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We present a multi-epoch Hubble Space Telescope (HST) study of stellar propermotions (PMs) for four fields spanning 200 degrees along the Sagittarius (Sgr)stream: one trailing arm field, one field near the Sgr dwarf spheroidal tidalradius, and two leading arm fields. We determine absolute PMs of dozens ofindividual stars per field, using established techniques that use distantbackground galaxies as stationary reference frame. Stream stars are identifiedbased on combined color-magnitude diagram and PM information. The results arebroadly consistent with the few existing PM measurements for the Sgr galaxy andthe trailing arm. However, our new results provide the highest PM accuracy forthe stream to date, the first PM measurements for the leading arm, and thefirst PM measurements for individual stream stars; we also serendipitouslydetermine the PM of the globular cluster NGC~6652. In the trailing-arm field,the individual PMs allow us to kinematically separate trailing-arm stars fromleading-arm stars that are 360 degrees further ahead in their orbit. Also, inthree of our fields we find indications that two distinct kinematicalcomponents may exist within the same arm and wrap of the stream. Qualitativecomparison of the HST data to the predictions of the Law & Majewski andPenarrubia et al. N-body models show that the PM measurements closely followthe predicted trend with Sgr longitude. This provides a successful consistencycheck on the PM measurements, as well as on these N-body approaches (which werenot tailored to fit any PM data).

Hubble Space Telescope Proper Motions along the Sagittarius Stream: I. Observations and Results for Stars in Four Fields