Multiplicity among solar–type stars^*

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

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

Corresponding author: J. L. Halbwachs, halbwachs@astro.u-strasbg.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 to , and a sharp peak for (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 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 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 %. 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.