Date of this Version
THE ASTROPHYSICAL JOURNAL, 311:305--316,1986 December 1
Fourier decomposition parameters for the photoelectric light curves of 20 short-period type II Cepheids in the field are compared with the models of Hodson, Cox, and King, also subjected to Fourier analysis. The hydrodynamic light curves display sequences in the Fourier phases very reminiscent of the resonance progressions among classical Population I Cepheids with periods less than 10 days. However a handful of type II models with small radii and relatively high period ratios, P2/P0, give Fourier phases which do not seem to follow the resonance sequence. Comparing the models with the type II Cepheid observations, we find very good agreement in both the phase-period and phase-phase diagrams. As in the models, there are a number of observed stars which obviously display a resonance progression and others which may not. Thus, based on the Fourier diagrams, we are led to distinguish three classes of short-period type II Cepheids with periods and period ratios as follows: type II S (1.d0 ≤ P ≤ 1.d6; P2/P0 ≥ 0.53); type II M (1. d 4 ≤ P0 ≤ 2. d0; P2/P0< 0.53); and type II L (P ≥ 2.d0d; P2/P0 ≤ 0.49). An apparent resonance sequence among the II M stars leads us to propose that the resonance center (P2/P0 = 0.5) occurs near 2 days, rather than 1.6 days as suggested previously. We discuss the problems posed for theories of stellar and galactic evolution by the existence of disk, halo, and cluster pulsators with very similar light curves. Comparison of our results with the work of Carson, Stothers, and Vemury, Carson and Stothers, and Petersen and Diethelm, finds a number of points of agreement as well as some contradiction. Further calculations and additional observations, particularly CCD photometry of globular cluster Cepheids, will be necessary to resolve these problems.