Semiregular Variable Stars

Studies of semiregular variable stars by different authors are examined and the main theoretical and observational problems associated with these stars are reviewed. Their evolutionary status and a possible connection with long-period variables such as Mira Ceti are discussed. Individual objects belonging to different types of semiregular variables are described in detail. The periods of all the stars considered here have been studied by periodogram and/or wavelet analysis. The previously developed additional photometric classification criteria are supplemented by the study of the phase portraits of the smoothed brightness oscillations. It is also noted that all the methods for analyzing photometric behavior applied to stars such as Mira Ceti can also be applied to semiregular variables. On leaving the Main sequence the stars pass through a Cepheid instability stage, turning into radially pulsating variables of a delta Cepheus type. Semiregular variable giants and supergiants of spectral classes F-K, which are customarily denoted by the symbol SRd, may be related to these stars. As they evolve further, the high luminosity variables fall into the region of red supergiants and are converted into SRc (or Lc) variables, while the variables with lower luminosity are converted into SRab semiregular (or irregular Lb) variables of later spectral classes. The RV Tau variables are a class of low-mass (with masses on the order of one solar mass) pulsating F-K supergiants ( M-v=-3(m)divided by 5(m)) which may lie on the short-time evolutionary stage of a transition from red giant to protoplanetary nebula, which explains the scarcity of this type of variability.