Pulsating Red Giants in a Globular Cluster: ω Centauri

摘要

We have carried out light-curve and time-series analysis of a sample of 16 pulsating red giants (PRGs) in the globular cluster ω Cen, using observations from the ASAS-SN database, and the AAVSO software package VStar. Of the 16 stars, 1 was classified by ASAS-SN as Mira (M), 5 as semiregular (SR), and 10 as "long secondary period" (LSP), i.e. the dominant period was an LSP. We have determined pulsation periods (P) for all of them, secondary pulsation periods for 3, possible secondary pulsation periods for 4, and LSPs for 8. This confirms that LSPs are common in Population II stars. In the context of a recent model for LSPs, this implies that many Population II PRGs had planetary companions which accreted gas and dust to become brown dwarfs or low-mass stars, now enshrouded by dust. In this model, the LSP is the orbital period of the hypothetical companion. The amplitudes of the pulsation periods vary by up to a factor of 3.4 on a median time scale of 18 pulsation periods, for reasons unknown. The ratios of LSP/P cluster around 4 and 8, presumably depending on whether P is a fundamental mode or first overtone period. We have augmented our sample with a few stars from the literature to plot period-luminosity relations. Sequences for LSPs, fundamental, and first-overtone pulsation periods are visible. Our results show that the complex variability of the PRGs in ω Cen is similar to that of red giants in other stellar systems, and in the field. In Appendix A, we give results for a few red giants in NGC 6712, which we obtained as a prelude to the ω Cen project.