Multiperiodicity in Mira Stars (Abstract)

Volume 24 number 2 (1996)

John R. Percy

Dalia Bagby

Abstract

(Abstract only) Mira stars are red giants pulsating with periods of 80-1000 days, and with amplitudes of 2.5 magnitudes or more. The pulsation mode of the Mira stars has been controversial for many years; some evidence favors the fundamental mode; other evidence favors the first overtone. If two or more pulsation modes are excited in the same star, it may be possible to deduce from the ratio the pulsation modes which are apparently too long to be normal pulsation periods. Such long periods are found in several other types of cool pulsating stars, including the semi-regular (SR) variables and the RV Tauri stars. The cause of these long term variations is not known. In the present study, we have used the AAVSO's database of times and magnitudes of maximum and minimum brightness of almost 400 bright Mira stars (Kowalsky et al. 1986) to investigate multiperiodicity in these stars. We worked exclusively with the times and magnitudes of maximum, subjecting these to Fourier analysis (and realizing that there would be significant aliasing problems because the data were spaced at regular intervals which were equal to the dominant pulsation period of the star). We found that the power spectra (power versus frequency) of the data could be divided into several types: (i) one dominant peak between 0 and f (the dominant frequency, corresponding to the dominant period P); (ii) one dominant peak at or very close to f; (iii) one dominant peak very close to 0 (corresponding to a very long secondary period); (iv) random peaks indicating no significant secondary period; (v) two peaks of roughly equal height, between 0 and f (of which one is an alias peak). We have looked for correlations between the power spectrum type, and other properties of the stars. We have analyzed the type (iii) stars in some detail, constructing phase diagrams for the long-period modulation of the magnitude of maximum. We have compared the type (v) models of Mira stars, assuming that the peaks are due to interference between two adjacent radial pulsation modes. This assumption appears to be a promising one. The nature of the long secondary periods (the type (iii) power spectra) is still not clear. We acknowledge the role of hundreds of AAVSO observers over the years, and the role of AAVSO Headquarters staff in making their data available. This project was supported by NSERC Canada, the Ontario Work-Study Program, and the Summer Youth Service Canada Program. Reference: Kowalsky, P.A., Percy, J. R., Mattei, J. A., and Waagen, E. O. 1986, J. Amer. Assoc. Var. Star Obs., 15, 236.