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The Binary Fraction of Stars in Dwarf Galaxies: The Cases of Draco and Ursa Minor

Spencer, Meghin E. and Mateo, Mario and Olszewski, Edward W. and Walker, Matthew G. and McConnachie, Alan W. and Kirby, Evan N. (2018) The Binary Fraction of Stars in Dwarf Galaxies: The Cases of Draco and Ursa Minor. Astronomical Journal, 156 (6). Art. No. 257. ISSN 1538-3881. doi:10.3847/1538-3881/aae3e4.

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Measuring the frequency of binary stars in dwarf spheroidal galaxies (dSphs) requires data taken over long time intervals. We combine radial velocity measurements from five literature sources taken over the course of ~30 years to yield the largest multi-epoch kinematic sample for stars in the dSphs Draco and Ursa Minor. With this data set, we are able to implement an improved version of the Bayesian technique described in Spencer et al. to evaluate the binary fraction of red giant stars in these dwarf galaxies. Assuming Duquennoy & Mayor period and mass ratio distributions, the binary fractions in Draco and Ursa Minor are 0.50_(-0.06)^(+0.04) and 0.78_(-0.08)^(+0.09), respectively. We find that a normal mass ratio distribution is preferred over a flat distribution, and that log-normal period distributions centered on long periods µ_(log P > 3.5) are preferred over distributions centered on short ones. We reanalyzed the binary fractions in Leo II, Carina, Fornax, Sculptor, and Sextans, and find that there is <1% chance that binary fraction is a constant quantity across all seven dwarfs, unless the period distribution varies greatly. This indicates that the binary populations in Milky Way dSphs are not identical in regard to their binary fractions, period distributions, or both. We consider many different properties of the dwarfs (e.g., mass, radius, luminosity, etc.) and find that binary fraction might be larger in dwarfs that formed their stars quickly and/or have high velocity dispersions.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Spencer, Meghin E.0000-0003-1240-1939
Walker, Matthew G.0000-0003-2496-1925
Kirby, Evan N.0000-0001-6196-5162
Additional Information:© 2018. The American Astronomical Society. Received 2018 June 25; revised 2018 August 27; accepted 2018 August 27; published 2018 November 9. The Hectochelle observations reported here were obtained at the MMT Observatory, a joint facility of the University of Arizona and the Smithsonian Institution. The authors would like to thank Jan Kleyna and Mark Wilkinson for sharing their velocity data from K02, K03, and W04 with us. We would also like to thank Josh Simon for allowing us to use his Keck spectra to obtain velocities for the K10 data set. We warmly thank Andrew Szentgyorgyi, Nelson Caldwell, and the rest of the Hectochelle builders and staff. We also thank Marc Lacasse and the MMT operators, robot operators, and technical staff. We thank the anonymous referee for helpful comments that improved this work. M.E.S. is supported by the National Science Foundation Graduate Research Fellowship under grant number DGE1256260. M.M. acknowledges support from NSF grants AST-1312997 and AST-1815403. M.G.W. acknowledges support from National Science Foundation grants AST-1313045 and AST-1813881. E.O. acknowledges support from NSF grants AST-1313006 and AST-1815767.
Group:Astronomy Department
Funding AgencyGrant Number
NSF Graduate Research FellowshipDGE-1256260
Subject Keywords:binaries: general; galaxies: dwarf; galaxies: individual (Draco, Ursa Minor) ; galaxies: kinematics and dynamics
Issue or Number:6
Record Number:CaltechAUTHORS:20181205-145346940
Persistent URL:
Official Citation:Meghin E. Spencer et al 2018 AJ 156 257
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:91518
Deposited By: George Porter
Deposited On:06 Dec 2018 15:57
Last Modified:16 Nov 2021 03:41

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