A Cluster’s Last Stand: The Death of Palomar 13

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Abstract
We present a proper-motion, CCD photometric study of stars in the distant halo globular cluster Palomar 13. The absolute proper motion of Pal 13 with respect to the background galaxies, derived from moderate-scale photographic plates separated by a 40 year baseline, is (μα cos δ, μδ) = (2.30, 0.27) ± (0.26, 0.25) mas yr-1. The resultant total space velocity (315 km s-1) implies that Pal 13 is in the inner part of its orbit near perigalacticon. Orbital integration reveals that the cluster possesses an inclined, very eccentric, retrograde orbit. These data confirm that Pal 13 is a paradigm "young halo" globular cluster. The derived proper motions for cluster stars are used to produce membership probabilities and a cleaned CCD UBV catalog for Pal 13. With this data set we have made small revisions to Pal 13's distance, metallicity, position, and light profile. The membership of four previously reported RR Lyrae variables and a disproportionately large group of blue straggler stars is confirmed. As expected, the blue stragglers are centrally concentrated. The small size of this cluster, combined with the shape of its light profile, which shows a clear departure from a classical King function beyond the tidal radius, suggests that Pal 13 is in the final throes of destruction. This could explain the large blue straggler specific frequency, as destructive processes would preferentially strip less massive stars.
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