The RR Lyrae Instability Strip in the Split Horizontal Branch Globular Cluster NGC 6569
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Date
2015
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Abstract
The first calibrated broadband BVI time-series photometry is obtained for the RR Lyrae variable stars in the split horizontal branch (HB) cluster NGC 6569. As a result, a sizeable population of 27 RR Lyrae variables has been identified. An eclipsing binary in the RR Lyrae instability strip was also found, which is most likely a field star. The 12 fundamental-mode RR Lyrae variables (RR0) have an average period of [P]RR0= 0.57±0.02 d and the 15 first-overtone variables (RR1) have an average period of [P]RR1=0.30±0.01 d. The mean periods of the RR Lyrae stars are consistent with an Oosterhoff I designation, and the number ratio of RR1- to RR0-type variables, N1/NRR=0.56, is more consistent with an Oosterhoff II designation. Compared to the M3 variables, the NGC 6569 RR Lyrae variables are not shifted to a longer periods at a given amplitude, making it unlikely that these stars are He-enhanced. The similarity in the periods and amplitudes between the RR Lyrae stars in NGC 6569 and the bulge field RR Lyrae population may indicate similar formation conditions.