BL Her Stars are primarily Population II Cepheids with periods between one and three days. On the H-R diagram they are about 1m magnitude brighter than the RR Lyrae variables but below the W Virginis variables. They are not as blue as the so-called anomalous Cepheids. Typically these stars are metal deficient, but notable exceptions are the prototype BL Her and V553 Cen with solar type abundances. BL Herculis stars are frequently observed to have light curve bumps on increasing or decreasing light. These bumps appear to follow a Hertsprung progression much like the more massive classical bump Cepheids. They may have the same mechanism, either echos from the stellar center, or a near resonance between the funamental (period pi0) and second overtone (period pi2) radial pulsation modes. This resonance hypothesis based on nonlinear calculations of Stobie predicts bumps before maximum light for pi2/pi sub 0 less than or equal to 0.50, and bumps after maximum light for Pi2/Pi sub 0 greater than or equal to 0.50.
NASA STI/Recon Technical Report N
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- Cepheid Variables;
- Stellar Luminosity;
- Celestial Mechanics;