Theoretical periods and masses of double-mode dwarf cepheids

With newly calculated opacities for Population I and II stellar compositions, models with radial fundamental mode periods between 0.05 and 0.22 days have been constructed. These models have been used to study the first overtone to fundamental period ratios to compare with dwarf Cepheids (AI Velorum stars). Contrary to previous results, the observed period ratios match the theoretical ones for masses between 1.1 and 2.2 Msun, and these variables, with T/sub e/ between 7000 and 8000 K, all seem to be in the post--main-sequence evolution stage. For the shortest known period dwarf Cepheid, SX Phe, it appears that only Population II compositions with Z=0.001 can produce the observed very large II1/II0 of 0.778 at a mass of 1.1 Msun. The periods for these models have been used to determine the pulsation constant as a function of M, R, L, and T/sub e/. With observed periods and at least approximate surface effective temperatures, the period--mean density and mass-luminosity relations based on the evolution tracks of Mengel and Sweigart for various compositions give theoretical masses also between 1.1 and 2.2 M/sub sun/. As several others have recently concluded, the dwarf Cepheids are apparently only large-amplitude delta Scuti variables possibly with more helium in their surface ionization zones which gives them their larger light variation amplitude. For masses up to about 1.5 M/sub sun/ and for periods less than 0.1 days, it appears possible to have pulsation in most of the instability strip with Y> or =0.1 resulting in no convection zone and the possibility of some diffusion-separated metallicism in pulsating stars