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.

arXiv comment10 pages, 7 figures, 3 tables, accepted for publication in ApJ

References in corpus (44)

• Maps of Dust IR Emission for Use in Estimation of Reddening and CMBR Foregrounds
• Measuring Reddening with SDSS Stellar Spectra and Recalibrating SFD
• Local Kinematics and the Local Standard of Rest
• The Dartmouth Stellar Evolution Database
• The Photometric Performance and Calibration of the HST Advanced Camera For Surveys
• A 2MASS All-Sky View of the Sagittarius Dwarf Galaxy: I. Morphology of the Sagittarius Core and Tidal Arms
• The Field of Streams: Sagittarius and its Siblings
• Mass models of the Milky Way
• The Sagittarius Dwarf Galaxy: a Model for Evolution in a Triaxial Milky Way Halo
• Constraining the Milky Way potential with a 6-D phase-space map of the GD-1 stellar stream
• Detection of a 63 Degree Cold Stellar Stream in the Sloan Digital Sky Survey
• An Orphan in the "Field of Streams"
• Four New Stellar Debris Streams in the Galactic Halo
• How Lumpy is the Milky Way's Dark Matter Halo?
• Precession of the Sagittarius stream
• Tidal Streams as Probes of the Galactic Potential
• Clumpy Streams from Clumpy Halos: Detecting Missing Satellites with Cold Stellar Structures
• The Sagittarius Streams in the Southern Galactic Hemisphere
• Evidence for a Triaxial Milky Way Dark Matter Halo from the Sagittarius Stellar Tidal Stream
• Dark Matter Sub-Halo Counts via Star Stream Crossings
• Substructure in Tidal Streams; Tributaries in the Anticenter Ring
• The M31 Velocity Vector. I. Hubble Space Telescope Proper Motion Measurements
• The Orbit of the Orphan Stream
• Forensics of Subhalo-Stream Encounters: The Three Phases of Gap Growth
• Stellar Streams and Clouds in the Galactic Halo
• Properties of Dark Subhaloes from Gaps in Tidal Streams
• The Pal 5 Star Stream Gaps
• Discovery of a cold stellar stream in the ATLAS DR1 data
• What's Up in the Milky Way? The Orientation of the Disc Relative to the Triaxial Halo
• Is Ursa Major II the Progenitor of the Orphan Stream?
• The Sagittarius Stream and Halo Triaxiality
• Tracing the Orphan Stream to 55 kpc with RR Lyrae Stars
• Kinematics and Chemistry of Stars Along the Sagittarius Trailing Tidal Tail and Constraints on the Milky Way Mass Distribution
• The Velocity Anisotropy of Distant Milky Way Halo Stars from Hubble Space Telescope Proper Motions
• A Pan-STARRS1 View of the Bifurcated Sagittarius Stream
• Detecting the disruption of dark-matter halos with stellar streams
• On the genealogy of the Orphan Stream
• A statistical method for measuring the Galactic potential and testing gravity with cold tidal streams
• Hubble Space Telescope Proper Motions along the Sagittarius Stream: I. Observations and Results for Stars in Four Fields
• Exposing Sgr tidal debris behind the Galactic disk with M giants selected in WISE$\cap$2MASS
• Hunting the Parent of the Orphan Stream: Identifying Stream Members from Low-Resolution Spectroscopy
• Hunting the Parent of the Orphan Stream II: The First High-Resolution Spectroscopic Study
• Are Complex A and the Orphan Stream related?
• An Orphan No Longer? Detection of the Southern Orphan Stream and a Candidate Progenitor

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• The Spur and the Gap in GD-1: Dynamical evidence for a dark substructure in the Milky Way halo
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• Absolute HST Proper Motion (HSTPROMO) of Distant Milky Way Globular Clusters: Galactocentric Space Velocities and the Milky Way Mass
• The slight spin of the old stellar halo
• The Proper Motion Field Along the Magellanic Bridge: a New Probe of the LMC-SMC Interaction
• 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
• Connecting the Milky Way potential profile to the orbital timescales and spatial structure of the Sagittarius Stream
• HALO7D II: The Halo Velocity Ellipsoid and Velocity Anisotropy with Distant Main Sequence Stars
• The Virgo Overdensity Explained
• The course of the Orphan Stream in the Northern Galactic Hemisphere traced with Gaia DR2
• SMHASH: Anatomy of the Orphan Stream using RR Lyrae stars
• Detecting the Figure Rotation of Dark Matter Halos with Tidal Streams
• Astrometry with the Wide-Field InfraRed Space Telescope
• A tangle of stellar streams in the north Galactic cap
• Fitting the density substructure of the stellar halo with MilkyWay@home