Showing papers on "Mass segregation published in 1999"

Evaporation of Compact Young Clusters near the Galactic Center

Abstract: We investigate the dynamical evolution of compact young clusters (CYCs) near the Galactic center (GC) using Fokker-Planck models. CYCs are very young (<5 Myr), compact (<1 pc), and only a few tens of parsecs away from the GC, while they appear to be as massive as the smallest Galactic globular clusters (~104 M?). A survey of cluster lifetimes for various initial mass functions, cluster masses, and Galactocentric radii is presented. Short relaxation times due to the compactness of CYCs and the strong tidal fields near the GC make clusters evaporate fairly quickly. Depending on cluster parameters, mass segregation may occur on a timescale shorter than the lifetimes of most massive stars, which accelerates the cluster's dynamical evolution even more. When the difference between the upper and lower mass boundaries of the initial mass function is large enough, strongly selective ejection of lighter stars makes massive stars dominate even in the outer regions of the cluster, so the dynamical evolution of those clusters is weakly dependent on the lower mass boundary. The mass bins for Fokker-Planck simulations were carefully chosen to properly account for a relatively small number of the most massive stars. We find that clusters with a mass 2 ? 104 M? evaporate in 10 Myr. Two CYCs observed near the GC?the Arches cluster (G0.121+0.17) and the Quintuplet cluster (AFGL 2004)?are interpreted in terms of the models; their central densities and apparent ages are consistent with the hypothesis that they represent successive stages of cluster evolution along a common track, with both undergoing rapid evaporation. A simple calculation based on the total masses in observed CYCs and the lifetimes obtained here indicates that the massive CYCs make up only a fraction of the star formation rate (SFR) in the inner bulge estimated from Lyman continuum photons and far-IR observations. This is consistent with the observation that many stars in the inner bulge form outside the large clusters.

The Luminosity Function of M3

Abstract: We present a high-precision, large-sample luminosity function (LF) for the Galactic globular cluster M3. With a combination of ground-based and Hubble Space Telescope data we cover the entire radial extent of the cluster. The observed LF is well fitted by canonical standard stellar models from the red giant branch (RGB) tip to below the main-sequence turnoff point. Specifically, neither the RGB LF bump nor subgiant branch LF indicate any breakdown in the standard models. On the main sequence we find evidence for a flat initial mass function and for mass segregation due to the dynamical evolution of the cluster.

The Internal Dynamics of Globular Clusters

Abstract: Galactic globular clusters are ancient building blocks of our Galaxy. They represent a very interesting family of stellar systems in which some fundamental dynamical processes have been taking place for more than 10 Gyr, but on time scales shorter than the age of the universe. In contrast with galaxies, these star clusters represent unique laboratories for learning about two-body relaxation, mass segregation from equipartition of energy, stellar collisions, stellar mergers, core collapse, and tidal disruption. This review briefly summarizes some of the tremendous developments that have taken place during the last two decades. It ends with some recent results on tidal tails around galactic globular clusters and on a very massive globular cluster in M31.

HST Observations of the Central-Cusp Globular Cluster NGC 6752. The Effect of Binary Stars on the Luminosity Function in the Core

Abstract: We consider the effect of binary stars on the main-sequence luminosity functions observed in the core of globular clusters, with specific reference to NGC 6752. We find that mass segregation results in an increased binary fraction at fainter magnitudes along the main-sequence. If this effect is not taken into account when analyzing luminosity functions, erroneous conclusions can be drawn regarding the distribution of single stars, and the dynamical state of the cluster. In the core of NGC 6752, our HST data reveal a flat luminosity function, in agreement with previous results. However, when we correct for the increasing binary fraction at faint magnitudes, the LF begins to fall immediately below the turn-off. This effect appears to be confined to the inner core radius of the cluster.

Hubble Space Telescope Observations of the Central-Cusp Globular Cluster NGC 6752: the Effect of Binary Stars on the Luminosity Function in the Core

Abstract: We consider the effect of binary stars on the main-sequence luminosity functions observed in the core of globular clusters, with specific reference to NGC 6752. We find that mass segregation results in an increased binary fraction at fainter magnitudes along the main sequence. If this effect is not taken into account when analyzing luminosity functions, erroneous conclusions can be drawn regarding the distribution of single stars and the dynamical state of the cluster. In the core of NGC 6752, our Hubble Space Telescope data reveal a flat luminosity function (LF), in agreement with previous results. However, when we correct for the increasing binary fraction at faint magnitudes, the LF begins to fall immediately below the turnoff. This effect appears to be confined to the inner core radius of the cluster.

N-body simulations of globular cluster tides

Abstract: We present N-body simulations of globular clusters, in orbits around the Galaxy, in order to study quantitatively and geometrically the tidal effects they encounter. The clusters are modelised with multi-mass King-Michie models (Michie 1963), including mass segregation at initial conditions. The Galaxy is modelled as realistic as possible, with three components: bulge, disk and dark halo. The main finding is that there exist two giant tidal tails around the globuler cluster in permanence along its orbit, whatever this orbit. The length of these tails is of the order of 5 tidal radii, or greater. The escaped stars are dis- tributed radially as a power law in density, with a slope of -4. The tails present substructures, or clumps, that are the relics of the strongest shocks. Due to the compressive disk-shocking, the clusters display a prolate shape whose major axis is precessing around the z axis. The tails are preferentially formed by the low- est mass stars, as expected, so that the tidal truncation increases mass segregation. Internal rotation of the cluster increases the mass loss. The flattening of dark matter cannot influence sig- nificantly the dynamics of the clusters. The orientation and the strength of the tidal tails are signatures of the last disk cross- ing, so that observed tidal tails can constrain strongly the cluster orbit and the galactic model (vertical scale of the disc).

Radial Color Gradient and Main Sequence Mass Segregation in M30 (NGC 7099)

Abstract: It has long been known that the post-core-collapse globular cluster M30 has a bluer-inward color gradient, and recent work suggests that the central deficiency of bright red giants does not fully account for this gradient. This study uses Hubble Space Telescope WFPC2 images in the F439W and F555W bands, along with ground-based CCD images with a wider field of view for normalization of the non-cluster background contribution, and finds \Delta(B-V) ~ +0.3 mag for the overall cluster starlight over the range r = 2" to >~ 1'. The slope of the color profile is: \Delta(B-V)/\Delta\log(r) = 0.20 +/- 0.07 mag dex^{-1}. We explore various algorithms for artificially redistributing the light of bright red giants and horizontal branch stars uniformly across the cluster. The traditional method of redistribution in proportion to the cluster brightness profile is shown to be inaccurate. There is no significant residual color gradient in M30 after proper uniform redistribution of all bright evolved stars; thus the color gradient in M30's central region appears to be due entirely to post-main-sequence stars. Two classes of plausible dynamical models, Fokker-Planck and multimass King models, are combined with theoretical stellar isochrones from Bergbusch & VandenBerg (1992) and from D'Antona et al. to quantify the effect of mass segregation of main sequence stars. In all cases, mass segregation of main sequence stars results in \Delta(B-V) ~ -0.06 to +0.02 mag over the range r = 20" - 80"; this is consistent with M30's residual color gradient within measurement error. The observed fraction of evolved star light in the B and V bands agrees with the corresponding model predictions at small radii but drops below it for r >~ 20".

The Formation of Stellar Clusters

Abstract: We review recent work that investigates the formation of stellar clusters, ranging in scale from globular clusters through open clusters to the small scale aggregates of stars observed in T associations. In all cases, recent advances in understanding have been achieved through the use of state of the art stellar dynamical and gas dynamical calculations, combined with the possibility of intercomparison with an increasingly large dataset on young clusters. Among the subjects that are highlighted are the frequency of cluster-mode star formation, the possible relationship between cluster density and the highest stellar mass, subclustering and the dynamical interactions that occur in compact aggregates, such as binary star formation. We also consider how the spectrum of stellar masses may be shaped by the process of competitive accretion in dense environments and how cluster properties, such as mass segregation and cluster morphology, can be used in conjunction with numerical simulations to investigate the initial conditions for cluster formation. Lastly, we contrast bottom-up and top-down scenarios for cluster formation and discuss their applicability to the formation of clusters on a range of scales.

Hst luminosity functions of the globular clusters m10, m22, and m55. a comparison with other clusters

Abstract: From a combination of deep Hubble Space Telescope V and I images with groundbased images in the same bands, we have obtained color-magnitude diagrams of M10, M22, and M55, extending from just above the hydrogen burning limit to the tip of the red giant branch, down to the white dwarf cooling sequence. We have used the color-magnitude arrays to extract main sequence luminosity functions (LFs) from the turnoff to about 0.13 solar masses. The LFs of M10 is significantly steeper than that for the other two clusters. The difference cannot be due to a difference in metallicity. A comparison with the LFs from Piotto, Cool, and King (1997), shows a large spread in the LF slopes. This spread is also present in the local mass functions (MFs) obtained from the observed LFs using different theoretical mass--luminosity relations. The dispersion in the MF slopes remains also after removing the mass segregation effects by using multimass King-Michie models. The globular cluster MF slopes are also flatter than the MF slope of the field stars and of the Galactic clusters in the same mass interval. We interpret the MF slope dispersion and the MF flatness as an evidence of dynamical evolution which makes the present day globular cluster stellar MFs different from the intial MFs. The slopes of the present day MFs exclude that the low mass star can be dynamically relevant for the Galactic globular clusters.

Mass segregation in very young open clusters.

Abstract: The study of the very young open cluster NGC 6231 clearly shows the presence of a mass segregation for the most massive stars. These observations, combined with those concerning other young objects and very recent numerical simulations, strongly support the hypothesis of an initial origin for the mass segregation of the most massive stars. These results led to the conclusion that massive stars form near the center of clusters. They are strong constraints for scenarii of star and stellar cluster formation.

Early Evolution of Stellar Clusters

Abstract: Observations have revealed that most stars are born in clusters. These systems, containing from tens to thousands of stars and typically significant mass in gas in the youngest systems, evolve due to a combination of stellar and star-gas interactions. Simulations of pure stellar systems are used to investigate possible initial configurations including ellipticity, substructure and mass segregation. Simulations of gas-rich clusters investigate the effects of accretion on the cluster dynamics and on the individual masses that result in a stellar mass spectrum. Further stellar interactions, including binary destruction and eventually cluster dissolution are also discussed.

The Formation and Evolution of LMC Globular Clusters: The Database

Abstract: We present details of the database from a large Cycle 7 HST project to study the formation and evolution of rich star clusters in the LMC (see Elson et al., this volume). Our data set, which includes NICMOS, WFPC2 and STIS images of 8 clusters, will enable us to derive deep luminosity functions for the clusters and to investigate the universality of the stellar IMF. We will look for age spreads in the youngest clusters, quantify the population of binary stars in the cores of the clusters and at the half-mass radii, and follow the development of mass segregation.

Deep STIS Luminosity Functions for LMC Clusters

Abstract: We present deep luminosity functions derived from HST STIS data for three rich LMC clusters (NGC 1805, NGC 1868, and NGC 2209), and for one Galactic globular cluster (NGC 6553). All of the LMC cluster luminosity functions are roughly consistent with a Salpeter IMF or with the solar neighbourhood IMF from Kroupa, Tout & Gilmore (1993). They continue to rise at least to 0.7M ⊙. NGC 1868 shows evidence for mass segregation which may be primordial. A comparison of deep luminsoisty functions for seven Galactic globulars shows that the luminosity functions are eroded at low masses by amounts that are strongly correlated with distance from the Galactic plane.

II. Spectral classication for 175 stars

Abstract: We present spectral types for 175 stars in the ionising cluster of 30 Doradus derived from multislit observations of 231 stars. Comparison with published classications for 70 stars in common with other authors reveals excellent agreement with the exception of a few cases which are discussed in detail. Our new observations raise to 261 the total number of stars in 30 Dor with known spectral types, outside R136. We analyse the spatial distribution of these stars according to their spectral types, and nd evidence for mass segregation.

The Luminosity Function of M3

Abstract: We present a high precision, large sample luminosity function (LF) for the Galactic globular cluster M3. With a combination of ground based and Hubble Space Telescope data we cover the entire radial extent of the cluster. The observed LF is well fit by canonical standard stellar models from the red giant branch (RGB) tip to below the main sequence turnoff point. Specifically, neither the RGB LF-bump nor subgiant branch LF indicate any breakdown in the standard models. On the main sequence we find evidence for a flat initial mass function and for mass segregation due to the dynamical evolution of the cluster.