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Petersen Diagram for RRd Stars in the Magellanic Clouds

Popielski B. L., Dziembowski W. A., Cassisi S.

Acta Astronomica

2000

Vol. 50, No. 4

491--507

RRd stars from the Magellanic Clouds form a well-defined band in the Petersen diagram. We explain this observed band with our evolutionary and pulsation calculations with assumed metallicity [Fe/H]=(-2,-1.3). Vast majority of RRd stars from LMC is confined to a narrower range of (-1.7,-1.3). The width of the band, at specified fundamental mode period, may be explained by mass spread at given metallicity. The shape of the band reflects the path of RRd stars within the RR Lyr instability strip. We regard the success in explaining the Petersen diagram as a support for our evolutionary models, which yield mean absolute magnitude in the mid of the instability strip, ⟨MV⟩, in the range 0.4 to 0.65 mag implying distance modulus to the LMC of 18.4 mag.

Nonradial Modes in RR Lyr Stars

Dziembowski W. A., Cassisi S.

Acta Astronomica

1999

Vol. 49, No. 3

371--382

We present a survey of nonradial mode properties in evolutionary sequences of RR Lyr star models. Attention is focused on the modes that may be driven by the opacity mechanism and on those that may be excited as a consequence of the 1:1 resonance with the radial pulsation. Qualitatively, all the models share the same properties of nonradial modes. At the quantitative level, the properties are to a large extent determined by the radial mode periods. There is only weak dependence on the star metallicity and no apparent dependence on the evolutionary status, that is on the helium exhaustion in the convective core. In the whole range of RRab and RRc star parameters we find unstable nonradial modes driven by the opacity mechanism. An instability of radial pulsation to a resonant excitation of nonradial oscillations is also a common phenomenon in both types. We discuss a possible role of nonradial modes in amplitude modulation observed in certain RR Lyr stars.