THE PULSATION CONSTANTS OF R-CORONAE-BOREALIS STARS

Results of hydrodynamic computations of nonlinear radial pulsations ar e presented for carbon-rich helium stars with masses 0.7 less than or equal to M/M. less than or equal to 0.9, effective temperatures 6600 l ess than or equal to T-eff less than or equal to 10000 K, and absolute bolometric magnitudes M-bol from -4.(m)5 to -5.(m)0. The opacity and equation of state were calculated for an elemental mixture with relati ve mass abundances of helium Y = 0.88 and heavy elements Z = 0.02, wit h an excess carbon mass abundance Delta Z(C) = 0.1. The pulsation prop erties of the models were determined using the discrete Fourier transf orms of the radii r(t) and velocities U(t) of all Lagrangian layers ov er a time interval covering several hundred pulsation cycles. Radially pulsating carbon-rich helium stars separate into two groups according to their pulsation constant Q. The first group (0.10 < Q < 0.13 days) contains fundamental-mode pulsators. The instability of these stars i s excited in the iron-ionization zone, in layers with temperatures sim ilar to 2 x 10(5) K. Stars belonging to the second group (0.25 < Q < 0 .55 days) pulsate in overtones with k greater than or equal to 1, and their instability is excited in the helium-ionization zone. The radial pulsations become semiregular at T-eff less than or equal to 7500 K. The power spectra of the oscillation moment: of inertia and kinetic en ergy art: dominated by the contributions of the fundamental mode and f irst overtone, whose contributions are roughly comparable. The simulta neous excitation of two pulsation modes is due to the presence of two instability regions: the zones of ionization of iron and helium.