Research Note

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

¹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
e-mail: nkains@stsci.edu
² Qatar Environment and Energy Research Institute, Qatar Foundation, PO Box 5825 Doha, Qatar
³ Instituto de Astronomía, Universidad Nacional Autónoma de Mexico, Ciudad Universitaria CP 04510, Mexico
⁴ SUPA School of Physics & Astronomy, University of St. Andrews, North Haugh, St. Andrews, KY16 9SS, UK
⁵ European Southern Observatory, Karl-Schwarzschild Straße 2, 85748 Garching bei München, Germany

Received: 23 June 2015
Accepted: 30 August 2015

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. 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 alone, 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.