A&A 397, 159–175 (2003) Astronomy DOI: 10.1051/0004-6361:20021507 & c ESO 2003 Astrophysics 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’Universit´e, 67 000 Strasbourg, France 2 Geneva Observatory, 51 chemin des Maillettes, 1290 Sauverny, Switzerland e-mail: [email protected];[email protected] 3 Observatoire de Paris–Meudon, bˆat. 11, 5 place J. Janssen, 92195 Meudon Cedex, France e-mail: [email protected] 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 0:2toq 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. +4 Therefore, the frequency of planetary systems among stars is presently 7 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 1. Introduction 1.1. Binary formation The distributions of fundamental parameters of binary stars The formation process of binary stars is still poorly under- have been searched for and debated over for three quarters of stood, and various mechanisms could intervene (see the recent century (since Opik¨ 1924), but we still did not have a definitive IAU symposium 200, and the conclusion by Mathieu 2001). answer. This controversy comes essentially from its importance Whatever the model, the complete formation process consists for our understanding of star formation. It is well–known that of two phases: the fragmentation producing condensing cores the majority of stars belong to double or multiple systems, and which will become the components of the binary, and the later any realistic model of star formation must allow the genera- evolution, consisting in accretion and migration, which will fix tion of binary stars having the statistical properties which are the final masses of the components and the orbital parameters observed. This interest increased after the discovery of the ex- of the systems. trasolar planets, when it was suggested that these new objects could have the same origin as double stars (Stepinski & Black 2000). 1.1.1. Fragmentation Send offprint requests to: J. L. Halbwachs, Fragmentation may occur at three different phases of the con- e-mail: [email protected] densation of a molecular cloud (see the reviews by Bonnell ? Based on photoelectric radial–velocity measurements collected at 2001a,b). The earliest one is the first phase of the gravitational Haute-Provence observatory and on observations made with the ESA collapse. The cloud is split in several condensing fragments, Hipparcos astrometry satellite. with centers separated by thousands of AU. Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20021507 160 J. L. Halbwachs et al.: Statistical properties of F7–K binaries The first phase of the condensation ends when the gas is no ratios. Bate (2000) also found that, unlike migration led by a longer optically thin. The gravitational collapse is then stopped, disk, it seems that accretion generates low eccentricities. and the temperature grows, impeding fragmentation. In a very recent paper, Bate et al. (2002b) simulated the for- A second collapse begins when the core is hot enough mation of binary stars in a small cluster as a hydrodynamical to induce the dissociation of the molecular hydrogen. process, taking all these effects into account. They concluded Fragmentation during or after the second collapse could gen- that equal–mass systems should be frequent among close bina- erate a seed binary with components as close as 0.1 AU. ries, and that the frequency of close binaries should increase However, this process requires very specific conditions, and with the mass of the primary. They predicted also that close it is unlikely to happen in reality. Another process leading binaries should often have wide companions. to close condensing cores is the formation of a disk around the protostar, and its fragmentation (Bonnell & Bate 1994). 1.1.3. Minimum secondary mass Whatever the process, the masses of the two cores are roughly proportional to the separation (Bate 2000): when the separation It follows from the above that the final mass ratio of a binary is about 0.1 AU, the component mass is only about 0.005 . is the product of two consecutive processes: fragmentation and M accretion. Since a large amount of material is accreted, Bate et al. (2002a) estimate that the final mass of any component 1.1.2. Evolution should usually be above the hydrogen–burning limit. Brown dwarfs are then failed stars which were ejected from dynami- The continuation of binary formation, from condensing cores cally unstable systems, and they are expected to be rare among to the final binary, could follow three different scenarios. binary components. The evolution of a cluster of protostars is led by dynamical interactions (see Bodenheimer & Burkert 2001; Clarke 2001). 1.2. History of binary statistic Some binaries or multiple systems may be formed when the cluster is disrupted. Although the tail of the distribution may It results from the previous section that the distributions of contain close binaries, separations below 10 AU are expected mass ratios, of periods and of eccentricities of main sequence to be rather rare. The orbits of these systems should be eccen- binaries are important observational clues for validating the tric. The final mass ratios depend on dynamical exchange in- models. However, these distributions are difficult to derive in teractions: the encounter of a binary with a single star heavier practice: the statistics of binary samples are biased by selection than the secondary component typically results in the ejection effects, and the relevant physical parameters are sometimes not of the secondary and the capture of the originally single star. directly observed. Therefore, the results of the investigations The distribution of masses of the components depends on the look curiously unstable and varying. This concerns not only number of cores generated by fragmentation. When it is larger distributions derived from different classes of binaries, such as than 2, the distribution of secondary masses does not depend visual binaries or spectroscopic binaries, but also results ob- on the primary mass. In other words, the distribution of mass tained by different studies related to the same kind of objects. ratios varies with the mass of the primary components. About this aspect, the most critical parameter is the mass ratio, hereafter defined as the ratio of the masses, q = = . When a circumbinary disk is formed, the transfer of an- M2 M1 gular momentum from the binary to the disk results in a de- crease of the semi–major axis and an increase of the eccentric- 1.2.1. Mass ratios ity (Artymowicz et al. 1991). If material is transferred from the When only spectroscopic binaries are considered, the distribu- disk to the binary, the mass ratio may increase (Artymowicz & tions of mass ratios that were derived may be distributed in two Lubow 1994; G¨unther & Kley 2002) or decrease (G¨unther & categories : Kley 2002). Close binaries could thus result from migration of wide pairs, but the investigations into this process are not ad- – The double–peak distributions. This distribution was of- vanced enough to evaluate whether it is sufficiently efficient to ten found in studies based on all known spectroscopic bi- explain the frequency of close binaries (Artymowicz & Lubow naries (Trimble 1974, 1978; Staniucha 1979). It consists 2001). of one peak of equal–mass components, and one peak When a close seed binary is formed in situ, the components around q 0:2 or 0.3. The former corresponds to the have low initial masses and they still receive a large part of their double–lined≈ spectroscopic binaries (SB2), and the latter final mass. The result of accretion from a gaseous envelope de- to the single–lined binaries (SB1).