Pulsating Red Giants in a Globular Cluster: 47 Tucanae

Volume 49 number 2 (2021)

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John R. Percy

Department of Astronomy and Astrophysics, and Dunlap Institute for Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4, Canada; john.percy@utoronto.ca

Prateek Gupta

Department of Astronomy and Astrophysics, and Dunlap Institute for Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4, Canada; john.percy@utoronto.ca

Abstract

We have carried out time-series analysis of a sample of 12 pulsating red giants (PRGs) in the globular cluster 47 Tuc, using observations from the ASAS-SN database, and the AAVSO software package VStar. Most (11/12) of the stars were classified by ASAS-SN as semiregular (SR). We have determined pulsation periods (P) for all 12 of them, and “long secondary periods” (LSPs) for 11 of them. This confirms that LSPs are common in Population II stars. In the context of recent explanations for LSPs, our results imply that many Population II red giants have accreting planetary companions, surrounded by dust. In over half the stars, the period given in the ASAS-SN catalogue is actually the LSP, not the pulsation period. About half the stars show some evidence of a second pulsation period, presumably a second pulsation mode. The amplitudes of the pulsation periods vary by up to a factor of 3.4, on time scales of 10 to 35 pulsation periods (median value 18). The average ratio of LSP to P is 9.0, but the values cluster around 5 and 10. This suggests that some of the stars are pulsating in a lower-order mode, but most are pulsating in a higher-order mode, and half are pulsating in both. The complex variability of the stars in our sample is similar to that of nearby PRGs with a solar composition. The fact that there are about 150 Galactic globular clusters, each with potentially-variable red giants, means that there are many opportunities for studies, like ours, by students and by amateur astronomers with an interest in data analysis, as well as by professional astronomers.