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Multiplicity of the Galactic Senior Citizens: A High-resolution Search for Cool Subdwarf Companions

Ziegler, Carl and Law, Nicholas M. and Baranec, Christoph and Riddle, Reed L. and Fuchs, Joshua T. (2015) Multiplicity of the Galactic Senior Citizens: A High-resolution Search for Cool Subdwarf Companions. Astrophysical Journal, 804 (1). Art. No. 30. ISSN 0004-637X.

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Cool subdwarfs are the oldest members of the low-mass stellar population. Mostly present in the galactic halo, subdwarfs are characterized by their low-metallicity. Measuring their binary fraction and comparing it to solar-metallicity stars could give key insights into the star formation process early in the Milky Way's history. However, because of their low luminosity and relative rarity in the solar neighborhood, binarity surveys of cool subdwarfs have suffered from small sample sizes and incompleteness. Previous surveys have suggested that the binary fraction of red subdwarfs is much lower than for their main-sequence cousins. Using the highly efficient Robo-AO system, we present the largest high-resolution survey of subdwarfs, sensitive to angular separations (ρ ≥ 0."15) and contrast ratios (Δ_(m)_i ≤ 6) invisible in past surveys. Of 344 target cool subdwarfs, 43 are in multiple systems, 19 of which are newly discovered, for a binary fraction of 12.5 ± 1.9%. We also discovered seven triple star systems for a triplet fraction of 2.0 ± 0.8%. Comparisons to similar surveys of solar-metallicity dwarf stars gives a ~3σ disparity in luminosity between companion stars, with subdwarfs displaying a shortage of low-contrast companions. We also observe a lack of close subdwarf companions in comparison to similar-mass dwarf multiple systems.

Item Type:Article
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URLURL TypeDescription Paper
Ziegler, Carl0000-0002-0619-7639
Law, Nicholas M.0000-0001-9380-6457
Baranec, Christoph0000-0002-1917-9157
Riddle, Reed L.0000-0002-0387-370X
Additional Information:© 2015 American Astronomical Society. Received 2014 November 11; accepted 2015 February 16; published 2015 April 27. Samuel Oschin. We are grateful to the Palomar Observatory staff for their ongoing support of Robo-AO on the 60 inch telescope, particularly S. Kunsman, M. Doyle, J. Henning, R. Walters, G. Van Idsinga, B. Baker, K. Dunscombe, and D. Roderick. The SOAR telescope is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement between the CNPq, Brazil, the National Observatory for Optical Astronomy (NOAO), the University of North Carolina, and Michigan State University, USA. We also thank the SOAR operators, notably Sergio Pizarro. We recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. C.B. acknowledges support from the Alfred P. Sloan Foundation. This research has made use of the SIMBAD database, operated by Centre des Données Stellaires (Strasbourg, France), and bibliographic references from the Astrophysics Data System maintained by SAO/NASA. Facilities: PO:1.5m (Robo-AO), Keck:II (NIRC2-LGS), SOAR (Goodman)
Funding AgencyGrant Number
Alfred P. Sloan FoundationUNSPECIFIED
Inter-University Centre for Astronomy and AstrophysicsUNSPECIFIED
Mount Cuba Astronomical FoundationUNSPECIFIED
Subject Keywords:binaries: close; instrumentation: adaptive optics; methods: data analysis; stars: late-type; subdwarfs; techniques: high angular resolution
Record Number:CaltechAUTHORS:20150608-082946346
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Official Citation:Multiplicity of the Galactic Senior Citizens: A High-resolution Search for Cool Subdwarf Companions Carl Ziegler et al. 2015 ApJ 804 30
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:58070
Deposited By: Ruth Sustaita
Deposited On:08 Jun 2015 17:31
Last Modified:19 Aug 2017 00:03

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