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Register a new userThe origin of very wide binary systems
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Added by
M. B. N. Kouwenhoven
Publication date
2009
fields
Physics
and research's language is
English
Authors
M.B.N. Kouwenhoven
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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.
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A large population of fragile, wide (> 1000 AU) binary systems exists in the Galactic field and halo. These wide binary stars cannot be primordial because of the high stellar density in star forming regions, while formation by capture in the Galactic field is highly improbable. We propose that these binary systems were formed during the dissolution phase of star clusters (see Kouwenhoven et al. 2010, for details). Stars escaping from a dissolving star cluster can have very similar velocities, which can lead to the formation of a wide binary systems. We carry out N-body simulations to test this hypothesis. The results indicate that this mechanism explains the origin of wide binary systems in the Galaxy. The resulting wide binary fraction and semi-major axis distribution depend on the initial conditions of the dissolving star cluster, while the distributions in eccentricity and mass ratio are universal. Finally, since most stars are formed in (relatively tight) primordial binaries, we predict that a large fraction of the wide binary stars are in fact higher-order multiple systems.
The Pluto-Charon binary system is the best-studied representative of the binary Kuiper-belt population. Its origins are vital to understanding the formation of other Kupier-belt objects (KBO) and binaries, and the evolution of the outer solar-system. The Pluto-Charon system is believed to form following a giant impact between two massive KBOs at relatively low velocities. However, the likelihood of a random direct collision between two of the most massive KBOs is low, and is further constrained by the requirement of a low-velocity collision, making this a potentially fine-tuned scenario. Here we expand our previous studies and suggest that the proto-Pluto-Charon system was formed as a highly inclined wide-binary, which was then driven through secular/quasi-secular evolution into a direct impact. Since wide-binaries are ubiquitous in the Kuiper-belt with many expected to be highly inclined, our scenario is expected to be robust. We use analytic tools and few-body simulations of the triple Sun-(proto-)Pluto-Charon system to show that a large parameter-space of initial conditions leads to such collisions. The velocity of such an impact is the escape velocity of a bound system, which naturally explains the low-velocity impact. The dynamical evolution and the origins of the Pluto-Charon system could therefore be traced to similar secular origins as those of other binaries and contact-binaries (e.g. Arrokoth), and suggest they play a key role in the evolution of KBOs.
Over the last decades, numerous wide (>1000 AU) binaries have been discovered in the Galactic field and halo. The origin of these wide binaries cannot be explained by star formation or by dynamical interactions in the Galactic field. We explain their existence by wide binary formation during the dissolution phase of young star clusters. In this scenario, two single stars that leave the dissolving cluster at the same time, in the same direction, and with similar velocities, form a new, very wide binary. Using N-body simulations we study how frequently this occurs, and how the orbital parameters of such binaries depend on the properties of the cluster from which they originate. The resulting wide binary fraction for individual star clusters is 1-30%, depending on the initial conditions. As most stars form as part of a binary or multiple system, we predict that a large fraction of these wide binaries are in fact wide triple and quadruple systems.
We report on our large scale search of 2MASS and SuperCOSMOS in the southern hemisphere for very widely separated white dwarf / L-dwarf binary systems and present our findings, including 8 widely separated candidate systems, and proper motion analysis confirming one of these as a widely separated white dwarf/ L-dwarf common proper motion binary candidate.
Binary stars can inflate the observed velocity dispersion of stars in dark matter dominated systems such as ultra-faint dwarf galaxies (UFDs). However, the population of binaries in UFDs is poorly constrained by observations, with preferred binary fractions for individual galaxies ranging from a few percent to nearly unity. Searching for wide binaries through nearest neighbor (NN) statistics (or the two-point correlation function) has been suggested in the literature, and we apply this method for the first time to detect wide binaries in a UFD. By analyzing the positions of stars in Reticulum~II (Ret~II) from Hubble Space Telescope images, we search for angularly resolved wide binaries in Ret~II. We find that the distribution of their NN distances shows an enhancement at projected separations of $lesssim8$ arc seconds relative to a model containing no binaries. We show that such an enhancement can be explained by a binary fraction of $f_bapprox0.07^{+0.04}_{-0.03}$, with modest evidence for a smaller mean separation than is seen in the solar neighborhood. We also use the observed magnitude distribution of stars in Ret~II to constrain the initial mass function over the mass range $0.34-0.78~M_{odot}$, finding that a shallow power-law slope of $alpha = 1.10^{+0.30}_{-0.09}$ matches the data.
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