The majority of stars in the Galactic field and halo are part of binary or
multiple systems. A significant fraction of these systems have orbital
separations in excess of thousands of astronomical units, and systems wider
than a parsec have been identified in the Galactic halo. These binary systems
cannot have formed through the 'normal' star-formation process, nor by capture
processes in the Galactic field. We propose that these wide systems were formed
during the dissolution phase of young star clusters. We test this hypothesis
using N-body simulations of evolving star clusters and find wide binary
fractions of 1-30%, depending on initial conditions. Moreover, given that most
stars form as part of a binary system, our theory predicts that a large
fraction of the known wide 'binaries' are, in fact, multiple systems.Comment: 4 pages, 1 figure, to appear in the proceedings of IAU Symposium 266,
  eds. R. de Grijs & J.R.D. Lepin

Bastian

D. Malmberg

de Grijs

Duquennoy

Goodwin

Kouwenhoven

M. B. Davies

M. B. N. Kouwenhoven

Mengel

P. Kroupa

Richard J. Parker

S. P. Goodwin

English

arXiv

Crossref

The origin of very wide binary systems

The majority of stars in the Galactic field and halo are part of binary or multiple systems. A significant fraction of these systems have orbital separations in excess of thousands of astronomical units, and systems wider than a parsec have been identified in the Galactic halo. These binary systems cannot have formed through the 'normal' star-formation process, nor by capture processes in the Galactic field. We propose that these wide systems were formed during the dissolution phase of young star clusters. We test this hypothesis using N-body simulations of evolving star clusters and find wide binary fractions of 1-30%, depending on initial conditions. Moreover, given that most stars form as part of a binary system, our theory predicts that a large fraction of the known wide 'binaries' are, in fact, multiple systems

Kouwenhoven, M.B.N.

Goodwin, S.P.

Parker, R.J.

Davies, M.B.

Malmberg, D.

Kroupa, P.

White Rose Research Online

This is an author produced version of The origin of very wide binary systems.White Rose Research Online URL for this paper:http://eprints.whiterose.ac.uk/120229/Article:Kouwenhoven, M.B.N., Goodwin, S.P., Parker, R.J. et al. (3 more authors) (2010) The origin of very wide binary systems. Proceedings of the International Astronomical Union, 5 (S266). pp. 438-441. ISSN 1743-9213 https://doi.org/10.1017/S1743921309991633promoting access toWhite Rose research paperseprints@whiterose.ac.ukhttp://eprints.whiterose.ac.uk/arXiv:0910.5796v1  [astro-ph.GA]  30 Oct 2009Star clusters: basic galactic building blocksProceedings IAU Symposium No. 266, 2010R. de Grijs & J. R. D. Le´pine, eds.c© 2010 International Astronomical UnionDOI: 00.0000/X000000000000000XThe origin of very wide binary systemsM. B. N. Kouwenhoven,1,2 S. P. Goodwin,2 Richard J. Parker,2M. B. Davies,3 D. Malmberg3 and P. Kroupa41Kavli Institute for Astronomy and Astrophysics, Peking University, Yi He Yuan Lu 5,Hai Dian District, Beijing 100871, P.R. Chinaemail: kouwenhoven@kiaa.pku.edu.cn2University of Sheffield, Hicks Building, Hounsfield Road, Sheffield S3 7RH, UK3Lund Observatory, Box 43, SE-221 00, Lund, Sweden4Argelander Institute for Astronomy, University of Bonn, Auf dem Hu¨gel 71, 53121, Bonn,GermanyAbstract. The majority of stars in the Galactic field and halo are part of binary or multiplesystems. A significant fraction of these systems have orbital separations in excess of thousandsof astronomical units, and systems wider than a parsec have been identified in the Galactic halo.These binary systems cannot have formed through the ‘normal’ star-formation process, nor bycapture processes in the Galactic field. We propose that these wide systems were formed duringthe dissolution phase of young star clusters. We test this hypothesis using N-body simulations ofevolving star clusters and find wide binary fractions of 1−30%, depending on initial conditions.Moreover, given that most stars form as part of a binary system, our theory predicts that alarge fraction of the known wide ‘binaries’ are, in fact, multiple systems.Keywords. binaries: general, stars: formation, stellar dynamics, methods: N-body simulations1. Observations and formation of very wide binary systemsNumerous wide (103 AU−0.1 pc) binary systems are known to exist in the Galacticfield and halo (e.g., Close et al. 1990; Chaname´ & Gould 2004). These binaries are usuallydetected as common-proper-motion pairs (e.g., Wasserman & Weinberg 1991; Le´pine &Bongiorno 2007) or through statistical methods (e.g., Bahcall & Soneira 1981; Garnavich1988). Approximately 15% of the wide binaries in the field have a semi-major axis in therange 103 AU < a < 0.1 pc (Duquennoy & Mayor 1991; see also Poveda et al. 2007).The origin of these wide binaries cannot be explained by star formation or by dynamicalinteractions in the Galactic field, and has long been a mystery.The majority of stars are known to form in embedded clusters (Lada & Lada 2003).Wide binary systems cannot have formed through clustered star formation, simply be-cause their semi-major axis is comparable to the size of a typical embedded cluster. Evenif it were possible to form wide binaries in star clusters, they would immediately be de-stroyed by dynamical interactions. Although most stars are believed to have formed instar clusters, a fraction (< 10−30%) may form through diffuse (isolated) star formation.However, this fraction makes it difficult to account for the ∼ 15% wide binaries in theGalactic field. If wide binaries are formed through diffuse star formation, then the ma-jority of stars formed in this process must form a wide binary, implying a star-formationprocess that is fundamentally different from that of clustered star formation.Binary systems of any separation may also form through dynamical capture processes.To form a binary system from two interacting stars, a third star should be present to serveas energy sink. Each of the three stars should be at the same location at the same time,and should have precisely those velocities and impact angles that allow the formation of12 M. B. N. Kouwenhoven et al.  100 101 102 103 104 105 106Semi-major axis (AU)0.00.20.40.60.81.0Semi-major axis distribution100 101 102 103 104 105 106Semi-major axis (AU)0.00.20.40.60.81.0Mass ratio0.1 1.0 10.0M1 (Msun)0.00.20.40.60.81.0Mass ratioFigure 1. The binary population resulting from a Plummer model with N = 1000 and avirial radius of 0.1 pc (50 runs), showing the semi-major-axis distribution (left), the correlationbetween mass ratio q and semi-major axis a (middle) and between primary mass and mass ratio(right). Binary and multiple systems are indicated with circles and triangles, respectively. Thevertical dashed lines indicate a = 103 AU and a = 0.1 pc, respectively, the semi-major-axis rangewe consider in our analysis. The dashed curve in the right-hand panel indicates the minimummass ratio qmin(M1) =Mmin/M1 (Kouwenhoven et al. 2009).a new binary system. These conditions are extremely rare in the Galactic field, resultingonly in a handful of binaries over the lifetime of the Galaxy (Goodman & Hut 1993).Note that binaries can form through the capture process in the cores of star clustersthrough this process. However, wide binary systems cannot form in these environments,simply because they have a semi-major axis comparable to (or even larger than) starcluster cores.We propose that wide binaries are formed during the dissolution of young star clusters(Kouwenhoven et al., in prep). Young clusters generally have a short lifetime and dissolveinto the field stellar population within typically 20 Myr (e.g., Mengel et al. 2005; Bastianet al. 2005; de Grijs & Parmentier 2007). During this dissolution process, two starsmay escape the cluster in the same direction, with similar velocities. Although initiallyunbound, these two stars may form a new binary system after having left the gravitationalpotential of the cluster. In this process, the separation between the two stars is typicallyon the order of the size of their natal cluster at the time of dissolution, i.e., of order aparsec.2. N-body simulations and resultsIt is theoretically possible to form a wide binary in the star cluster dissolution phaseand it is important to find out whether this process happens frequently enough to accountfor the observed wide binary population. To this end, we carry out N -body simulations ofevolving star clusters using the starlab package (Portegies Zwart et al. 2001). We ran-domly drawN single stars from the Kroupa (2002) initial mass function in the mass range0.1−50 M⊙. The stars are given positions and velocities according to two models, (i) thespherical and homogeneous Plummer (1911) model and (ii) a fractal stellar distributionwith fractal dimension 1.5, following the prescriptions of Goodwin & Whitworth (2004).The simulations are run until the star cluster is completely dissolved. We only considerthe wide binary population in the semi-major-axis range 103 AU < a < 0.1 pc, wherethe upper limit roughly corresponds to the stability limit in the Galactic field. We findthe dependence on initial conditions by varying the cluster mass, size and morphology.Our main results are the following.(a) The resulting wide binary fraction is 1− 30%, depending on the initial propertiesIAUS266. The origin of very wide binary systems 3of the star cluster. The wide binary fraction increases strongly with increasing initialsubstructure of the cluster, and decreases with increasing cluster mass.(b) The dissolution of star clusters results in a binary population with a bimodalsemi-major-axis distribution. At small separations, we find a dynamical peak, consistingof binaries formed dynamically in the centre of the star cluster. At large separations, wefind a dissolution peak, which contains the binary systems formed during the dissolutionprocess of the star cluster. As an example, we show in Figure 1 the binary populationresulting from a star cluster with N = 103 single stars and an initial radius of 0.1 pc.The resulting semi-major axis scales linearly with the initial size of the star cluster.(c) Wide-binary formation is a process where two random stars encounter each other.When two stars are nearby in phase space, their mutual gravitational attraction (i.e., theircombined mass) determines whether they form a new binary. The resulting mass-ratiodistribution can therefore be characterised by gravitationally focused random pairing(e.g., Gaburov et al. 2008). The resulting eccentricity distribution is thermal (e.g., Heggie1975) and there is no correlation between the orbital orientation and the stellar rotationvectors.3. Implications for higher-order multiplicityIn the previous section we described simulations of star clusters that initially consistof single stars only. In reality, however, most stars (possibly even all stars) are formedas part of binary or multiple systems (e.g., Duquennoy & Mayor 1991; Fischer & Marcy1992; Kroupa 1995; Kouwenhoven et al. 2005, 2007; Zinnecker & Yorke 2007). To firstorder, primordial binaries with separations much smaller than those of wide binaries canbe dynamically approximated as single stars. Therefore, for a star cluster with primordialbinaries we expect to find approximately the same wide-binary population as describedabove. But in this case, the wide ‘binaries’ are, in fact, wide triple or quadruple sys-tems, where one or both of the subsystems consists of a primordial binary. Our N -bodysimulations of clusters with primordial binaries show that this is indeed the case. Theratio between the newly formed wide binaries, triples and quadruples, is a function ofthe primordial binary fraction. For example, for a primordial binary fraction of 50%, weexpect a binary:triple:quadruple ratio of 1:2:1. By accurately measuring the multiplicityof wide-binary systems, we can therefore constrain the primordial binary fraction.4. SummaryA significant fraction of the stars in the Galactic field and halo are part of wide(a > 103 AU) binary systems. The origin of these binary systems cannot be explainedby clustered star formation, nor by dynamical capture in the Galactic field and halo.We propose that these binaries are formed during the dissolution process of young starclusters, where two initially unbound stars escape in a similar direction and form abound wide binary (Kouwenhoven et al., in prep). We test this hypothesis using N -bodysimulations and find that we are able to reproduce wide-binary fractions of 1− 30% (inthe semi-major-axis range 103 AU < a < 0.1 pc), depending on the initial conditions.Moreover, as most stars form as part of a binary system, we predict that a large fractionof the known wide ‘binaries’ are, in fact, triple or quadruple systems, and the ratio of widebinary, triple and quadruple systems therefore provides an indication of the primordialbinary fraction.4 M. B. N. Kouwenhoven et al.AcknowledgementsMBNK was supported by the Peter and Patricia Gruber Foundation through a PPGFfellowship, the Kavli Foundation, the Peking University One Hundred Talent Fund (985)and by STFC (grant PP/D002036/1). RJP acknowledges support from STFC.ReferencesBahcall, J. N., & Soneira, R. M. 1981, ApJ, 246, 122Bastian, N., Gieles, M., Lamers, H. J. G. L. M., Scheepmaker, R. A., & de Grijs, R. 2005, A&A,431, 905Chaname´, J., & Gould, A. 2004, ApJ, 601, 289Close, L. M., Richer, H. B., & Crabtree, D. R. 1990, AJ, 100, 1968Fischer, D. A., & Marcy, G. W. 1992, ApJ, 396, 178de Grijs, R., & Parmentier, G. 2007, ChJA&A, 7, 155Duquennoy, A., & Mayor, M. 1991, A&A, 248, 485Gaburov, E., Gualandris, A., & Portegies Zwart, S. 2008, MNRAS, 384, 376Garnavich, P. M. 1988, ApJ (Letters), 335, L47Goodman, J., & Hut, P. 1993, ApJ, 403, 271Goodwin, S. P., & Whitworth, A. P. 2004, A&A, 413, 929Heggie, D. C. 1975, MNRAS, 173, 729Kroupa, P. 2001, MNRAS, 322, 231Kouwenhoven, M. B. N., Brown, A. G. A., Zinnecker, H., Kaper, L., & Portegies Zwart, S. F.2005, A&A, 430, 137Kouwenhoven, M. B. N., Brown, A. G. A., & Kaper, L. 2007, A&A, 464, 581Kouwenhoven, M. B. N., Brown, A. G. A., Goodwin, S. P., Portegies Zwart, S. F., & Kaper, L.2009, A&A, 493, 979Kroupa, P. 1995, MNRAS, 277, 1491Lada, C. J., & Lada, E. A. 2003, ARA&A, 41, 57Le´pine, S., & Bongiorno, B. 2007, AJ, 133, 889Mengel, S., Lehnert, M. D., Thatte, N., & Genzel, R. 2005, A&A, 443, 41Poveda, A., Allen, C., & Herna´ndez–Alca´ntara, A. 2007, in: W. I. Hartkopf, E. F. Guinan and P.Harmanec (eds.), Binary Stars as Critical Tools and Tests in Contemporary Astrophysics,Proc. IAU Symp. 240, p. 417Plummer, H. C. 1911, MNRAS, 71, 460Portegies Zwart, S. F., McMillan, S. L. W., Hut, P., & Makino, J. 2001, MNRAS, 321, 199Wasserman, I., & Weinberg, M. D. 1991, ApJ, 382, 149Zinnecker, H., & Yorke, H. W. 2007, ARA&A, 45, 481

The majority of stars in the Galactic field and halo are part of binary or multiple systems. A significant fraction of these systems have orbital separations in excess of thousands of astronomical units, and systems wider than a parsec have been identified in the Galactic halo. These binary systems cannot have formed through the &apos;normal&apos; star-formation process, nor by capture processes in the Galactic field. We propose that these wide systems were formed during the dissolution phase of young star clusters. We test this hypothesis using N-body simulations of evolving star clusters and find wide binary fractions of 1 - 30%, depending on initial conditions. Moreover, given that most stars form as part of a binary system; our theory predicts that a large fraction of the known wide &apos;binaries&apos; are, in fact, multiple systems.Astronomy &amp; AstrophysicsCPCI-S(ISTP)

The origin of very wide binary systems

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