We present absolute astrometry of four radio sources in the Becklin-Neugebauer/Kleinman-Low (BN/KL) region, derived from archival data (taken in 1991, 1995, and 2000) as well as from new observations (taken in 2006). All data consist of 3.6 cm continuum emission and were taken with the Very Large Array in its highest angular resolution A configuration. We confirm the large proper motions of the BN object, the radio source I (GMR I) and the radio counterpart of the infrared source n (Orion-n), with values from 15 to 26 km s −1 . The three sources are receding from a point between them from where they seem to have been ejected about 500 years ago, probably via the disintegration of a multiple stellar system. We present simulations of very compact stellar groups that provide a plausible dynamical scenario for the observations. The radio source Orion-n appeared as a double in the first three epochs, but as single in 2006. We discuss this morphological change. The fourth source in the region, GMR D, shows no statistically significant proper motions. We also present new, accurate relative astrometry between BN and radio source I that restrict possible dynamical scenarios for the region. During the 2006 observations, the radio source GMR A, located about 1 ′ to the NW of the BN/KL region, exhibited an increase in its flux density of a factor of ∼3.5 over a timescale of one hour. This rapid variability at cm wavelengths is similar to that previously found during a flare at millimeter wavelengths that took place in 2003.

We present absolute astrometry of 35 radio sources in the Orion Trapezium and Becklin-Neugebauer/Kleinman-Low regions, obtained from Very Large Array archival observations collected over a period of 15 years. By averaging the results for all the sources, we estimate the mean absolute proper motion of Orion to be -in Galactic coordinatesµ ℓ cos b = +2.1 ± 0.2 mas yr −1 ; µ b = −0.1 ± 0.2 mas yr −1 . These values agree remarkably well with those expected from the differential rotation of the Milky Way. Subtraction of this mean motion from the individual measurements allows us to register all proper motions to the rest frame of the Orion nebula, and identify radio sources with large residual velocities. In the KL region, we find three sources in this situation: the BN object, the radio source I, and the radio counterpart of the infrared source n. All three objects appear to be moving away from a common point where they must all have been located about 500 years ago. This suggests that all three sources were originally part of a multiple massive stellar system that recently disintegrated as a result of a close dynamical interaction.

In order to determine an upper bound for the mass of the massive compact halo objets (MACHOs) we use the halo binaries contained in a recent catalog (Allen & Monroy-Rodríguez 2013). To dynamically model their interactions with massive perturbers a Monte Carlo simulation is conducted, using an impulsive approximation method and assuming a galactic halo constituted by massive particles of a characteristic mass. The results of such simulations are compared with several subsamples of our improved catalog of candidate halo wide binaries. In accordance with Quinn et al. (2009) we also find our results to be very sensitive to the widest binaries. However, our larger sample, together with the fact that we can obtain galactic orbits for 150 of our systems, allows a more reliable estimate of the maximum MACHO mass than that obtained previously. If we employ the entire sample of 211 candidate halo stars we obtain an upper limit of 112 M ⊙ . However, using the 150 binaries in our catalog with computed galactic orbits we are able to refine our fitting criteria. Thus, for the 100 most halo-like binaries we obtain a maximum MACHO mass of 21 − 68 M ⊙ . Furthermore, we can estimate the dynamical effects of the galactic disk using binary samples that spend progressively shorter times within the disk. By extrapolating the limits obtained for our most reliable -albeit smallest-sample we find that as the time spent within the disk tends to zero the upper bound of the MACHO mass tends to less than 5 M ⊙ . The non-uniform density of the halo has also been taken into account, but the limit obtained, less than 5 M ⊙ , does not differ much from the previous one. Together with microlensing studies that provide lower limits on the MACHO mass, our results essentially exclude the existence of such objects in the galactic halo.

We present absolute astrometry of the core of the Orion molecular cloud, made with Very Large Array archive data taken over the last two decades. Our analysis reveals that both the BN object and the radio source I have proper motions: the BN object has a proper motion of mas yr Ϫ1 (corresponding to a velocity of 12.6 ‫ע‬ 0.6 km s Ϫ1 at an adopted distance of 450 pc) to the northwest, while the radio source I has a proper motion 27 ‫ע‬ 1 of mas yr Ϫ1 (corresponding to a velocity of km s Ϫ1 ) to the southeast. The motion of the two 5.6 ‫ע‬ 0.7 12 ‫ע‬ 2 sources is nearly antiparallel, diverging from a point in between them, where they were located about 500 years ago. These results suggest that the BN object and the radio source I were part of a multiple young stellar system that disintegrated in the recent past.

The effect of a barred potential (such as the one of the Milky Way) on the Galactic orbits of 48 globular clusters for which absolute proper motions are known is studied. The orbital characteristics are compared with those obtained for the case of an axisymmetric Galactic potential. Tidal radii are computed and discussed for both the better known axisymmetric case and that including a bar. The destruction rates due to bulge and disk shocking are calculated and compared in both Galactic potentials.

Modified gravity scenarios where a change of regime appears at acceleration scales a < a0 have been proposed. Since for 1M⊙ systems the acceleration drops below a0 at scales of around 7000 AU, a statistical survey of wide binaries with relative velocities and separations reaching 10 4 AU and beyond should prove useful to the above debate. We apply the proposed test to the best currently available data. Results show a constant upper limit to the relative velocities in wide binaries which is independent of separation for over three orders of magnitude, in analogy with galactic flat rotation curves in the same a < a0 acceleration regime. Our results are suggestive of a breakdown of Kepler's third law beyond a ≈ a0 scales, in accordance with generic predictions of modified gravity theories designed not to require any dark matter at galactic scales and beyond.

Using the most recent proper-motion determination of the old, solar-metallicity, Galactic open cluster M67 in orbital computations in a non-axisymmetric model of the Milky Way, including a bar and three-dimensional spiral arms, we explore the possibility that the Sun once belonged to this cluster. We have performed Monte Carlo numerical simulations to generate the present-day orbital conditions of the Sun and M67, and all the parameters in the Galactic model. We compute 3.5 × 10 5 pairs of orbits Sun-M67 looking for close encounters in the past with a minimum distance approach within the tidal radius of M67. In these encounters we find that the relative velocity between the Sun and M67 is larger than 20 km s −1 . If the Sun had been ejected from M67 with this high velocity by means of a three-body encounter, this interaction would have either destroyed an initial circumstellar disk around the Sun or dispersed its already formed planets. We also find a very low probability, much lower than 10 −7 , that the Sun was ejected from M67 by an encounter of this cluster with a giant molecular cloud. This study also excludes the possibility that the Sun and M67 were born in the same molecular cloud. Our dynamical results convincingly demonstrate that M67 could not have been the birth cluster of our solar system.

No modern catalogue of wide binary and multiple systems (WBMS) exists for the region of the solar vicinity (stars nearer than ~22 pc), that include updated information on their membership to moving clusters and an age classification. With the aim of filling this gap, and also because of its importance for investigating the dynamical evolution of double and multiple systems, we have compiled a WBMS list extracted from the catalogues of nearby stars of Gliese (1969, also Gliese & Jahreiss 1979), as well as Luyten’s NLTT (Luyten 1979–1980, Luyten & Hughes 1980, see also Warren et al. 1989), the Bright Star Catalogue (Hoffleit 1982), the catalogue of Woolley et al. (1970) and other sources.By comparing the transverse velocities of WBM systems with those expected for members of the Hyades and Sirius Superclusters, membership of some WBMS to them has been established. We have classified as probably young systems (PYS) those with one component satisfying at least one of various criteria of youth. A system that is not a PYS is classified as probably old (POS).The catalogue contains 385 binaries and 85 systems of higher multiplicity. Out of these, 129 binaries are PYS and 256 are POS, while 32 multiples are PYS and 53 are POS.