Apparent discrepancy between pulsation and evolution masses for Cepheids

Results of new theoretical pulsation calculations in the linear nonadiabatic approximation are reported. Both Q-values and the location of blue edges in the H-R diagram are presented for masses and compositions that are of interest in connection with classical Cepheids. It is shown that estimates of Cepheid masses obtained by comparing observational data with theoretical Q-values can be made to agree with mass estimates obtained by comparing with results of theoretical evolution calculations if any one of the following is true: (a) absolute magnitudes assigned to Cepheids in open clusters and associations are systematically too dim by 0.3 mag, (b) the conversion from a raw mean color to surface temperature T, overestimates log T, by A log T 0.025, (c) the helium abundance assigned to evolutionary models is too low by A Y 0.14. Comparison with theoretical "blue edges" in both the H-R diagram and the period-magnitude diagram suggests that a combination of solutions (a) and (b) with emphasis on (a) provides the most likely resolution of the apparent discrepancy. One consequence of this resolution is a possible increase by 0.2 .3 mag in distance moduli for external galaxies and therefore possibly an increase of perhaps 10-15 percent in estimates of the Hubble time.