The RR Lyrae variables and Horizontal Branch of NGC6656 (M22)

TL;DR

This study presents calibrated UBVI photometry and revised classifications of RR Lyrae stars in NGC6656 (M22), confirming its Oosterhoff II status and providing insights into its horizontal branch morphology and reddening.

Contribution

First calibrated broadband photometry and updated RR Lyrae classifications for M22, with detailed analysis of its Oosterhoff type and horizontal branch characteristics.

Findings

RR0 mean period = 0.66 days

RR1 mean period = 0.33 days

E(B-V) = 0.36 mag

Abstract

The first calibrated broadband UBVI time-series photometry is presented for the RR Lyrae variable stars in NGC6656 (M22), with observations spanning a range of twenty-two years. We have also redetermined the variability types and periods for the RR Lyrae stars identified previously by photographic observations, revising the number of fundamental-mode RR Lyrae variables (RR0) to 10 and the number of first-overtone variables (RR1) to 16. The mean periods of the RR0 and RR1 variables are <P>_{RR0}=0.66 \pm 0.02 d and <P>_{RR1}=0.33 \pm 0.01 d, respectively, supporting an Oosterhoff II classification for the cluster. The number ratio of RR1- to all RR-type variables is N_1/N_{RR}=0.61, also consistent with an Oosterhoff II designation. Both the RR Lyrae stars' minimum light colors and the blue edge of the RR Lyrae instability strip suggest E(B-V)=0.36 \pm 0.02 mag toward M22. Regarding the HB morphology of M22, we find (B-R)/(B+V+R)=+0.97 \pm 0.1 and at least one "gap" located in an unusual part of the blue HB, in the middle of the so-called hot HB stars.