Variable Stars in the LMC Globular Cluster NGC 2257: New Results Based on 2007-08 B,V Photometry

摘要

The variable star population in the Large Magellanic Cloud outer-halo globular cluster NGC 2257 has been reinvestigated using photometry (to~20~(th) mag) of over 400 new B,V CCD images taken with the CTIO 0.9-m telescope on 14 nights in Dec'07 and Jan'08. The derived periods of most of the stars are consistent with the pulsationperiods derived previously, and where there were discrepancies these have been resolved. Accurate Fourier coefficients and parameters were computed for the B and V light curves. Six new variable stars were discovered (V45-V50), including a bright candidate long-period variable star showing secondary oscillations (V45). Examination of archival HST images and previously-published photometry shows that the excess brightness of two bright RR Lyrae stars (V48, V50) is due to contamination from close red giant branch stars. Among the previously known variable stars three doublemode (RRd) RR Lyrae stars (V8, V16 and V34) and several Blazhko variables were discovered. The total number of cluster variable stars now stands at forty-seven: 23 RRab stars, four of which show Blazhko light-curve variations; 20 RRc stars, one showing clear Blazhko variations and another showing possible Blazhko variations; the three RRd stars, all having the dominant period –0.36 day and period ratios P_1/P_0~0.7450, and the LPV located near the tip of the red giant branch. A comparison with similar stars in other environments shows that the RRd stars in NGC 2257 are most similar to those in IC 4499.