The Frequency of Binary Stars in the Core of 47 Tucanae

Differential time series photometry has been derived for 46422 main-sequencestars in the core of 47 Tucanae. The observations consisted of near-continuous160-s exposures alternating between the F555W and F814W filters for 8.3 days in1999 July with WFPC2 on the Hubble Space Telescope. Using Fourier and othersearch methods, eleven detached eclipsing binaries and fifteen W UMa stars havebeen discovered, plus an additional ten contact or near-contact non-eclipsingsystems. After correction for non-uniform area coverage of the survey, theobserved frequencies of detached eclipsing binaries and W UMa's within 90arcseconds of the cluster center are 0.022% and 0.031% respectively. Theobserved detached eclipsing binary frequency, the assumptions of a flat binarydistribution with log period and that the eclipsing binaries with periodslonger than about 4 days have essentially their primordial periods, imply anoverall binary frequency of 13 +/- 6 %. The observed W UMa frequency and theadditional assumptions that W UMa's have been brought to contact according totidal circularization and angular momentum loss theory and that the contactbinary lifetime is 10^{9} years, imply an overall binary frequency of 14 +/- 4%. An additional 71 variables with periods from 0.4 - 10 days have been foundwhich are likely to be BY Draconis stars in binary systems. The radialdistribution of these stars is the same as that of the eclipsing binaries and WUMa stars and is more centrally concentrated than average stars, but less sothan the blue straggler stars. A distinct subset of six of these stars fall inan unexpected domain of the CMD, comprising what we propose to call redstragglers.Comment: Accepted for publication in the Astrophysical Journal, 65 pages including 26 figure