Hubble Space Telescope Proper Motions along the Sagittarius Stream: I.
  Observations and Results for Stars in Four Fields
  We present a multi-epoch Hubble Space Telescope (HST) study of stellar proper
motions (PMs) for four fields spanning 200 degrees along the Sagittarius (Sgr)
stream: one trailing arm field, one field near the Sgr dwarf spheroidal tidal
radius, and two leading arm fields. We determine absolute PMs of dozens of
individual stars per field, using established techniques that use distant
background galaxies as stationary reference frame. Stream stars are identified
based on combined color-magnitude diagram and PM information. The results are
broadly consistent with the few existing PM measurements for the Sgr galaxy and
the trailing arm. However, our new results provide the highest PM accuracy for
the stream to date, the first PM measurements for the leading arm, and the
first PM measurements for individual stream stars; we also serendipitously
determine the PM of the globular cluster NGC~6652. In the trailing-arm field,
the individual PMs allow us to kinematically separate trailing-arm stars from
leading-arm stars that are 360 degrees further ahead in their orbit. Also, in
three of our fields we find indications that two distinct kinematical
components may exist within the same arm and wrap of the stream. Qualitative
comparison of the HST data to the predictions of the Law & Majewski and
Penarrubia et al. N-body models show that the PM measurements closely follow
the predicted trend with Sgr longitude. This provides a successful consistency
check on the PM measurements, as well as on these N-body approaches (which were
not tailored to fit any PM data).
