Showing papers on "Mass segregation published in 1992"

Binaries in globular-clusters

Abstract: Binary stars in a globular cluster (hereafter, GC) may be primordial (i.e. formed along with the cluster), or the result of cluster dynamics. “Dynamical” binaries can result from conservative three-body encounters (e.g. Spitzer, 1987) if a third star can carry away enough kinetic energy to leave two others bound, or from dissipative two-body encounters, if two stars happen to pass within a few stellar radii of one other (Fabian, Pringle, & Rees, 1975). Such non-primordial systems are likely to be found primarily in evolved GC cores, both because conditions are more favorable for making them there, and because of mass segregation. Knowledge of the formation process allows reasonable estimates to be made of their mass and energy distributions. The initial spatial, mass, and energy distributions of primordial binaries, on the other hand, are largely unknown.

The Evolution of a primordial binary population in a globular cluster

Abstract: A simple model is presented for the evolution of a primordial binary population in a globular cluster. Monte Carlo simulations are given for an initial population of 50,000 binaries against a fixed background population of 500,000 single stars in a tidally truncated cluster model. Individual histories of all binaries are followed through mass segregation, scattering recoil, escape from the cluster, or coalescence. It is found that most binaries are destroyed by binary-binary interactions, with the rest escaping in the point-mass approximation. In a more realistic model, the majority of the rest merge. At any instant, most of the remaining binaries are drifting in toward the center before their first strong encounter. A typical binary spends most of its active life in or near the cluster core. The few binaries which receive a recoil sufficient to place them in the halo past the half-mass radius remain there long enough to make a significant contribution to the radial binary distribution.

Fokker-Planck models and globular cluster evolution : the problem of M71

Abstract: Comparisons are made between star-count mass functions and surface density profiles for the globular cluster M71 and multimass, tidally truncated Fokker-Planck simulations reheated by three-body binaries. The degree of mass segregation and the short relaxation time observed for the cluster suggest that M71 should be a post-core-collapse cluster. In the standard models, the tidal boundary and the degree of mass segregation were approximately reproducible. However, we note that the inferred tidal radius, about 10 pc, is distinctly less than that based on consideration of the cluster's Galactic orbit

The blue stragglers of m67 and other open clusters

Abstract: Results are presented of a study aimed at establishing the frequency of spectroscopic binaries in a sample of blue stragglers, belonging to five old- and intermediate-age open clusters: NGC 752, NGC 2360, NGC 2420, NGC 2682 or M67, and NGC 7789. The observational material, which for the M67 subsample spans 10 years, consists of high-resolution, low signal-to-noise echelle spectra obtained with equivalent systmes at the Multiple Mirror Telescope and at the 1.5-m Tillinghast Reflector, both atop Mt. Hopkins (Arizona). In order to obtain reliable radial velocities for these early-type and/or rapidly rotating objects with the cross-correlation method, synthetic spectra matching the observed spectra were employed as templates. The goodness of the grid of computed spectra wsa estimated to be excellent judging from the degree of agreement in high-resolution, high signal-to-noise observed spectra of the Sun, Procyon, and Sirius with their theoretical counterparts. Using this technique it was possible to determine for the first time the spectroscopic orbit for the M67 blue straggler Fagerholm 190 (P 4.2 days). It is proposed that this is a case where mass transfer between the components has been involved, that the system is still in the final stages of mass transfer, and that the original primary--now the secondary--is an abnormal subgiant evolving towards a white dwarf. The rather large orbital eccentricity (e 0.21) remains an open question, since according to current theories a system with such a period should have been circularized by now. If the mass-transfer process that made F190 into a blue straggler started recently, by using the additional information provided by the turnoff mass we can put very strict limits on the original and current masses of both components. Very long-period variations (several thousand days) were detected in three other blue stragglers of M67; since only one of these has a preliminary orbit available, it is not clear whether these systems could also be explained by the mass-transfer mechanism, or a binary-binary interaction scenario should be invoked. Three large-amplitude spectroscopic binaries were discovered among the blue stragglers of the remaining clusters. Some blue stragglers do not show any signifianct variation at all. It is found that the contamination of the blue-straggler sample by field stars is significant in the case of NGC 7789, and to a lesser extent also for NGC 2420. The frequency of spectroscopic binaries among the blue stragglers studied exceeds 40%, approximately twice the current value accepted for the field or normal cluster members. Blue stragglers tend to be more concentrated towards the center of the clusters than single stars, where the frequency of binaries is expected to increase due to mass segregation; in that case, the derived value would be within the expected range. Nevertheless, more than one mechanism seems to be required to account for all the blue stragglers.

Effect of mass segregation on mass function of young open clusters

Abstract: The effect of mass segregation on cluster mass function has been studied and it is found that the slope of the mass function is steeper in the outer part of the open clusters. Since derived dynamical relaxation time is always larger than the age of the cluster, it is inferred that the observed mass segregation might have taken place at the time of cluster formation.

Density profiles and luminosity functions of the globular clusters M3 and M92

Abstract: Tautenburg Schmidt plates with the globular clusters M3 and M92 were measured on the APM facility in Cambridge, mainly for astrometric purposes. The data were used to derive the radial density profiles and the luminosity functions of these clusters. There is a good agreement of our results with formerly ones derived by other authors. In both clusters effects of mass segregation are observed.

Globular Clusters as Probes of the Galactic Potential

Abstract: Capaccioli, Ortolani and Piotto (1990) have shown that the slope of the mass function of galactic globular clusters (GCs) correlates with the cluster distance from the galactic plane. We report on the interpretation (Stiavelli et al., 1990) of this rather surprising result as the cumulative effect of disk-shocking. The leading idea is that, during a passage through the disk, the cluster loses preferentially stars at large radii and, due to mass segregation, these are mainly low mass stars. As a start, we have tackled the problem by a semi-analytical approach and modeled the disk-shocking in the impulse approximation (Ostriker, Spitzer and Chevalier, 1972). We have taken into consideration the model parameters describing the galactic mass-distribution and its effect on GC tidal radii, and GC structural characteristics (concentration, IMF slope, etc.), as well as those accounting for observational selection effects and for some of the theoretical shortcuts. We find that, in addition to selection effects which may spoil the observed correlation, the results are strongly dependent on i) the way tidal radius rt changes with galactocentric distance R g , and ii) the value of the surface mass density of the galactic disk in the solar neighbourhood ∑⊙.