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

Abstract

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

10 pages, 7 figures, 3 tables, accepted for publication in ApJ

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Cited by in corpus (5)

• galstreams: A Library of Milky Way Stellar Stream Footprints and Tracks
• An all-sky proper motion map of the Sagittarius stream using Gaia DR2
• The Milky Way's Shell Structure Reveals the Time of a Radial Collision
• The course of the Orphan Stream in the Northern Galactic Hemisphere traced with Gaia DR2
• Detecting the Figure Rotation of Dark Matter Halos with Tidal Streams