The unusually large population of Blazhko variables in the globular cluster NGC 5024 (M53)

TL;DR

This study identifies a remarkably high proportion of Blazhko effect RR Lyrae stars in NGC 5024, revealing the largest known population of such variables in a globular cluster and providing insights into their characteristics and distribution.

Contribution

It reports the discovery of a large population of Blazhko RR Lyrae stars in NGC 5024, significantly increasing known cases and analyzing their properties and implications for stellar evolution.

Findings

66% of RRc stars show Blazhko modulation

37% of RRab stars show Blazhko modulation

Blazhko amplitudes range from 0.02 to 0.39 mag

Abstract

We report the discovery of amplitude and phase modulations typical of the Blazhko effect in 22 RRc and 9 RRab type RR Lyrae stars in NGC 5024 (M53). This brings the confirmed Blazhko variables in this cluster to 23 RRc and 11 RRab, that represent 66% and 37% of the total population of RRc and RRab stars in the cluster respectively, making NGC 5024 the globular cluster with the largest presently known population of Blazhko RRc stars. We place a lower limit on the overall incidence rate of the Blazhko effect among the RR Lyrae population in this cluster of 52%. New data have allowed us to refine the pulsation periods. The limitations imposed by the time span and sampling of our data prevents reliable estimations of the modulation periods. The amplitudes of the modulations range between 0.02 and 0.39 mag. The RRab and RRc are neatly separated in the CMD, and the RRc Blazhko variables are on averge redder than their stable couterparts; these two facts may support the hypothesis that the HB evolution in this cluster is towards the red and that the Blazhko modulations in the RRc stars are connected with the pulsation mode switch.