Hunting the Parent of the Orphan Stream II: The First High-Resolution Spectroscopic Study

Abstract

We present the first high-resolution spectroscopic study on the Orphan Stream for five stream candidates, observed with the Magellan Inamori Kyocera Echelle (MIKE) spectrograph on the Magellan Clay telescope. The targets were selected from the low-resolution catalog of Casey et al (2013a): 3 high-probability members, 1 medium and 1 low-probability stream candidate were observed. Our analysis indicates the low and medium-probability target are metal-rich field stars. The remaining three high-probability targets range over $\sim$1 dex in metallicity, and are chemically distinct compared to the other 2 targets and all standard stars: low [$\alpha$/Fe] abundance ratios are observed, and lower limits are ascertained for [Ba/Y], which sit well above the Milky Way trend. These chemical signatures demonstrate that the undiscovered parent system is unequivocally a dwarf spheroidal galaxy, consistent with dynamical constraints inferred from the stream width and arc. As such, we firmly exclude the proposed association between NGC 2419 and the Orphan stream. A wide range in metallicities adds to the similarities between the Orphan stream and Segue 1, although low [$\alpha$/Fe] abundance ratios in the Orphan stream are in tension with high [$\alpha$/Fe] values observed in Segue 1. Open questions remain before Segue 1 could possibly be claimed as the `parent' of the Orphan stream. The parent system could well remain undiscovered in the southern sky.

14 pages, 3 figures. Journal version

References in corpus (15)

• The Milky Way's Circular Velocity Curve to 60 kpc and an Estimate of the Dark Matter Halo Mass from Kinematics of ~2400 SDSS Blue Horizontal Branch Stars
• New Grids of ATLAS9 Model Atmospheres
• The accretion origin of the Milky Way's stellar halo
• Detection of a 63 Degree Cold Stellar Stream in the Sloan Digital Sky Survey
• An Orphan in the "Field of Streams"
• The stellar halo of the Galaxy
• Detection of a 60 Degree-Long Dwarf Galaxy Debris Stream
• Chemical abundances of the Milky Way thick disk and stellar halo II.: sodium, iron-peak and neutron-capture elements
• A 22 Degree Tidal Tail for Palomar 5
• Chemically Tagging the HR1614 Moving Group
• Is Ursa Major II the Progenitor of the Orphan Stream?
• Indirect Dark Matter Detection Limits from the Ultra-Faint Milky Way Satellite Segue 1
• Tracing the Orphan Stream to 55 kpc with RR Lyrae Stars
• Spitzer, Gaia, and the Potential of the Milky Way
• The Aquarius Co-Moving Group is Not a Disrupted Classical Globular Cluster

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• The R-Process Alliance: Chemo-Dynamically Tagged Groups II. An Extended Sample of Halo $r$-Process-Enhanced Stars
• Hubble Space Telescope Proper Motions of Individual Stars in Stellar Streams: Orphan, Sagittarius, Lethe, and the New "Parallel" Stream
• The course of the Orphan Stream in the Northern Galactic Hemisphere traced with Gaia DR2
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• Milky Way archaeology using RR Lyrae and type II Cepheids I. The Orphan stream in 7D using RR Lyrae stars
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