Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998arep...42..662f&link_type=abstract
Astronomy Reports, Volume 42, Issue 5, September 1998, pp.662-670
Astronomy and Astrophysics
Astronomy
2
Scientific paper
Results of hydrodynamic computations of nonlinear radial pulsations are presented for carbon-rich helium stars with masses 0.7 <= M/M <= 0.9, effective temperatures 6600 <= T_eff <= 10000 K, and absolute bolometric magnitudes M_bol from -4.5m to -5.0m. The opacity and equation of state were calculated for an elemental mixture with relative mass abundances of helium Y = 0.88 and heavy elements Z = 0.02, with an excess carbon mass abundance Z_C = 0.1. The pulsation properties of the models were determined using the discrete Fourier transforms of the radii r(t) and velocities U(t) of all Lagrangian layers over 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 is excited in the iron-ionization zone, in layers with temperatures ~2 x 10^5 K. Stars belong- ing to the second group (0.25 < Q < 0.55 days) pulsate in overtones with k 1, and their instability is excited in the helium-ionization zone. The radial pulsations become semiregular at T_eff <= 7500 K. The power spectra of the oscillation moment of inertia and kinetic energy are dominated by the contributions of the fundamental mode and first overtone, whose contributions are roughly comparable. The simultaneous excitation of two pulsation modes is due to the presence of two instability regions: the zones of ionization of iron and helium.
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