R Centauri: an Unusual Mira Variable in a He-Shell Flash (Abstract)

Volume 29 number 1 (2001)

George Hawkins

AAVSO Headquarters 25 Birch Street Cambridge, MA 02138

Janet A. Mattei

AAVSO Headquarters 25 Birch Street Cambridge, MA 02138

Grant Foster

AAVSO Headquarters 25 Birch Street Cambridge, MA 02138

Abstract

(Abstract only) R Cen is an oxygen-rich Mira variable with a period of 546 days, amplitude of 5.3– 11.8 V, and spectral type of M4e-M8IIe, as listed in the General Catalogue of Variable Stars (Kholopov et al. 1985). It is one of only a few Miras with doublepeaked maxima in its light curve (e.g., Keenan et al. 1974). Our power spectrum shows harmonics up to 8 times the main mode at 546 days. The most likely explanation for the double-peaked light curve is a resonance between the fundamental and an overtone mode, with fo = 2ff similar to that observed in bump Cepheids (Buchler et al. 1990), or a half integer resonance as suspected in Cepheids with period doubling (Kovács and Buchler 1988). Visual observations from 1918 to 2000 from the AAVSO International Database show two unusual properties: 1) the pulsational amplitude has decreased by 3 magnitudes since 1950 (Figure 1), and 2) the period of the dominant mode has been steadily decreasing from 550 days at JD 2434000 (1951) to its present value of 505– 510 days (Figure 2). Figure 2 shows the results of a statistical analysis using the weighted wavelet Z-transform (Foster 1996) and a Cleanest Fourier analysis (Foster 1995). We suggest that the recent behavior is due to R Cen entering a He-shell flash in its interior, as the period decrease of 1 day/yr is similar to that of other Miras thought to be undergoing a He-shell flash, such as R Hya and R Aql (Wood and Zarro 1981), and T UMi (Mattei and Foster 1995; Gál and Szátmary 1995). A star in a He-shell flash may show a period change lasting only a few tens of years if it is caught right after the flash begins, or the change could last a few hundred to a few thousand years if it appears after the luminosity of the He-shell flash reaches the surface (Wood and Zarro 1981). However, R Cen should be monitored closely in the future, as some Mira variables (such as S Her and T Cep, see Wood and Zarro 1981) show short term period changes that are unrelated to a He-shell flash. We give further discussion of this unusual Mira variable in Hawkins, Mattei, and Foster (2001).