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 eects 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 0: 2t oq 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 dierence 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%. The extension of the distribution of mass ratios in
Highlights
IntroductionThe distributions of fundamental parameters of binary stars have been searched for and debated over for three quarters of century (since Opik 1924), but we still did not have a definitive answer
The first one is a peak around q = 1. This peak is not an artefact coming from an excessive correction of the bias against SB2 twin components, since it is already visible in the distribution of mass ratios of some 15 SB of the unbiased sample with q ≥ 0.7
The rejection was only 3%, and, since the selection effects play against the detection of twins, this indicates that this feature is probably real
Summary
The distributions of fundamental parameters of binary stars have been searched for and debated over for three quarters of century (since Opik 1924), but we still did not have a definitive answer. This controversy comes essentially from its importance for our understanding of star formation. It is well–known that the majority of stars belong to double or multiple systems, and any realistic model of star formation must allow the generation of binary stars having the statistical properties which are observed. The complete formation process consists of two phases: the fragmentation producing condensing cores which will become the components of the binary, and the later evolution, consisting in accretion and migration, which will fix the final masses of the components and the orbital parameters of the systems
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