[ITAL]BV[/ITAL] Photometry of RR Lyrae Variables in the Globular Cluster M2 (NGC 7089)

Open Access
Publisher Website
Google Scholar
Abstract
We present BV CCD photometry for 30 RR Lyrae variables, including 13 newly discovered, in the Oosterhoff type II globular cluster M2. New light curves and ephemerides are presented. Our observations cover magnitudes ranging from the red giant branch tip to ≈3 mag below the turnoff, allowing us to determine accurate fundamental photometric parameters. With the discovery of new RR Lyrae variables, the mean periods of the RRab and RRc variables are Pab = 0.685 ± 0.022 days and Pc =0.333 ± 0.013 days, respectively. The number ratio of the RRc type variables to the total number of RR Lyrae type variables is n(c)/n(ab + c) = 0.40. The RR Lyrae pulsational mode transition occurs between the fundamental period of 0.528 days and 0.556 days. From star counts along the horizontal branch (HB), we obtain two HB color distribution indices, (B - R)/(B + V + R) = 0.92 and B2/(B + V + R) = 0.46. The period-change rates are also derived. The mean period-change rate from six RRab variables is β = 0.06 ± 0.04 days Myr-1, which agrees well with the predictions of Lee's 1991 model. Our color-magnitude diagram shows a very extended blue HB tail, and there are no significant blue HB gaps in M2.