Modeling the Disruption of the Globular Cluster Palomar 5 by Galactic Tides

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Abstract
In order to understand the extended massive tidal tails of the globular cluster Pal5, its very low mass and velocity dispersion, and its size, which is much larger than the theoretical tidal radius, we performed more than 1000 N-body simulations. Tidal shocks at disk crossings near perigalacticon dominate the evolution of extended low-concentration clusters, resulting in massive tidal tails and often in a quick destruction of the cluster. The overlarge size of Pal5 can be explained as the result of an expansion following the heating induced by the last strong disk shock ~150Myr ago. The models can reproduce the low observed velocity dispersion and the relative fractions of stars in the tails and between the inner and outer parts of the tails. The tidal tails of Pal5 show substantial structure not seen in our simulations. We argue that this structure is probably caused by Galactic substructure, such as giant molecular clouds, spiral arms, and dark-matter clumps. Clusters initially larger than their theoretical tidal limit remain so, because, after being shocked, they settle into a new equilibrium near apogalacticon, where they are unaffected by the perigalactic tidal field. This implies that, contrary to previous wisdom, globular clusters on eccentric orbits may well remain super-tidally limited and hence vulnerable to strong disk shocks, which dominate their evolution until destruction. Our simulations unambiguously predict the destruction of Pal5 at its next disk crossing in \~110Myr. This corresponds to only 1% of the cluster lifetime, suggesting that many more similar systems could once have populated the inner parts of the Milky Way, but have been transformed into debris streams by the Galactic tidal field. (Abridged)Comment: accepted for publication in The Astronomical Journa
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