The evolutionary status of Population II Cepheids

Several grids of post-horizontal-branch evolutionary models have been constructed assuming a range in either total mass or initial core mass on the horizontal branch. The less luminous Population II Cepheids (BL Herculis stars) are identified with stars of small envelope mass undergoing 'blueward noses' from the asymptotic giant branch due to readjustments between the hydrogen- and helium-burning shell luminosities following the exhaustion of helium in the core. The more luminous Cepheids (W Virginis stars) are identified with stars undergoing blueward loops from the second giant branch in response to helium shell flashes or finally evolving to the blue as the hydrogen-burning shell nears the stellar surface. The models predict relatively too few variables spread over too large a range in luminosity in the brighter group. Mass loss on the second giant branch is suggested to help resolve this problem. The assumption of a range in total mass on the zero-age horizontal branch produces a slightly better fit to the observed luminosity distribution of Population II Cepheids than does the assumption of a range in initial core mass.