Dynamics of Type II Cepheids
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
To further understand the variations of Type II Cepheids, a selection of BL Her, W Vir and RV Tauri stars have been examined photometrically and spectroscopically, focusing on their dynamical variations. These three star types have been found to vary in a more similar fashion at the photospheric level, as traced by their spectral behaviour, than in their photometry. A selection of twenty Type II Cepheids have been monitored from 1999-2002 at Mount John University Observatory. High resolution spectroscopic and BVRI photometric observations have been made. The photometry has been used to monitor colour changes, long-term variations and establish periods for the stars. For the brighter stars, the photometry has been supplemented by visual estimates from a variety of sources. Period changes have been found for ST Pup and U Mon. Changes of the phase of light minimum, depending on the photometric bandpass used, have necessitated an alternate zero-point for establishing phase. Mean Fe I radial velocity curves have been phased on the photometric periods and a spline curve fitted. The derivative of this curve has then been found and the resulting acceleration curve minimum associated with minimum photospheric radius used to rephase the data. Under this phasing the shock-associated effects of line broadening, splitting, emission and the B magnitude peak occur around phase 0.0 for all the stars examined. The metallic lines velocities observed are also very similar for the W Vir and RV Tauri stars, despite the differences in the depths of the light minima. Where the differences enter is in the strength of the shock-related effects. In the W Virginis stars, the shock effects of splitting, emission and strong peaks in B magnitude become more pronounced at longer periods and these same shock effects are observed in the RV Tauri stars. However) the strength of these effects the RV Tauri stars, as seen by the emission and B magnitude peaks varies with the depths of the light curve minima. This does not however correlate overall with the occurrence of a deep or shallow minimum but varies depending on the star. Line level effects are observed in all the Type II Cepheids examined, with the Hα absorption line velocities having a higher amplitude and reaching maximum velocity later in phase that the metallic lines. Within the metallic velocity differences and phase lags are also observed between species. The magnitude of these differences increases the longer-period stars. Long-term photometric variations were observed in a selection of RVb stars. Reddening during the long-term minima was observed in SX Cen, AI Sco, U Mon and IW Car but not AR Pup. AI Sco and U Mon were found to show enhanced Hα emission during their long-term minima. RU Cen was found to show a strong blue absorption component in Hα over a single pulsational cycle, but this was not found to correlate with any photometric changes.