Using BV photometry to distinguish between type I and type II cepheids.
FERNIE J.D. and EHLERS P.
Abstract (from CDS):
We use Johnson BV photometry of Galactic type I and type II Cepheids in a variety of diagrams to see whether the two populations can be adequately separated. The diagrams include the Fourier parameters φ21, φ31, φ41, R21, R31, R41, the loop width in the V-(B-V) plane, the relative phasing of V and B-V extrema, and the amplitude ratios of B-V to V, all versus period. We found the R_i1_ diagrams useless in this regard, while a plot of the phasing parameter versus amplitude ratio was best, closely followed by the φ_i1_ diagrams. A major difficulty was arriving at a sample of Cepheids that could be reliably taken to be definitely type II. We found it necessary to use only stars that would have been more than 2 kpc from the Galactic plane had they been type I for this sample, and even then it is uncertain whether such overlap between types as we find in our diagrams is due to inadequacy of the tests or is due to type I stars' having contaminated our type II sample. We point out evidence strongly suggesting that type I stars can be found at least up to 2 kpc from the plane. Selection effects in our sample of type II Cepheids resulted in only one BL Herculis star (short-period type II) being included, so our conclusions basically apply only to W Virginis stars.
Stars: Variables: Cepheids - Stars: Population II