J/ApJ/804/30 Robo-AO observed cool subdwarf companions (Ziegler+, 2015)
Multiplicity of the Galactic senior citizens: a high-resolution search for cool
subdwarf companions.
Ziegler C., Law N.M., Baranec C., Riddle R.L., Fuchs J.T.
<Astrophys. J., 804, 30 (2015)>
=2015ApJ...804...30Z 2015ApJ...804...30Z (SIMBAD/NED BibCode)
ADC_Keywords: Stars, subdwarf ; Stars, double and multiple ; Stars, late-type ;
Magnitudes
Keywords: binaries: close - instrumentation: adaptive optics -
methods: data analysis - stars: late-type - subdwarfs -
techniques: high angular resolution
Abstract:
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 (Δmi≤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.
Description:
We selected targets from the 564 spectral type F- through M-subdwarf
candidates studied by Marshall (2007, J/AJ/134/778). These targets
were selected from the New Luyten Two-Tenths catalog (NLTT; Luyten,
1979nlcs.book.....L 1979nlcs.book.....L; Luyten & Hughes 1980, Proper-Motion Survey with
the Forty-Eight Inch Schmidt Telescope LV First Supplement to the NLTT
Catalogue (Minneapolis, MN: Univ. Minnesota Press)) of high proper
motion stars (>0.18 arcsec/yr) using a reduced proper motion diagram
(RPM). Of the 552 subdwarfs confirmed by Marshall, a randomly selected
sample of 348 G-, K-, and M-subdwarfs were observed by Robo-AO when
available between other high priority surveys. We obtained
high-angular-resolution images of the 348 subdwarfs during 32 separate
nights of observations between 2012 September 3 and 2013 August 21 (UT).
The observations were performed using the Robo-AO laser adaptive
optics system (Riddle et al. 2012SPIE.8447E..2OR; Baranec et al. 2013,
J. Visualized Exp. 72 e50021; 2014ApJ...790L...8B 2014ApJ...790L...8B) mounted on the
Palomar 60 inch telescope.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table5.dat 33 344 Full Robo-AO Observation List
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See also:
I/98 : NLTT Catalogue (Luyten, 1979)
J/ApJ/582/1011 : Revised NLTT Catalog (Salim+, 2003)
J/PASP/115/22 : Stellar parameters for 134 cool subdwarfs (Yong+, 2003)
J/AJ/134/778 : BVRI photometry of candidate subdwarfs (Marshall, 2007)
J/ApJ/799/4 : Robo-AO observations of binary stars (Riddle+, 2015)
Byte-by-byte Description of file: table5.dat
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Bytes Format Units Label Explanations
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1- 5 I5 --- NLTT [69/58812] NLTT number (NNNNN)
7- 10 F4.1 --- Vmag V-band magnitude
12- 22 A11 "date" Date Date of the observation
24- 29 A6 --- Qual [High/Medium/Low ] Observation quality
31- 33 A3 --- Comp [Yes-] Presence of detected companion
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History:
From electronic version of the journal
(End) Prepared by Tiphaine Pouvreau [CDS] 14-Sep-2017