Spectra of population II cepheid variable stars

Abstract

The physical properties of the population II cepheids have been studied using the following material: photoelectric observations of Arp, low dispersion spectra of Joy, high dispersion spectra taken by Sanford and Abt, and numerous moderate dispersion spectra obtained by the author. Radial velocity curves for ten cepheids are shown. It is shown that the velocity curves for all population II cepheids with periods greater than 15 days are probably discontinuous. The difference between the RV tauri star an the star whose velocity and light curves repeat well is shown to be due to a delay in the new outrush of gas during alternate cycles of the RV tauri star. Displacement curves are derived for eight cepheids that probably have discontinuous velocity curves. The photoelectric observations are interpreted to show that the surface temperature M5 No. 42 varies from 7200°K to 4800°K and of W Virginis from 6200°K to below 4800°K. The electron pressure for the two stars varies from 100 dynes/cm2 to 0.1 dynes/cm2 and 10 dynes/cm2 to less than 0.1 dynes/cm2 respectively. The changes in radii obtained applying Steffan's Law to the changes in temperature and luminosity agree reasonably well with integration of the velocity curve. Values of the surface gravity obtained from the colors at maximum radius and from the deceleration of the velocity curve yield masses in the vicinity of one to three solar masses. The use of the period density relation is consistent with masses between 1.2 and 2.0 solar masses. The spectra of the population II cepheids are generally of type A5 to F0 at earliest a F5 to G0 at latest. No correlation of spectral type with period can found. A correlation between spectral type and light curve seems to be present. Consideration of the emission lines and of certain absorption lines that are effected by dilution leads to the conclusion that proper model for W Virginis must contain a shock wave moving out through the atmosphere of the star.