6 v 1 2 3 Ju l 1 99 6 A SPECTROSCOPIC SURVEY FOR BINARY STARS IN THE GLOBULAR CLUSTER NGC 5053

We carried out a radial velocity survey for spectroscopic binaries in the low density globular cluster NGC 5053. 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 ∼3 km s −1. 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 lines than that of a typical low-metallicity giant star. Using a Monte-Carlo simulation method, we estimated the fraction of binary systems which may have been missed from our detection due to unfavorable orbital configurations. With our survey, the binary discovery efficiency is 29% for systems with 3 d ≤ P ≤ 10 yr, 0.125 ≤ q ≤ 1.75 and eccentric orbits (0 ≤ e ≤ 1). This yields a binary frequency of 29%. We also applied Kolmogorov-Smirnov (K-S) tests to the cumulative distributions of maximum velocity variations from the actual measurements and the synthetic velocity data. The results from these tests are consistent with 21−29% binary population with 3 d ≤ P ≤ 10 yr, 0.125 ≤ q ≤ 1.75 in NGC 5053. The hypothesis of a binary frequency in NGC 5053 higher than 50% is rejected with a confidence level higher than 85%. The binary frequency in NGC 5053 derived from our survey is somewhat higher than estimates for other clusters by various surveys. This is perhaps related to the fact that NGC 5053 is relatively dynamically young compared to other clusters. We also argue that the binary population in globular clusters is not significantly deficient compared to binaries in other stellar environments such as open clusters, or to field and low metallicity halo stars.