Milky Way archaeology using RR Lyrae and type II Cepheids I. The Orphan stream in 7D using RR Lyrae stars

Abstract

We present a chemo-dynamical study of the Orphan stellar stream using a catalog of RR~Lyrae pulsating variable stars for which photometric, astrometric, and spectroscopic data are available. Employing low-resolution spectra from the Sloan Digital Sky Survey (SDSS), we determined line-of-sight velocities for individual exposures and derived the systemic velocities of the RR~Lyrae stars. In combination with the stars' spectroscopic metallicities and \textit{Gaia} EDR3 astrometry, we investigated the northern part of the Orphan stream. In our probabilistic approach, we found 20 single mode RR~Lyrae variables likely associated with the Orphan stream based on their positions, proper motions, and distances. The acquired sample permitted us to expand our search to nonvariable stars in the SDSS dataset, utilizing line-of-sight velocities determined by the SDSS. We found 54 additional nonvariable stars linked to the Orphan stream. The metallicity distribution for the identified red giant branch stars and blue horizontal branch stars is, on average, $-2.13\pm 0.05$ dex and $-1.87\pm 0.14$ dex, with dispersions of 0.23 and 0.43dex, respectively. The metallicity distribution of the RR~Lyrae variables peaks at $-1.80\pm 0.06$ dex and a dispersion of 0.25dex. Using the collected stellar sample, we investigated a possible link between the ultra-faint dwarf galaxy Grus II and the Orphan stream. Based on their kinematics, we found that both the stream RR~Lyrae and Grus II are on a prograde orbit with similar orbital properties, although the large uncertainties on the dynamical properties render an unambiguous claim of connection difficult. At the same time, the chemical analysis strongly weakens the connection between both. We argue that Grus II in combination with the Orphan stream would have to exhibit a strong inverse metallicity gradient, which to date has not been detected in any Local Group system.

22 pages, 12 figures, Accepted for publication in A&A, final version

References in corpus (43)

• Array Programming with NumPy
• Modules for Experiments in Stellar Astrophysics (MESA)
• galpy: A Python Library for Galactic Dynamics
• Determining stellar atmospheric parameters and chemical abundances of FGK stars with iSpec
• The SEGUE Stellar Parameter Pipeline. I. Description and Initial Validation Tests
• The Observed properties of Dark Matter on small spatial scales
• The Catalina Surveys Periodic Variable Star Catalog
• The accretion origin of the Milky Way's stellar halo
• Detection of a 63 Degree Cold Stellar Stream in the Sloan Digital Sky Survey
• Modern stellar spectroscopy caveats
• The SEGUE Stellar Parameter Pipeline. II. Validation with Galactic Globular and Open Clusters
• An Orphan in the "Field of Streams"
• The SEGUE Stellar Parameter Pipeline. III. Comparison with High-Resolution Spectroscopy of SDSS/SEGUE Field Stars
• Detection of a 60 Degree-Long Dwarf Galaxy Debris Stream
• Globular Cluster Streams as Galactic High-Precision Scales - The Poster Child Palomar 5
• The Streams of the Gaping Abyss: A population of entangled stellar streams surrounding the Inner Galaxy
• The SEGUE Stellar Parameter Pipeline. IV. Validation with an Extended Sample of Galactic Globular and Open Clusters
• Forensics of Subhalo-Stream Encounters: The Three Phases of Gap Growth
• Gaps in globular cluster streams: giant molecular clouds can cause them too
• Weak Galactic halo--dwarf spheroidal connection from RR Lyrae stars
• Fourteen candidate RR Lyrae star streams in the inner Galaxy
• Spectroscopic Comparison of Metal-Rich RRab Stars of the Galactic Field with Their Metal-Poor Counterparts
• Metallicity and $\alpha$-element Abundance Gradients along the Sagittarius Stream as Seen by APOGEE
• Is Ursa Major II the Progenitor of the Orphan Stream?
• The age of the Milky Way inner stellar spheroid from RR Lyrae population synthesis
• On the RR Lyrae stars in globulars: IV. $\omega$ Centauri Optical UBVRI Photometry
• Standard Galactic Field RR Lyrae II: A Gaia DR2 calibration of the period-Wesenheit-metallicity relation
• On the Use of Field RR Lyrae as Galactic Probes. II. A new $\Delta$S calibration to estimate their metallicity
• Insights from the Outskirts: Chemical and Dynamical Properties in the outer Parts of the Fornax Dwarf Spheroidal Galaxy
• Chemical abundances of solar neighborhood RR Lyrae stars
• The RRC Stars: Chemical Abundances and Envelope Kinematics
• Kinematics of the Palomar 5 stellar stream from RR Lyrae stars
• Anomalous Double-Mode RR Lyrae Stars in the Magellanic Clouds
• Blazhko-type modulation in the double-mode RR Lyrae stars of the OGLE Galactic bulge collection
• Dwarf spheroidals in the M81 Group - Metallicity distribution functions and population gradients
• Chemical Analysis of the Ultra-Faint Dwarf Galaxy Grus~II. Signature of high-mass stellar nucleosynthesis
• Purveyors of fine halos. II. Chemodynamical association of halo stars with Milky Way globular clusters
• Newly Discovered RR Lyrae Stars in the SDSSXPanXSTARRS1XCatalina Footprint
• Blazhko modulation in the infrared
• Peculiar double-periodic pulsation in RR Lyrae stars of the OGLE collection. II. Short-period stars with dominant radial fundamental mode
• The course of the Orphan Stream in the Northern Galactic Hemisphere traced with Gaia DR2
• Inverted metallicity gradients in two Virgo cluster star-forming dwarf galaxies: evidence of recent merging?
• The Axial Rotation and Variable Macroturbulence of RR Lyrae and Red Horizontal Branch Stars

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