RR Lyrae mode switching in globular cluster M 68 (NGC 4590)

TL;DR

This study investigates the mode-switching behavior of RR Lyrae stars in globular cluster M 68, revealing new cases and revisiting previous ones through homogeneous analysis of observational data.

Contribution

It provides a detailed, homogeneous analysis of RR Lyrae mode switching in M 68, identifying new cases and clarifying previous reports of pulsation behavior.

Findings

V45 switched from double-mode to first-overtone mode in 2013.

V7's fundamental mode amplitude is increasing over time.

V33 lost a pulsation mode between 1950 and 1986.

Abstract

We build on our detailed analysis of time-series observations of the globular cluster M 68 to investigate the irregular pulsational behaviour of four of the RR Lyrae stars in this cluster. M 68 is one of only two globular clusters in which mode-switching of RR Lyrae stars has previously been reported, and we discuss one additional case, as well as a case of irregular behaviour, and we briefly revisit the two previously reported cases with a homogeneous analysis. We find that in 2013, V45 was pulsating in the first-overtone mode only, despite being previously reported as a double-mode (fundamental and first overtone) pulsator in 1994, and that the amplitude of the fundamental mode in V7 is increasing with time. We also suggest that V21 might not have switched pulsation modes as previously reported, although the first overtone seems to be becoming less dominant. Finally, our analysis of available archival data confirms that V33 lost a pulsation mode between 1950 and 1986.