The proper motion of Palomar 5
  Palomar 5 (Pal 5) is a faint halo globular cluster associated with narrow
tidal tails. It is a useful system to understand the process of tidal
dissolution, as well as to constrain the potential of the Milky Way. A
well-determined orbit for Pal 5 would enable detailed study of these open
questions. We present here the first CCD-based proper motion measurement of Pal
5 obtained using SDSS as a first epoch and new LBT/LBC images as a second,
giving a baseline of 15 years. We perform relative astrometry, using SDSS as a
distortion-free reference, and images of the cluster and also of the Pal 5
stream for the derivation of the distortion correction for LBC. The reference
frame is made up of background galaxies. We correct for differential chromatic
refraction using relations obtained from SDSS colors as well as from
flux-calibrated spectra, finding that the correction relations for stars and
for galaxies are different. We obtain mu_alpha=-2.296+/-0.186 mas/yr and
mu_delta=-2.257+/-0.181 mas/yr for the proper motion of Pal 5. We use this
motion, and the publicly available code galpy, to model the disruption of Pal 5
in different Milky Way models consisting of a bulge, a disk and a spherical
dark matter halo. Our fits to the observed stream properties (streak and radial
velocity gradient) result in a preference for a relatively large Pal 5 distance
of around 24 kpc. A slightly larger absolute proper motion than what we measure
also results in better matches but the best solutions need a change in
distance. We find that a spherical Milky Way model, with V_0=220 km/s and V_(20
kpc), i.e., approximately at the apocenter of Pal 5, of 218 km/s, can match the
data well, at least for our choice of disk and bulge parametrization.
