Showing papers on "Mass segregation published in 2000"
TL;DR: In this paper, the authors used the observed magnitudes, colors, and star counts to constrain the shape of the inner ONC stellar mass function across the hydrogen-burning limit.
Abstract: We present the results of a 0."5-0."9 FWHM imaging survey at K (2.2 μm) and H (1.6 μm) covering ~5.'1 × 5.'1 centered on θ^1C Ori, the most massive star in the Orion Nebula Cluster (ONC). At the age and distance of this cluster, and in the absence of extinction, the hydrogen-burning limit (0.08 M_☉) occurs at K ≈ 13.5 mag, while an object of mass 0.02 M_☉ has K ≈ 16.2 mag. Our photometry is complete for source detection at the 7 σ level to K ≈ 17.5 mag and thus is sensitive to objects as low-mass as 0.02 M_☉ seen through visual extinction values as high as 10 mag. We use the observed magnitudes, colors, and star counts to constrain the shape of the inner ONC stellar mass function across the hydrogen-burning limit. After determining the stellar age and near-infrared excess properties of the optically visible stars in this same inner ONC region, we present a new technique that incorporates these distributions when extracting the mass function from the observed density of stars in the K-(H-K) diagram. We find that our data are inconsistent with a mass function that rises across the stellar/substellar boundary. Instead, we find that the most likely form of the inner ONC mass function is one that rises to a peak around 0.15 M_☉, and then declines across the hydrogen-burning limit with slope N(log M) ∝ M^(0.57). We emphasize that our conclusions apply to the inner 0.71 pc × 0.71 pc of the ONC only; they may not apply to the ONC as a whole where some evidence for general mass segregation has been found.
338 citations
TL;DR: The evolution of star clusters is studied using N-body simulations in which the evolution of single stars and binaries are taken self-consistently into account as mentioned in this paper, and initial conditions are chosen to represent relatively young Galactic open clusters, such as the Pleiades, Praesepe and the Hyades.
Abstract: The evolution of star clusters is studied using N-body simulations in which the evolution of single stars and binaries are taken self-consistently into account. Initial conditions are chosen to represent relatively young Galactic open clusters, such as the Pleiades, Praesepe and the Hyades. The calculations include a realistic mass function, primordial binaries and the external potential of the parent Galaxy. Our model clusters are generally significantly flattened in the Galactic tidal field, and dissolve before deep core collapse occurs. The binary fraction decreases initially due to the destruction of soft binaries, but increases later because lower mass single stars escape more easily than the more massive binaries. At late times, the cluster core is quite rich in giants and white dwarfs. There is no evidence for preferential evaporation of old white dwarfs, on the contrary the formed white dwarfs are likely to remain in the cluster. Stars tend to escape from the cluster through the first and second Lagrange points, in the direction of and away from the Galactic center. Mass segregation manifests itself in our models well within an initial relaxation time. As expected, giants and white dwarfs are much more strongly affected by mass segregation than main-sequence stars. Open clusters are dynamically rather inactive. However, the combined effect of stellar mass loss and evaporation of stars from the cluster potential drives its dissolution on a much shorter timescale than if these effects are neglected. The often-used argument that a star cluster is barely older than its relaxation time and therefore cannot be dynamically evolved is clearly in error for the majority of star clusters.
327 citations
TL;DR: In this article, the authors present the results of a 0.9" FWHM imaging survey at K 2.2 micron and H 1.6 micron covering 5.1' x 5.5' in the Orion Nebula Cluster (ONC).
Abstract: We present the results of a 0.5-0.9" FWHM imaging survey at K (2.2 micron) and H (1.6 micron) covering 5.1' x 5.1' centered on Theta 1C Ori, the most massive star in the Orion Nebula Cluster (ONC). At the age and distance of this cluster, and in the absence of extinction, the hydrogen burning limit (0.08 Mo) occurs at K~13.5 mag while an object of mass 0.02 Mo has K~16.2 mag. Our photometry is complete for source detection at the 7 sigma level to K~17.5 mag and thus is sensitive to objects as low-mass as 0.02 Mo seen through visual extinction values as high as 10 magnitudes. We use the observed magnitudes, colors, and star counts to constrain the shape of the inner ONC stellar mass function across the hydrogen burning limit. After determining the stellar age and near-infrared excess properties of the optically visible stars in this same inner ONC region, we present a new technique that incorporates these distributions when extracting the mass function from the observed density of stars in the K-(H-K) diagram. We find that our data are inconsistent with a mass function that rises across the stellar/sub-stellar boundary. Instead, we find that the most likely form of the inner ONC mass function is one that rises to a peak around 0.15 Mo, and then declines across the hydrogen-burning limit with slope N(log M) ~ M^(0.57+/-0.05). We emphasize that our conclusions apply to the inner 0.71 pc x 0.71 pc of the ONC only; they may not apply to the ONC as a whole where some evidence for general mass segregation has been found.
308 citations
TL;DR: In this article, the mass-luminosity (MF) distributions of globular clusters are transformed into mass functions (MFs) by means of mass luminosity relations that are consistent with all presently available data on the physical properties of low-mass, low-metallicity stars.
Abstract: Accurate luminosity functions (LFs) for a dozen globular clusters have now been measured at or just beyond their half-light radius using HST. They span almost the entire cluster main sequence (MS) below 0.75 M☉. All these clusters exhibit LFs that rise continuously from an absolute I magnitude MI 6 to a peak at MI 8.5-9 and then drop with increasing MI. Transformation of the LFs into mass functions (MFs) by means of mass-luminosity (ML) relations that are consistent with all presently available data on the physical properties of low-mass, low-metallicity stars shows that all the LFs observed so far can be obtained from MFs having the shape of a lognormal distribution with characteristic mass mc = 0.33 ± 0.03 M☉ and standard deviation σ = 0.34 ± 0.04. In particular, the LFs of the four clusters in the sample that extend well beyond the peak luminosity down to close to the hydrogen-burning limit (NGC 6341, NGC 6397, NGC 6752, and NGC 6809) can only be reproduced by such distributions and not by a single power law in the 0.1-0.6 M☉ range. After correction for the effects of mass segregation, the variation of the ratio of the number of higher to lower mass stars with cluster mass or any simple orbital parameter or the expected time to disruption recently computed for these clusters shows no statistically significant trend over a range of this last parameter of more than a factor of ~100. We conclude that the global MFs of these clusters have not been measurably modified by evaporation and tidal interactions with the Galaxy and, thus, should reflect the initial distribution of stellar masses. Since the lognormal function that we find is also very similar to the one obtained independently for much younger clusters and to the form expected theoretically, the implication seems to be unavoidable that it represents the true stellar initial mass function for this type of star in this mass range.
120 citations
TL;DR: In this paper, the authors investigated the dynamical evolution of compact young star clusters near the Galactic center using Aarseth's Nbody6 codes and found that, in most cases, the CYC lifetimes of previous Fokker-Planck (F-P) calculations are 5%-30% shorter than those from the present N-body simulations.
Abstract: We investigate the dynamical evolution of compact young star clusters (CYCs) near the Galactic center using Aarseth's Nbody6 codes. The relatively small number of stars in the cluster (5000-20,000) makes real-number N-body simulations for these clusters feasible on current workstations. Using Fokker-Planck (F-P) models, Kim, Morris, & Lee made a survey of cluster lifetimes for various initial conditions and have found that clusters with a mass 2 × 104 M☉ evaporate in ~10 Myr. These results were, however, to be confirmed by N-body simulations because some extreme cluster conditions, such as strong tidal forces and a large stellar mass range participating in the dynamical evolution, might violate assumptions made in F-P models. Here we find that, in most cases, the CYC lifetimes of previous F-P calculations are 5%-30% shorter than those from the present N-body simulations. The comparison of projected number density profiles and stellar mass functions between N-body simulations and Hubble Space Telescope/NICMOS observations by Figer and colleagues in 1999 suggests that the current tidal radius of the Arches cluster is ~1.0 pc and that the parameters for the initial conditions of that cluster are as follows: total mass of 2 × 104 M☉ and mass function slope for intermediate to massive stars of 1.75 (the Salpeter function has 2.35). We also find that the lower stellar mass limit, the presence of primordial binaries, the amount of initial mass segregation, and the choice of initial density profile (King or Plummer models) do not significantly affect the dynamical evolution of CYCs.
92 citations
TL;DR: In this paper, the color gradient in the post-core-collapse globular cluster M30 (NGC 7099) has been investigated using Wide Field Planetary Camera 2 images in the F439W and F555W bands.
Abstract: It has long been known that the post–core-collapse globular cluster M30 (NGC 7099) has a bluer-inward color gradient, and recent work suggests that the central deficiency of bright red giant stars does not fully account for this gradient. This study uses Hubble Space Telescope Wide Field Planetary Camera 2 images in the F439W and F555W bands, along with ground-based CCD images with a wider field of view for normalization of the noncluster background contribution, and finds Δ(B-V) ~ 0.3 mag for the overall cluster starlight over the range 2'' to 1' in radius. The slope of the color profile in this radial range is Δ(B-V)/Δ log r = +0.20 ± 0.07 mag dex-1, where the quoted uncertainty accounts for Poisson fluctuations in the small number of bright evolved stars that dominate the cluster light. 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 caused entirely by post–main-sequence stars. Two classes of plausible dynamical models, Fokker-Planck and multimass King models, are combined with theoretical stellar isochrones from Bergbusch V 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''.
14 citations
TL;DR: In this article, the luminosity function of the globular cluster Palomar 1 was determined together with its confidence interval by applying an adaptive kernel estimator to data from the Isaac Newton Telescope.
Abstract: The luminosity function of the globular cluster Palomar 1 is determined together with its confidence interval by applying an adaptive kernel estimator to data from the Isaac Newton Telescope. The luminosity function of Palomar 1 has two minima in the V band and one in the I band. The cluster core contains a relatively high fraction of bright stars, indicative of mass segregation.
10 citations
TL;DR: In this article, the authors investigated the dynamical evolution of compact young star clusters (CYCs) near the Galactic center (GC) using Aarseth's Nbody6 codes.
Abstract: We investigate the dynamical evolution of compact young star clusters (CYCs) near the Galactic center (GC) using Aarseth's Nbody6 codes. The relatively small number of stars in the cluster (5,000-20,000) makes real-number N-body simulations for these clusters feasible on current workstations. Using Fokker-Planck (F-P) models, Kim, Morris, & Lee (1999) have made a survey of cluster lifetimes for various initial conditions, and have found that clusters with a mass <~ 2x10^4 Msun evaporate in ~10 Myr. These results were, however, to be confirmed by N-body simulations because some extreme cluster conditions, such as strong tidal forces and a large stellar mass range participating in the dynamical evolution, might violate assumptions made in F-P models. Here we find that, in most cases, the CYC lifetimes of previous F-P calculations are 5-30% shorter than those from the present N-body simulations. The comparison of projected number density profiles and stellar mass functions between N-body simulations and HST/NICMOS observations by Figer et al. (1999) suggests that the current tidal radius of the Arches cluster is ~1.0 pc, and the following parameters for the initial conditions of that cluster: total mass of 2x10^4 Msun and mass function slope for intermediate-to-massive stars of 1.75 (the Salpeter function has 2.35). We also find that the lower stellar mass limit, the presence of primordial binaries, the amount of initial mass segregation, and the choice of initial density profile (King or Plummer models) do not significantly affect the dynamical evolution of CYCs.
6 citations
Posted Content•
TL;DR: In this paper, the evolution with angular velocity proportional to the square root of the density is predicted for self-similar core collapse in the Dynamical Main Sequence of globular clusters.
Abstract: Evolution with mass segregation and the evolution of the rotation of cores are both discussed for self-similar core collapse. Evolution with angular velocity proportional to the square root of the density is predicted. On the Dynamical Main Sequence of globular clusters the energy emission from binaries balances the energy expended in expanding the halo.
Newton's exactly solved N-body problem is then given, along with recent generalisations, all of which have no violent relaxation, but a new type of statistical equilibrium is discussed.
Finally, we set the creation of streams in the Galaxy's halo in the historical context of their discovery.
3 citations
Posted Content•
TL;DR: In this paper, the authors review some elements related to the theoretical expectation of mass segregation, the results from N-body and other computer simulations, as well as the now substantial clear observational evidence.
Abstract: Star clusters - open and globulars - experience dynamical evolution on time scales shorter than their age Consequently, open and globular clusters provide us with unique dynamical laboratories for learning about two-body relaxation, mass segregation from equipartition of energy, and core collapse We review briefly, in the framework of star clusters, some elements related to the theoretical expectation of mass segregation, the results from N-body and other computer simulations, as well as the now substantial clear observational evidence
3 citations
Posted Content•
TL;DR: A dense-enough gas-accumulation evolves, over a few Myr of intensifying star formation, to an embedded cluster If it contains a sufficient amount of mass, O stars form and explosively expel the remaining gas, whereas poorer clusters reduce their embryonic gas content more gradually as mentioned in this paper.
Abstract: A dense-enough gas-accumulation evolves, over a few Myr of intensifying star formation, to an embedded cluster If it contains a sufficient amount of mass, O stars form and explosively expel the remaining gas, whereas poorer clusters reduce their embryonic gas content more gradually The sudden expulsion of gas unbinds most of a rich cluster, but a significant fraction of it can condense by two-body interactions to become an open cluster despite a star-formation efficiency as low as 30 per cent Poorer clusters survive their gradual mass loss more easily, but have short, relaxation-limited life-times Pleiades-like clusters may thus form as nuclei of expanding OB associations, by filling their tidal radii and having large (1-2 pc) core-radii A 'main-sequence' of clusters is thus established Ultimately, a cluster dies an explosive death through the ever shortening relaxation time, and leaves a remnant that consists of about 4-10 stars arranged in a highly hierarchical and thus long-lived system Dynamical mass segregation in very young clusters is extremely rapid, and heats a cluster substantially, which is partially off-set by the cooling from the disruption of primordial binaries
01 Jan 2000
TL;DR: In this article, the mass luminosity functions (LF) of a dozen globular clusters have been measured at or just beyond their half-light radius using HST and it was shown that the global LF of these clusters have not been measurably modified by evaporation and tidal interactions with the Galaxy and, thus, should reflect the initial distribution of stellar masses.
Abstract: Accurate luminosity functions (LF) for a dozen globular clusters have now been measured at or just beyond their half-light radius using HST. They span almost the entire cluster main sequence (MS) below ti 0.75 Mo. Transformation of the LF into mass functions (MF) by means of the available mass luminosity (ML) relations that are consistent with all presently available data on the physical properties of low mass, low metallicity stars shows that all the LF observed so far can be obtained from MF having the shape of a log-normal distribution with characteristic mass m, = 0.33 + 0.03 Mo and standard deviation a = 0.34 + 0.04. After correction for the effects of mass segregation, the variation of the ratio of the number of higher to lower mass stars with cluster mass or any simple orbital parameter or the expected time to disruption shows no statistically significant trend over a range of this last parameter of more than a factor of ti 100. We conclude that the global MF of these clusters have not been measurably modified by evaporation and tidal interactions with the Galaxy and, thus, should reflect the initial distribution of stellar masses.
Posted Content•
TL;DR: In this paper, large-field multi-color images of 20 galactic globular clusters are used to investigate the presence of tidal tails around these stellar systems, and the authors associate these stellar overdensities with the stars evaporation, tidal shocking, tidal torquing, and bulge shocking.
Abstract: Large-field multi-color images of 20 galactic globular clusters are used to investigate the presence of tidal tails around these stellar systems Field and cluster stars are sorted with the help of color-magnitude diagrams, and star-count analysis is performed on the selected cluster stars in order to increase the signal-to-noise ratio of their surface density We study the overdensities of these stars using the wavelet transform of the star counts in order to filter the background density noise and to detect the weak structures, at large scale, formed by the numerous stars previously members of the clusters We associate these stellar overdensities with the stars evaporated from the clusters because of dynamical relaxation and/or tidal stripping from the clusters by the galactic gravitational field Most of the globular clusters in our sample display strong evidence of tidal interactions with the galactic plane in the form of large and extended deformations These tidal tails exhibit projected directions preferentially towards the galactic center All the clusters observed, which do not suffer from strong observational biases, present such tidal tails, tracing their dynamical evolution (evaporation, tidal shocking, tidal torquing, and bulge shocking) in the Galaxy The clusters exhibit different regimes of mass loss rate, detected using the radial density slope in the outer parts of the clusters For NGC 5139 ($\omega$ Centauri), we estimate, taking into account the possible presence of mass segregation in its outer parts, that about 06 to 1% of its mass has been lost during the crossing (Abridged)
Posted Content•
TL;DR: In this article, a review discusses dynamical methods for determining the masses of star clusters, including evaporation, mass segregation, core collapse, tidal shocks, dynamical friction and merging.
Abstract: This review discusses (i) dynamical methods for determining the masses of Galactic and extragalactic star clusters, (ii) dynamical processes and their time-scales for the evolution of clusters, including evaporation, mass segregation, core collapse, tidal shocks, dynamical friction and merging. These processes lead to significant evolution of globular cluster systems after their formation.
01 Jan 2000
TL;DR: In this paper, a review discusses dynamical methods for determining the masses of star clusters, including evaporation, mass segregation, core collapse, tidal shocks, dynamical friction and merging.
Abstract: This review discusses (i) dynamical methods for determining the masses of Galactic and extragalactic star clusters, (ii) dynamical processes and their time-scales for the evolution of clusters, including evaporation, mass segregation, core collapse, tidal shocks, dynamical friction and merging. These processes lead to significant evolution of globular cluster systems after their formation.
01 Jan 2000
TL;DR: In this paper, the effects of the mass segregation corrections on the correlations among Global Mass Functions (GMF) of the Globular Clusters and their kinematical parameters are discussed.
Abstract: This communication will be focused upon two main topics: IR observations of Globular Clusters (GC) from the space and the reliability of the mass segregation corrections for Luminosity Functions (LF), observed far away from the half mass radius. The effects of the mass segregation corrections on the correlations among Global Mass Functions (GMF) of the GCs and their kinematical parameters will be finally discussed
Posted Content•
TL;DR: In this article, it has been shown that the stars compete for the available reservoir of gas, resulting in a mass spectrum and is potentially the dominant mechanism for producing the initial mass function, which may result in formation of massive stars through collisions.
Abstract: Observations have revealed that most stars are born in clusters. As these clusters typically contain more mass in gas than in stars, accretion can play an important role in determining the final stellar masses. Numerical simulations of gas accretion in stellar clusters have found that the stars compete for the available reservoir of gas. The accretion rates are highly nonuniform and are determined primarily by each star's position in the cluster. Stars in the centre accrete more gas, resulting in initial mass segregation. This competitive accretion naturally results in a mass spectrum and is potentially the dominant mechanism for producing the initial mass function. Furthermore, accretion on to the core of a cluster forces it to shrink, which may result in formation of massive stars through collisions.
Journal Article•
TL;DR: In this paper, the results of BV(RI)KC CCD photom- etry down to V = 2 1mag in the region of NGC 6994 were presented.
Abstract: We present the results of BV(RI)KC CCD photom- etry down to V =2 1mag in the region of NGC 6994. To our knowledge, no photometry has previously been reported for this object and we find evidences that it is a poor and sparse old open cluster, with a minimum angular diameter of 9 arcmin, i.e. larger than the 3 arcmin originally assigned to it. We obtain a color excess EB V =0 :07 0:02 mag by means of the BV IC technique. Based on the theoretical isochrones from VandenBerg (1985) that are in better agree- ment with our data, we estimate for this cluster a distance from the Sun of 620 pc (V0 MV =9 0:25 mag) and an age lying within the range o f2-3G yr,adopting solar metallic- ity. Thus, the corresponding cluster's Galactocentric distance is 8.1 kpc and is placed at about 350 pc below the Galactic plane. According to these results, NGC 6994 belongs to the old open cluster population located in the outer disk and at large distances from the Galactic plane, and must have suffered significant in- dividual dynamical evolution, resulting in mass segregation and evaporation of low mass stars.
01 Jan 2000
TL;DR: Kroupa et al. as mentioned in this paper showed that a dense enough gas-accumulation evolves, over a few Myrof intensifying star formation, to an embedded cluster.
Abstract: Pavel KroupaInstitut fu¨r Theoretische AstrophysikTiergartenstr. 15, D-69121 Heidelberg, GermanyAbstract. A dense-enough gas-accumulation evolves, over a few Myrof intensifying star formation, to an embedded cluster. If it contains asufficient amount of mass, O stars form and explosively expel the remain-ing gas, whereas poorer clusters reduce their embryonic gas content moregradually. The sudden expulsion of gas unbinds most of a rich cluster,but a significant fraction of it can condense by two-body interactionsto become an open cluster despite a star-formation efficiency as low as30 per cent. Poorer clusters survive their gradual mass loss more easily,but have short, relaxation-limited life-times. Pleiades-like clusters maythus form as nuclei of expanding OB associations, by filling their tidalradii and having large (1–2 pc) core-radii. A ’main-sequence’ of clustersis thus established. Ultimately, a cluster dies an explosive death throughthe ever shortening relaxation time, and leaves a remnant that consistsof about 4–10 stars arranged in a highly hierarchical and thus long-livedsystem. Dynamical mass segregation in very young clusters is extremelyrapid, and heats a cluster substantially, which is partially off-set by thecooling from the disruption of primordial binaries.1. IntroductionStar clusters fascinate because some are beautifully evident to the unaided eye,and because their birth, life and death remain mysterious. Wonderful examples’magically’ lying together in one quadrant on the summer sky (southern hemi-sphere) are M42 (the Orion Nebula Cluster, ONC), and the Pleiades and HyadesClusters. Their ages are τ
01 Jan 2000
TL;DR: In this article, WFPC2-HST observations of the lower main sequence of the galactic globular cluster M92 (NGC6341), extending down to V'27.
Abstract: We present WFPC2-HST observations of the lower main sequence of the galactic globular cluster M92 (NGC6341), extending down to V ' 27. Our data, which cover a region of the cluster extending from the very center out to r ' 380 arcsec, show that the slope of the luminosity function becomes flatter going towards the center of the cluster, as expected if M92 is a dynamically relaxed system. The slope of the inferred power-law mass function we com- puted ranges from x ' 0.7 for the field centered a t r = 18.61 core radii, to x ' 0.3 for the annular field centered a t r = 10.61 core radii, where a value of 14 arcsec has been used for the core radius of M92. Such a result, obtained without making any use of dynamical theoretical models, shows that segregation effects are at work in M92.