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A Study of Amplitude Variability in a Sample of Pulsating Red Giants

Volume 53 number 2 (2025)

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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
Ana Florella Pago
Department of Astronomy and Astrophysics, and Dunlap Institute for Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4, Canada; ana.pago@mail.utoronto.ca

Abstract

Amplitude variability is one of several non-periodic phenomena found in pulsating red giants (Long Period Variables, LPVs). In this paper, a sample of 19 LPVs with significant and/or complex amplitude variability was analyzed using long-term observations (mostly visual) from the AAVSO International Database. Properties such as the stars’ amplitude variation, mean magnitude variation, presence of overtone periods (and aliases and harmonics) were quantified using the Fourier and wavelet routines in the AAVSO’s VStar time-series analysis package. The stars were then categorized on the basis of three possible causes of amplitude variability as proposed by Kiss et al. (A&A Suppl., 145, 283, 2000): closely-spaced beating periods; rotation-pulsation interaction; and pulsation mode switching, either one time only or repeatedly. It was found that nine stars showed beating; two showed rotation-pulsation interaction; four showed both beating and rotation-pulsation interaction; one showed both beating and mode-switching; one showed evidence of all three causes; and two did not appear to fit any of the three causes. Beating therefore appears to be the most common cause of amplitude variation. Our analysis was complicated by observational effects such as limited accuracy of visual data, and the presence of alias periods caused by the seasonal gaps in the data. Nevertheless, it appears that the nature and causes of amplitude are many and complex. Systematic, sustained observations of these stars should definitely continue.