A Survey of Proper-Motion Stars. XVII. A Deficiency of Binary Stars on Retrograde Galactic Orbits and the Possibility that ω Centauri is Related to the Effect

摘要

We compare the frequency of field binary stars as a function of Galactic velocity vectors and find a deficiency of such stars on strongly retrograde orbits. Metal-poor stars moving on prograde Galactic orbits have a binary frequency of 28% ± 3%, whereas the retrograde stars' binary frequency is only 10% ± 2% for V ≤ -300 km s-1. No such binary deficiencies are seen for the U or W velocities, nor for [Fe/H]. Some mechanism exists that either disrupts binary systems or preferentially adds single stars moving primarily on retrograde orbits. Theoretical analyses and critical evaluations of our observational data appear to rule out preferential disruption of preexisting binary stars due to such causes as tidal interactions with massive gravitational perturbers, including giant molecular clouds, black holes, or the Galactic center. Dynamically evolved stellar ensembles, such as globular clusters, provide a possible source of single stars. Three lines of evidence rule out this explanation. First, there is no mechanism to significantly enhance dissolution of clusters moving on retrograde orbits. Second, a study of globular clusters moving on prograde and retrograde orbits and with perigalacticon distances such that they are unlikely to be affected strongly by central tidal effects shows that clusters moving on prograde Galactic orbits may be more evolved dynamically than clusters moving on retrograde orbits. Finally, we have undertaken a comprehensive search for star streams that might be discernible. Monte Carlo modeling suggests that our sample may include one moving group, but it contains only five stars. Although the Galactic orbit of this group passes near the Galactic center, it is not moving on a retrograde Galactic orbit and falls short by a factor of at least 20 in supplying the necessary number of single stars. There is one intriguing possibility to explain our results. A dissolved dwarf galaxy may have too large a velocity spread to be easily detected in our sample using our technique. However, dwarf galaxies appear to often show element-to-iron versus [Fe/H] abundance patterns that are not similar to the bulk of the stellar field and cluster halo stars. We explore the s-process elements Y and Ba. Eight stars in our sample have such elemental abundances already measured and also lie in the critical domain with -1.6 ≤ [Fe/H] ≤ -1.0 and V ≤ -300 km s-1. The admittedly small samples appears to show a bimodal distribution in [Y/Fe], [Ba/Fe], and [α/Fe], where "α" represents an average abundance of Mg, Si, Ca, and Ti. This behavior is reminiscent of the difference in the abundances found between the globular cluster ω Centauri and other globular clusters. It is also intriguing that the stars most similar to ω Cen in their chemical abundances show a relatively coherent set of kinematic properties, with a modest velocity dispersion. The stars less like ω Cen define a dynamically hot population. The binary frequency of the stars in ω Cen does not appear to be enhanced, but detailed modeling of the radial velocity data remains to be done.