Nonlinear convective pulsation models of type II Cepheids

We present a grid of nonlinear convective pulsation models of type-II Cepheids: BL Her stars, W Vir stars and RV Tau stars. The models have 0.6M⊙ and cover a wide range of luminosities, effective temperatures and chemical compositions. The most important result is the detection of two domains in the HR diagram, in which period doubling effect is present. First domain, with pulsation periods in between 2 d and 5.5 d, was also detected in earlier radiative computations. The first star showing the period doubling effect with period in the predicted range (≈ 2.4 d) was discovered recently. The second domain extends for periods longer than 9.5 d but so far no period-doubled type-II Cepheid that could be associated with this domain was discovered. We also detect very small domains in which modulation of pulsation is possible. The origin of these phenomena may be assocaited with half-integer resonances between radial pulsation modes. In the most luminous models a dynamical instability is detected, which indicates that pulsation driven mass loss is important process occurring in type-II Cepheids.

The Extended Atmosphere and Evolution of the RV Tau Star R Scuti

We identify H2O, CO, SiO and CO2 bands in near-infrared ISO/SWS spectra of the RV Tau star R Sct. These molecules originate from an atmosphere around the photosphere. RV Tau stars are commonly considered as post-AGB stars. We argue that R Sct may be a thermal-pulsing AGB star, observed in a helium-burning phase.

Spectral Characteristics of RV Tauri Stars

Spectroscopic properties of RV Tau variables are summarized. We report on our detailed spectroscopic investigation of a sample of RV Tau stars covering all three RV Tau subgroups. Though the observed abundance pattern is similar to those of a few post-AGB stars, we find considerable variation in the observed chemical compositions indicating that there is some non-homogeneity among subgroups of RV Tau variables. Possible scenarios of dust-gas separation in the material ejected by the dusty wind are discussed to explain the observed abundances. This mechanism would work better if the star happens to possess a binary companion. Interestingly, one of our sample stars, EP Lyr, shows long-term variations in radial velocity possibly caused by the presence of a companion.

Irregularity Interpreted as Low Dimension Chaos for Convective Models of W VIR Variables

Linear and nonlinear pulsational properties of convective stellar envelopes relevant for W Vir and RV Tau stars are surveyed. All models show the same trend to pass from regular to irregular behavior when a control parameter is changed (the effective temperature). The transition to irregular pulsation follows well known systematic routes to chaos (as in the radiative case). Some rich structures were found in special cases; they deserve further research. We show that the chaotic behavior is sustained even when convection is taken into account. The effect of the inclusion of time dependent convection shows up mostly as a shift of Kovacs and Buchler (Ap.J 1988) results in the parameters plane (L,Teff) towards more realistic models.