The Blue Straggler and Main-Sequence Binary Population of the Low-Mass Globular Cluster Palomar 13

We present high-precision VI photometry of stars from the middle of the giantbranch to about 5 magnitudes below the main-sequence turnoff in the globularcluster Palomar 13 based on images obtained with the Keck II 10m telescope. Wetabulate a complete sample of blue stragglers in the cluster out to about 18core radii. The blue straggler population is significantly more centrallyconcentrated than the giant star sample, which is in turn significantly morecentrally concentrated than the main-sequence star sample. Palomar 13 has oneof the highest specific frequencies of blue stragglers of any known globularcluster, but the specific frequency of blue stragglers in the outskirts of thecluster does not increase as has been seen in denser clusters. We also identifya group of faint blue stragglers (bluer than the turnoff, but having about thesame magnitude) that outnumbers the brighter stragglers by more than a factorof 2. The cluster's color-magnitude diagram shows a large excess of stars tothe red of the main sequence, indicating that the cluster's binary fraction isat least 30% +/- 4%, which appears to be similar to that of the low-masscluster E3 but significantly higher than that of the more massive clusters Pal5 and NGC 288.Comment: 36 pages, 13 figures, 1 electronic table, accepted to A