Multiplicity among solar–type stars - III. Statistical properties of the F7–K binaries with periods up to 10 years

Free access article

Issue		A&A Volume 397, Number 1, January I 2003


Page(s)		159 - 175
Section		Stellar clusters and associations
DOI		http://dx.doi.org/10.1051/0004-6361:20021507

A&A 397, 159-175 (2003)
DOI: 10.1051/0004-6361:20021507

Multiplicity among solar-type stars

III. Statistical properties of the F7-K binaries with periods up to 10 years

J. L. Halbwachs¹, M. Mayor², S. Udry² and F. Arenou³

¹ Observatoire Astronomique de Strasbourg (UMR 7550), Observatoire Astronomique, 11 rue de l'Université, 67 000 Strasbourg, France
² Geneva Observatory, 51 chemin des Maillettes, 1290 Sauverny, Switzerland
e-mail: Michel.Mayor@obs.unige.ch;Stephane.Udry@obs.unige.ch
³ Observatoire de Paris-Meudon, bât. 11, 5 place J. Janssen, 92195 Meudon Cedex, France
e-mail: Frederic.Arenou@obspm.fr

(Received 4 July 2002 / Accepted 1 October 2002 )

Abstract
Two CORAVEL radial velocity surveys - one among stars in the solar neighbourhood, the other in the Pleiades and in Praesepe - are merged to derive the statistical properties of main-sequence binaries with spectral types F7 to K and with periods up to 10 years. A sample of 89 spectroscopic orbits was finally obtained. Among them, 52 relate to a free-of-bias selection of 405 stars (240 field stars and 165 cluster stars). The statistics corrected for selection effects yield the following results: (1) No discrepancy is found between the binaries among field stars and the binaries in open cluster. The distributions of mass ratios, of periods, the period-eccentricity diagram and the binary frequencies are all within the same error intervals. (2) The distribution of mass ratios presents two maxima: a broad peak from $q \approx 0.2$ to $q \approx 0.7$ , and a sharp peak for q > 0.8 (twins). Both are present among the early-type as well as among the late-type part of the sample, indicating a scale-free formation process. The peak for q > 0.8 gradually decreases when long-period binaries are considered. Whatever their periods, the twins have eccentricities significantly lower than the other binaries, confirming a difference in the formation processes. Twins could be generated by in situ formation followed by accretion from a gaseous envelope, whereas binaries with intermediate mass ratios could be formed at wide separations, but they are made closer by migration led by interactions with a circumbinary disk. (3) The frequency of binaries with P<10 years is about 14%. (4) About 0.3% of binaries are expected to appear as false positives in a planet search. Therefore, the frequency of planetary systems among stars is presently 7⁺⁴_-2%. The extension of the distribution of mass ratios in the planetary range would result in a very sharp and very high peak, well separated from the binary stars with low mass ratios.

Key words: stars: binaries: general -- stars: binaries: spectroscopic -- stars: formation -- stars: low-mass, brown dwarfs

Offprint request: J. L. Halbwachs, halbwachs@astro.u-strasbg.fr

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