Showing papers on "Mass segregation published in 1996"

A Catalog of Parameters for Globular Clusters in the Milky Way

Abstract: A database of parameters for globular star clusters in the Milky Way is described which is available in electronic form through the WorldWideWeb. The information in the catalog includes up-to-date measurements for cluster distance, reddening, luminosity, colors and spectral types, velocity, structural and dynamical parameters, horizontal branch morphology, metallicity, and other quantities. This catalog will be updated regularly and maintained in electronic form for widest possible accessibility. Associated Articles Source Paper Catalog Description Catalog Description

Internal Dynamics of Globular Clusters

Abstract: Galactic globular clusters, which are ancient building blocks of our Galaxy, represent a very interesting family of stellar systems in which some fundamental dynamical processes have taken place on time scales shorter than the age of the universe. In contrast with galaxies, these clusters represent unique laboratories for learning about two-body relaxation, mass segregation from equipartition of energy, stellar collisions, stellar mergers, and core collapse. In the present review, we summarize the tremendous developments, as much theoretical as observational, that have taken place during the last two decades, and which have led to a quantum jump in our understanding of these beautiful dynamical systems.

The abundance spread among giants and subgiants in the globular cluster omega centauri

Abstract: We present spectroscopic abundances and radial velocities for giant stars in the Galactic globular cluster omega Centauri based on the CaII infrared triplet. Two samples of stars were observed: 234 stars at M_V = 1.25 on the lower giant branch at radial distances between 8 and 23arcmin, and 145 stars at M_V = -1.3 at radial distances between 3 and 22arcmin. Previous metallicity studies found a non-gaussian metallicity distribution containing a tail of metal-rich stars. We confirm these results except our unbiased cluster metallicity distributions are significantly narrower. They contain the following key features: (1) No very metal-poor stars, (2) a sudden rise in the metal-poor distribution to a modal [Fe/H] value of --1.70 consistent with an homogeneous metallicity unresolved at the 0.07 dex level, (3) a tail to higher metallicities with more stars than predicted by simple chemical evolution models, and (4) a weak correlation between metallicity and radius such that the most metal-rich stars are concentrated to the cluster core. The unresolved metal-weak tail implies that the gas out of which omega Cen formed was well-mixed up to the modal metallicity of the cluster. Therefore, omega Cen like other Galactic globular clusters, seems to have formed in a pre-enriched and homogenized (up to the modal metallicity) environment. The existence of a weak metallicity gradient supports the idea that omega Cen self-enriched, with the enriched gas sinking to the cluster center due to gas dissipation processes. We also note, however, that the metal-rich stars are more massive than the bulk of the stars in the cluster, and may have sunk to the center by dynamical mass segregation over the lifetime of the cluster.

The Abundance Spread Among Giants and Subgiants in the Globular Cluster Omega Centauri

Abstract: We present spectroscopic abundances and radial velocities for giant stars in the Galactic globular cluster omega Centauri based on the CaII infrared triplet. Two samples of stars were observed: 234 stars at M_V = 1.25 on the lower giant branch at radial distances between 8 and 23arcmin, and 145 stars at M_V = -1.3 at radial distances between 3 and 22arcmin. Previous metallicity studies found a non-gaussian metallicity distribution containing a tail of metal-rich stars. We confirm these results except our unbiased cluster metallicity distributions are significantly narrower. They contain the following key features: (1) No very metal-poor stars, (2) a sudden rise in the metal-poor distribution to a modal [Fe/H] value of --1.70 consistent with an homogeneous metallicity unresolved at the 0.07 dex level, (3) a tail to higher metallicities with more stars than predicted by simple chemical evolution models, and (4) a weak correlation between metallicity and radius such that the most metal-rich stars are concentrated to the cluster core. The unresolved metal-weak tail implies that the gas out of which omega Cen formed was well-mixed up to the modal metallicity of the cluster. Therefore, omega Cen like other Galactic globular clusters, seems to have formed in a pre-enriched and homogenized (up to the modal metallicity) environment. The existence of a weak metallicity gradient supports the idea that omega Cen self-enriched, with the enriched gas sinking to the cluster center due to gas dissipation processes. We also note, however, that the metal-rich stars are more massive than the bulk of the stars in the cluster, and may have sunk to the center by dynamical mass segregation over the lifetime of the cluster.

Statistics of N-body simulations — III. Unequal masses

Abstract: We describe results from large numbers of $N$-body simulations containing from $250$ to $1000$ stars each. The distribution of stellar masses is a power law, and the systems are isolated. While the collapse of the core exhibits the expected segregation of different masses, we find that the post-collapse evolution is, at a first approximation, homologous. This is quite surprising because there is no reason for supposing that mass segregation should not continue to have a substantial effect on the evolution of the cluster. In fact the spatial distribution of the mean stellar mass is nearly static throughout the post-collapse regime, except for the overall expansion of the systems, and this helps to explain why the post-collapse evolution is nearly self-similar. Self-similarity is also exhibited by the distribution of anisotropy and the profile of departures from equipartition, which show little change during the post-collapse phase. The departures from energy equipartition and isotropy are small in the core and increase with radius. During post-collapse evolution massive stars (mainly) are removed from the system by binary activity. This effect dominates the preferential escape of low-mass stars due to standard two-body relaxation processes.

Discussions of the internal motions and mass segregation of M67 on the basis of accurate proper motions

Abstract: The radius and virial mass of the old open cluster M67 are presented. The internal motion and mass segregation of the cluster are also discussed on the basis of accurate stellar proper motions obtained combining three independent proper motion catalogues of the cluster. Increases of the mean proper motion and the intrinsic dispersion of member stars with radial distance from the cluster center might suggest that the stars are escaping from the cluster. The stars in both inner and outer regions appear to be in isotropic orbits. At last, it is found that both space and velocity mass segregations exist for the old open cluster due to the dynamical evolution.

Low-mass stars in an outer field in NGC 6397

Abstract: We have imaged a field 10' from the center of the globular cluster NGC 6397 in the visible and I bands with WFPC2 on the Η ST. At a level which is severely limited by counting statistics in the small area so far studied, this field is richer in dwarfs with />21.5 than the 4:6 radius parallel field discussed by Paresce and colleagues. This indicates that the dynamical process of mass segregation is occurring in the cluster.

Studies of Cold Gas in the Early Universe with Large Millimeter Arrays

Abstract: Our knowledge of the process by which cold gas in the early universe accumulates and forms stars is limited by our inability to image the gas. The next generation of millimeter and submillimeter wavelength arrays will allow us to explore the gas content of forming galaxies at the same resolution at which HST and the new 8 m class optical/IR telescopes will reveal early generations of stars. The dust that obscures our view of the early universe at optical wavelengths becomes an asset at millimeter/submillimeter wavelengths where the thermal dust emission can be imaged. With the new millimeter arrays astronomers will: (1) image the mass segregation and kinematics of hierarchical galaxy formation; (2) have the ability to detect thermal dust continuum emission from more than 15 million galaxies with an observation time of less than a minute per galaxy (indeed, such cosmological IR-luminous galaxies will be a source of confusion to every continuum observation); and (3) detect atomic and molecular spectral line emission from normal galaxies such as the Milky Way at redshifts greater than one.

Ubv ccd photometry of intermediate age open cluster m11 i. statistical analysis

Abstract: We present the color-magnitude diagrams (CMD) of more than 24,000 stars in the field of an intermediate age open cluster M11, based on wide field CCD imaging The morphology of the CMDs varies strikingly as the distance from the center of the cluster increases From the surface number density analysis, we confirm the mass segregation effect in this cluster: the bright, massive stars are centrally more concetrated than the faint, low mass stars The slope of the field-corrected surface density with respect to magnitude progressively increases as the radius increases, up to r = 5' Most of the field stars in or near the cluster main sequence band and in the bright part of the red stars in the CMDs appear to be nearly at the same distance as M11, and they are considered to be the major component of disk stars in the Sagittarius-Carina arm

Dynamics of embedded proto star clusters in clouds

Abstract: The dynamics of clumps and proto stars in a young proto cluster system embedded in its parent molecular cloud are studied here through numerical simulations. It is found that the presence of massive clumps and their dynamics strongly affects the motion of the lower mass protostars. Mass segregation of clumps due to dynamical drag by the interclump gas results in the formation of a dense central region, and the lower mass proto stars are preferentially ejected out of the cloud via gravitational encounters. The protostellar cluster is found to expand gradually with time, forming a more extended system than the clumps. Thus, embedded proto star clusters are probably dynamically evolved systems with large haloes of low-mass protostars. This explains observations of T Tauri stars, which appear to be isolated from active star-forming sites and are in regions devoid of dense gas. Since multiple star formation sites in clouds are common, outer members are subject to velocity perturbations due to massive clumps and other clusters, possibly leading to a faster removal of objects.