BL Herculis model pulsations III. Livermore opacities

Details BL Herculis model pulsations III. Livermore opacities BL Herculis model pulsations III. Livermore opacities

May 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...285..213b&link_type=abstract

Astronomy and Astrophysics 285, 213-219 (1994)

Astronomy and Astrophysics

Astrophysics

12

Stars: Cepheids, Stars: Oscillations Of Hydrodynamics

Scientific paper

We revisit the nonlinear pulsational properties of BL Her stars with the new Livermore opacities. In contrast to classical Cepheid and RR Lyrae models, the Petersen diagrams for the BL Her models show very little sensitivity to the switch from the now obsolete Los Alamos opacities to the new Livermore opacities. In addition, the light curves and radial velocity curves are very similar, as are therefore their Fourier decomposition coefficients. This work confirms the major results of a previous survey, viz. that the 2:1 resonance between the fundamental mode and the second overtone, which, in classical Cepheids, shapes the 'Hertzsprung progression' of the Fourier decomposition parameters, is also visible here, but much more in the radial velocity than in the magnitude coefficients. Because of the large sensitivity of the BL Her light and radial velocity curves to mass and luminosity (and to a lesser extent metallicity) the BL Her stars form a rather inhomogeneous group compared to the classical Cepheids. Consequently the resonance structure is washed out in a global sample of BL Her models. We confirm as well the presence of another interesting feature of the BL Her models, also related to a resonance, viz. the existence of a narrow, =~ 0.5deg wide window of RV Tau-like alternations, that are strictly periodic, however. The BL Her first overtone blue edge is close, to within =~-100 to +50 K of the fundamental blue edge, and all other overtones are stable. Their radial velocity Fourier coefficients merge with those of the fundamental pulsators making a discrimination difficult. Their magnitude Fourier coefficients, on the other hand, are perhaps slightly better separated. The few observational data on =~ 1deg period BL Her stars that are currently available do not allow a clear-cut dichotomy into a first overtone and a fundamental group. However, the two types of pulsators may be distinguished on the basis of their amplitudes which are very small for the first overtones.

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