A Spectroscopic Survey for Binary Stars in the Globular Cluster NGC5053

Abstract

We carried out a radial velocity survey for spectroscopic binaries in the low density globular cluster NGC5053. Our sample contains a total of 77 cluster member giant and subgiant stars with visual magnitudes of 14.5-18.6. Of these 77 stars, 66 stars have on average of 3-4 measurements with a total of 236 velocities. A typical velocity error per measurement is about 3 km/s. The stars in our sample are spatially distributed from the cluster center out to 10 arcminutes in radius (4.5 core radii). Among these 66 stars with multiple velocity measurements, we discovered 6 spectroscopic binary candidates. Of these six candidates, one was discovered as a binary previously by Pryor et al. (1991) and candidate ST is a binary with a very short-period of three to five days. We obtained three possible orbital solutions for binary candidate ST by fitting its radial velocity data. These orbital solutions are consistent with star ST being a cluster member, although its spectrum has much stronger MgI triplet absorption line than that of a typical low-metallicity giant star. We estimated the fraction of binary systems which may have been missed from our detection due to unfavorable binary orbital configurations. Our survey has a binary discovery efficiency of 29% for systems with 3d < P < 10yr, 0.125 < q < 1.75 and eccentric orbits (0 < e < 1). This yields a binary frequency of 29%. With Kolmogorov-Smirnov (K-S) tests, the hypothesis of a frequency in NGC5053 higher than 50% is rejected with a confidence level higher than 85%. We conclude that the binary frequency in NGC5053 is 21-29% with 3d < P < 10yr, 0.125 < q < 1.75. This value is somewhat higher than estimates for other clusters by various surveys. This is perhaps related to the fact that NGC5053 is relatively dynamically young compared to other clusters. (shortened version)