Another consequence is that the rates of period change are decreased, bringing them into better agreement with observed values.