HYDRODYNAMIC MODELS FOR PULSATING BL HERCULIS-TYPE VARIABLES

A grid of hydrodynamic limit-cycle models is computed for pulsating BL Her variables with masses M = 0.55, 0.6, 0.65M., luminosities 106 les s than or equal to L-bol/L. less than or equal to 169, and effective t emperatures 5500 less than or equal to T-eff less than or equal to 700 0 K. The OP tables are used to compute the opacity kappa(R) for the hy drogen and helium mass fractions X = 0.7 and Y = 0.299. Irrespective o f the stellar mass, the pulsation instability in the first overtone is typical of stars with pulsation periods Pi < 1 day, while the radial pulsations for Pi > 1 day are attributable to the instability of the f undamental mode. Approximation formulas are derived which relate the p ulsation constant Q to the mass-to-radius ratio (MIR) of the star. It is shown that alternating pulsations with variable amplitude may arise from the 2 Pi(0) = 3 Pi(1) resonance between the fundamental mode and the first overtone at certain mass-to-radius ratios. The pulsations o f these stars are also characterized by good repetition of the pulsati on motions, but only in an interval that covers two cycles of the fund amental mode. The secondary bump, which is observed in the light and r adial-velocity curves of BL Her stars with periods from 1.3 to 3 days, is associated with the pressure wave generated in the region of parti al ionization of HeII at maximum compression of the stellar envelope.