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We present a multi-epoch Hubble Space Telescope (HST) study of stellar propermotions (PMs) for four fields along the Orphan Stream. We determine absolutePMs of several individual stars per target field using established techniquesthat utilize distant background galaxies to define a stationary referenceframe. Five Orphan Stream stars are identified in one of the four fields basedon combined color-magnitude and PM information. The average PM is consistentwith the existing model of the Orphan stream by Newberg et al. In addition tothe Orphan stream stars, we detect stars that likely belong to other stellarstreams. To identify which stellar streams these stars belong to, we examinethe 2-d bulk motion of each group of stars on the sky by subtracting the PMcontribution of the solar motion (which is a function of position on the skyand distance) from the observed PMs, and comparing the vector of net motionwith the spatial extent of known stellar streams. By doing this, we identifycandidate stars in the Sagittarius and Lethe streams, and a newly-found stellarstream at a distance of ~17 kpc, which we tentatively name the "Parallelstream". Together with our Sagittarius stream study (Sohn et al., 2015, ApJ,803, 56), this work demonstrates that even in the Gaia era, HST will continueto be advantageous in measuring PMs of old stellar populations on astar-by-star basis, especially for distances beyond ~10 kpc.

Hubble Space Telescope Proper Motions of Individual Stars in Stellar Streams: Orphan, Sagittarius, Lethe, and the New "Parallel" Stream