LIMITS ON CLUSTER BINARIES

An unresolved binary system will be systematically brighter and redder than the primary component alone. These systems may measurably widen the main sequence in the color-magnitude diagram. In this paper we exploit the width of the main sequence in NGC 6341 (M92) and NGC 7099 (M30) from published high-quality photometry to derive limits on the fraction of unresolved binary systems in these cluster fields. We show analytically that a "second sequence" is expected for quite general assumptions about the binary population. Then, developing a maximum likelihood technique for using all the high-quality data in a color-magnitude plane, we analyze results for the two clusters, deriving limits on the binary fraction of approximately 9% and approximately 4%, respectively. In M92 in particular, there is a well-determined fraction, approximately 8% of binary candidate objects. We argue that photometric indices indicate that most of these are due to crowding, but propose that a small component consists of true binaries.