Long Secondary Periods in Pulsating Red Giants: A Century of Investigation

TL;DR

This study investigates long secondary periods in 103 red giant stars using observational data, confirming their prevalence, typical period ratios, and identifying some stars with bimodal pulsations, thus advancing understanding of stellar variability.

Contribution

The paper provides a comprehensive analysis of long secondary periods in red giants, confirming their characteristics and identifying new pulsation behaviors using historical observational data.

Findings

Long secondary periods peak at a ratio of 10 to pulsation periods.

33 of 37 stars' periods are consistent with previous data.

Identified 16 bimodal pulsating stars.

Abstract

Red giants are unstable to radial pulsation. About a third of them also show a long secondary period, 5 to 10 times the pulsation period. The long secondary periods were recently ascribed to eclipses of the red giant by a low-mass dust-enshrouded companion. Long secondary periods have been known for over a century. In this paper, I use primarily American Association of Variable Star Observers visual and photoelectric observations to look for evidence of long secondary periods in 103 red giant stars listed by Nancy Houk in 1963 as having long secondary periods, based mostly on photographic photometry. I have determined long secondary periods in 37 stars, and upper limits (some of them not very stringent) in 25. In the former, the ratio of long secondary period to pulsation period peaks strongly at 10, which suggests that most of the stars are pulsating in the first overtone. The loong secondary periods are consistent with those o0f Houk in 33 of the 37 stars. I have identified 16 stars as bimodal pulsaters;their period ratios are consistent with previous observational and theoretical results. For 14 stars,the periods in the General Catalogue of Variable Stars are incorrect or absent.