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Showing papers on "Mass segregation published in 2001"


Journal ArticleDOI
Pavel Kroupa1
TL;DR: In this paper, the uncertainty inherent in any observational estimate of the IMF is investigated by studying the scatter introduced by Poisson noise and the dynamical evolution of star clusters, and it is found that this apparent scatter reproduces quite well the observed scatter in power-law index determinations, thus defining the fundamental limit within which any true variation becomes undetectable.
Abstract: A universal initial mass function (IMF) is not intuitive, but so far no convincing evidence for a variable IMF exists. The detection of systematic variations of the IMF with star-forming conditions would be the Rosetta Stone for star formation. In this contribution an average or Galactic-field IMF is defined, stressing that there is evidence for a change in the power-law index at only two masses: near 0.5 M⊙ and near 0.08 M⊙. Using this supposed universal IMF, the uncertainty inherent in any observational estimate of the IMF is investigated by studying the scatter introduced by Poisson noise and the dynamical evolution of star clusters. It is found that this apparent scatter reproduces quite well the observed scatter in power-law index determinations, thus defining the fundamental limit within which any true variation becomes undetectable. The absence of evidence for a variable IMF means that any true variation of the IMF in well-studied populations must be smaller than this scatter. Determinations of the power-law indices α are subject to systematic errors arising mostly from unresolved binaries. The systematic bias is quantified here, with the result that the single-star IMFs for young star clusters are systematically steeper by Δα≈0.5 between 0.1 and 1 M⊙ than the Galactic-field IMF, which is populated by, on average, about 5-Gyr-old stars. The MFs in globular clusters appear to be, on average, systematically flatter than the Galactic-field IMF (Piotto & Zoccali; Paresce & De Marchi), and the recent detection of ancient white-dwarf candidates in the Galactic halo and the absence of associated low-mass stars (Ibata et al.; Mendez & Minniti) suggest a radically different IMF for this ancient population. Star formation in higher metallicity environments thus appears to produce relatively more low-mass stars. While still tentative, this is an interesting trend, being consistent with a systematic variation of the IMF as expected from theoretical arguments.

6,784 citations


Journal ArticleDOI
TL;DR: In this paper, the authors investigate the physics of gas accretion in young stellar clusters and show that the relative velocities of the stars and the gas are comparable in the presence of the same potential.
Abstract: We investigate the physics of gas accretion in young stellar clusters. Accretion in clusters is a dynamic phenomenon as both the stars and the gas respond to the same gravitational potential. Accretion rates are highly non-uniform with stars nearer the centre of the cluster, where gas densities are higher, accreting more than others. This competitive accretion naturally results in both initial mass segregation and a spectrum of stellar masses. Accretion in gas-dominated clusters is well modelled using a tidal-lobe radius instead of the commonly used Bondi–Hoyle accretion radius. This works as both the stellar and gas velocities are under the influence of the same gravitational potential and are thus comparable. The low relative velocity which results means that Rtidal

595 citations


Journal ArticleDOI
TL;DR: In this paper, the authors investigate the physics of gas accretion in young stellar clusters using a tidal-lobe radius instead of the commonly used Bondi-Hoyle accretion radius.
Abstract: We investigate the physics of gas accretion in young stellar clusters. Accretion in clusters is a dynamic phenomenon as both the stars and the gas respond to the same gravitational potential. Accretion rates are highly non-uniform with stars nearer the centre of the cluster, where gas densities are higher, accreting more than others. This competitive accretion naturally results in both initial mass segregation and a spectrum of stellar masses. Accretion in gas-dominated clusters is well modelled using a tidal-lobe radius instead of the commonly used Bondi-Hoyle accretion radius. This works as both the stellar and gas velocities are under the influence of the same gravitational potential and are thus comparable. The low relative velocity that results means that the tidal radius is smaller than the Bondi-Hoyle radius in these systems. In contrast, when the stars dominate the potential and are virialised, the Bondi-Hoyle radius is smaller than the tidal radius and thus Bondi-Hoyle accretion is a better fit to the accretion rates.

492 citations


Journal ArticleDOI
TL;DR: In this paper, direct N-body calculations are presented of the formation of Galactic clusters using GasEx, which is a variant of the code Nbody6, focusing on the possible evolution of the Orion Nebula Cluster (ONC) by assuming that the embedded OB stars explosively drove out 2/3 of its mass in the form of gas about 0.4 Myr ago.
Abstract: Summary Direct N-body calculations are presented of the formation of Galactic clusters using GasEx, which is a variant of the code Nbody6. The calculations focus on the possible evolution of the Orion Nebula Cluster (ONC) by assuming that the embedded OB stars explosively drove out 2/3 of its mass in the form of gas about 0.4 Myr ago. A bound cluster forms readily and survives for 150 Myr despite additional mass loss from the large number of massive stars, and the Galactic tidal field. This is the very first time that cluster formation is obtained under such realistic conditions. The cluster contains about 1/3 of the initial 10 4 stars, and resembles the Pleiades Cluster to a remarkable degree, implying that an ONC-like cluster may have been a precursor of the Pleiades. This scenario predicts the present expansion velocity of the ONC, which will be measurable by upcoming astrometric space missions. These missions should also detect the original Pleiades members as an associated expanding young Galactic-field sub-population. The results arrived at here suggest that Galactic clusters form as the nuclei of expanding OB associations. The results have wide implications, also for the formation of globular clusters and the Galactic field and halo stellar populations. In view of this, the distribution of binary orbital periods and the mass function within and outside the model ONC and Pleiades is quantified, finding consistency with observational constraints. Advanced mass segregation is evident in one of the ONC models. The calculations show that the primordial binary population of both clusters could have been much the same as is observed in the Taurus–Auriga star forming region. The computations also demonstrate that the binary proportion of brown dwarfs is depleted significantly for all periods, whereas massive stars attain a high binary fraction.

465 citations


Journal ArticleDOI
TL;DR: The evolution of star clusters is studied using N-body simulations in which the evolution of single stars and binaries is taken self-consistently into account as mentioned in this paper, showing that the binary fraction decreases initially because of the destruction of soft binaries, but increases later because lower mass single stars escape more easily than the more massive binaries.
Abstract: The evolution of star clusters is studied using N-body simulations in which the evolution of single stars and binaries is 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 by the Galactic tidal field, and dissolve before deep core collapse occurs. The binary fraction decreases initially because of 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 white dwarfs formed 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 Centre. 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 effects of stellar mass-loss and evaporation of stars from the cluster potential drive the dissolution of a cluster on a much shorter time-scale 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. An observation of a blue straggler in an eccentric orbit around an unevolved star or a blue straggler of more than twice the turn-off mass might indicate past dynamical activity. We find two distinct populations of blue stragglers: those formed above the main-sequence turn-off, and those which appear as blue stragglers as the cluster's turn-off drops below the mass of the rejuvenated star.

376 citations


Journal ArticleDOI
TL;DR: In this paper, high dispersion echelle spectroscopy of the very luminous, young super star cluster (SSC) in M82, obtained with the 4.2m William Herschel Telescope (WHT), for the purpose of deriving its dynamical mass and assessing whether it will survive to become an old globular cluster.
Abstract: We present high dispersion echelle spectroscopy of the very luminous, young super star cluster (SSC) ‘F’ in M82, obtained with the 4.2-m William Herschel Telescope (WHT), for the purpose of deriving its dynamical mass and assessing whether it will survive to become an old globular cluster. In addition to the stellar lines, the spectrum contains complex Na I absorption and broad emission lines from the ionized gas. We measure a stellar velocity dispersion of 13.4±0.7 kms −1 , a projected half-light radius of 2.8 ± 0.3 pc from archival HST/WFPC2 images, and derive a dynamical mass of 1.2±0.1×10 6 M⊙, demonstrating that M82-F is a very massive, compact cluster. We determine that the current luminosity-to-mass ratio (LV /M)⊙ for M82-F is 45 ± 13. Comparison with spectral synthesis models shows that (LV /M)⊙ is a factor of � 5 higher than that predicted for a standard Kroupa (2001) initial mass function (IMF) at the well-determined age for M82-F of 60 ± 20 Myr. This high value of (LV /M)⊙ indicates a deficit of low mass stars in M82-F; the current mass function (MF) evidently is ‘top-heavy’. We find that a lower mass cutoff of 2–3M⊙ is required to match the observations for a MF with a slope � = 2.3. Since the cluster apparently lacks longlived low mass stars, it will not become an old globular cluster, but probably will dissolve at an age of 62 Gyr. We also derive up-dated luminosity-to-mass ratios for the younger SSCs NGC 1569A and NGC 1705-1. For the first object, the observations are consistent with a slightly steeper MF (� = 2.5) whereas for NGC 1705-1, the observed ratio requires the MF to be truncated near 2 M⊙ for a slope of � = 2.3. We discuss the implications of our findings in the context of large scale IMF variations; with the present data the top-heavy MF could reflect a local mass segregation effect during the birth of the cluster. M82-F likely formed in a dense molecular cloud; however, its high radial velocity with respect to the centre of M82 (� 175 km s −1 ) suggests it is on an eccentric orbit and now far from its birthplace, so the environment of its formation is unknown.

207 citations


Journal ArticleDOI
TL;DR: In this paper, the authors present the results of a 0.86 square degree CCD photometric survey of the open cluster NGC 2516, which has an age of about 150 Myr and may have a much lower metallicity than the similarly-aged Pleiades.
Abstract: We present the results of a 0.86 square degree CCD photometric survey of the open cluster NGC 2516, which has an age of about 150 Myr and may have a much lower metallicity than the similarly-aged Pleiades. Our survey of cluster members is complete to and is used to select a preliminary catalogue of 1254 low mass () cluster candidates, of which about 70-80 percent are expected to be genuine. After applying corrections for contamination by non-members and adding data for higher mass stars from the literature, we investigate the cluster binarity, luminosity and mass function, mass segregation and total mass. We find a binary fraction of percent, for A to M-type systems with mass ratios between 0.6 and 1, which is very similar to the Pleiades. The mass function is metallicity and evolutionary-model dependent, but consistent with a Salpeter-like law (, or for the solar and half-solar metallicity models of Siess et al. [CITE], and for the solar metallicity models of D'Antona & Mazzitelli [CITE]), for . At lower masses () there is a sharp fall in the mass function, with or (for the solar and half-solar metallicity models of Siess et al.), and (for the solar metallicity models of D'Antona & Mazzitelli). The true stellar mass function might have α values up to 0.4 larger if account were taken of low mass stars in unresolved binary systems with mass ratios less than 0.6. The falling mass function of NGC 2516 at lower masses seems inconsistent with the much flatter mass functions derived from comparable data in the Pleiades and field populations. This deficit of lower mass, fainter stars is also seen in the observed luminosity function. We rule out incompleteness as the cause of this discrepancy, but demonstrate that mass segregation is clearly present in NGC 2516, with more than half the low-mass ( ) stars included. Taking this into account, it is probable that the whole-cluster mass functions for NGC 2516 and the Pleiades are similar down to 0.3. The mass of NGC 2516 stars with inside our survey is , depending on metallicity and what corrections are applied for unresolved binarity. Correcting for mass segregation increases this to ~, about twice the total mass of the Pleiades. If NGC 2516 and the Pleiades do have similar mass functions, then less massive stars and brown dwarfs contribute about a further 15 percent to the mass of NGC 2516 and we predict a cluster population of about 360-440 brown dwarfs with .

118 citations


Journal ArticleDOI
TL;DR: In this article, an analysis of deep CCD photometry for the very rich, old open star cluster NGC 6819 is presented, where the authors find a tight, very rich main-sequence and turnoff consisting of over 2900 cluster stars in the V, B-V color-magnitude diagram (CMD).
Abstract: We present analysis of deep CCD photometry for the very rich, old open star cluster NGC 6819. The science goals are to catalog the white dwarfs in the cluster and measure the cluster luminosity and mass functions. These CFH12K data results represent the first of nineteen open star clusters which were imaged as a part of the CFHT Open Star Cluster Survey. We find a tight, very rich, main-sequence and turnoff consisting of over 2900 cluster stars in the V, B-V color-magnitude diagram (CMD). Main-sequence fitting of the unevolved cluster stars with the Hyades star cluster yields a distance modulus of (m - M)V = 12.30 ± 0.12, for a reddening of E(B-V) = 0.10. These values are consistent with a newly calculated theoretical stellar isochrone of age 2.5 Gyr, which we take to be the age of the cluster. Both the depth gained in the photometry and the increased projected area of the CFH12K Mosaic CCD allow for detailed star counts in concentric annuli out to large angular radii. These indicate a much larger cluster extent (R = 95 ± 10), by a factor of ~2 over some previous estimates. Incompleteness tests confirm a slightly negatively sloped luminosity function extending to faint (V ~ 23) magnitudes which is indicative of a dynamically evolved cluster. Further luminosity function and mass segregation tests indicate that low-mass objects (M ≤ 0.65 M⊙) predominate in the outer regions of the cluster, 35 ≤ R ≤ 95. The estimation of the number of white dwarfs in NGC 6819, based on stellar evolution models, white dwarf cooling timescales, and conservation of star number arguments applied to the red giant stars of the cluster are in good agreement with the observed number. For those white dwarf candidates which pass both a statistical subtraction that removes background galaxies and field stars and a high star/galaxy confidence by using image classification, we show comparisons with white dwarf isochrones and cooling models which suggest the need for spectroscopy to confirm the white dwarf nature of the brighter objects. This is entirely feasible for all objects, before a statistical subtraction cut, with the current generation of 8 m–class telescopes and multiobject spectrometers.

109 citations


Journal ArticleDOI
TL;DR: In this article, the first results of a deep WFPC2 photometric survey of the loose galactic globular cluster NGC 288 were presented, where the fraction of binary systems is estimated from the color distribution of objects near the Main Sequence (MS) with a method analogous to that introduced by Rubenstein & Bailyn(1997).
Abstract: We present the first results of a deep WFPC2 photometric survey of the loose galactic globular cluster NGC 288. The fraction of binary systems is estimated from the color distribution of objects near the Main Sequence (MS) with a method analogous to that introduced by Rubenstein & Bailyn(1997). We have unequivocally detected a significant population of binary systems which has a radial distribution that has been significantly influenced by mass segregation. In the inner region of the cluster (r 1 r_h), f_b must be less than 0.10, and the most likely value is 0.0, independently of the adopted F(q). The detected population of binaries is dominated by primordial systems. The specific frequency of Blue Straggler Stars (BSS) is exceptionally high, suggesting that the BSS production mechanism via binary evolution can be very efficient. A large population of BSS is possible even in low density environments if a sufficient reservoir of primordial binaries is available. The observed distribution of BSS in the Color Magnitude Diagram is not compatible with a rate of BSS production which has been constant in time, if it is assumed that all the BSS are formed by the merging of two stars.

109 citations


Journal ArticleDOI
TL;DR: In this article, the authors presented deep CCD photometry of the very rich, intermediate-age (similar to the Hyades) open star cluster NGC 2099 (M37) and determined the age of the star cluster from the termination point (MV = 11.95 ± 0.30).
Abstract: We present deep CCD photometry of the very rich, intermediate-age (similar to the Hyades) open star cluster NGC 2099 (M37). The V, B-V color-magnitude diagram (CMD) for the cluster shows an extremely well-populated and very tightly constrained main sequence extending over 12 mag from the turnoff. Both a well-defined main-sequence turnoff and a red giant population are also clearly evident. The CFH12K photometry for this cluster is faint enough (V ~ 24.5) to detect the remnants of the most massive progenitor cluster stars under the Type I SNe limit. Therefore, the CMD of the cluster also exhibits a well defined white dwarf "clump" caused by the decreased rate of cooling of these stars as they age, and a subsequent gap with very few objects. By using source classification to eliminate faint blue galaxies and a statistical subtraction technique to eliminate foreground/background objects, we have determined the age of the star cluster from the termination point (MV = 11.95 ± 0.30) in the white dwarf luminosity function. The white dwarf cooling age is found to be 566 ± Myr from comparisons with white dwarf cooling models and is in excellent agreement with the main-sequence turnoff isochrone age (520 Myr). After carefully accounting for the uncertainties in the white dwarf limiting magnitude, we show that the cooling age confirms that models including convective core overshooting are preferred for young–intermediate-aged clusters. This is particularly important in the case of NGC 2099 as the age is similar to that of the Hyades cluster, for which current models have difficulties in reproducing the details of the main-sequence turnoff. We also derive the reddening [E(B-V) = 0.21 ± 0.03] and distance [(m - M)V = 11.55 ± 0.13] to NGC 2099 by matching main-sequence features in the cluster to a new fiducial main sequence for the Hyades, after correcting for small metallicity differences. As a continuing part of the goals of the CFHT Open Star Cluster Survey to better understand dynamical processes of open clusters, we also fit a King model to the cluster density distribution and investigate the cluster main-sequence luminosity and mass functions in increasing concentric annuli. We find some evidence for mass segregation within the boundary of NGC 2099 as expected given the cluster's age relative to the dynamical age. The present global mass function for the cluster is found to be shallower than a Salpeter IMF.

90 citations


Journal ArticleDOI
TL;DR: In this paper, a very deep and accurate photometry of the open cluster M35 has been presented, where the authors have observed this association in the Cousins R, I filters, together with the Johnson V filter.
Abstract: We present very deep and accurate photometry of the open cluster M35. We have observed this association in the Cousins R, I filters, together with the Johnson V filter. We have covered a region of 27.5 × 27.5 square arcmin, equivalent to a fifth of the total area of the cluster. The data range from Ic = 12.5 to 23.5 mag, and the color intervals are 0.4 ≤ (V-I)c ≤ 3.0, 0.5 ≤ (R-I)c ≤ 2.5. Roughly, these values span from 1.6 M☉ down to the substellar limit, in the case of cluster members. By using the location of the stars on color-magnitude and color-color diagrams, we have selected candidate members of this cluster. We have merged our sample with previously published data and obtained a color-magnitude diagram for the complete stellar population of the cluster, covering the spectral range early B-mid M. Based on the distribution of field and cluster stars in color-magnitude and color-color diagrams, we estimate that two-thirds of these candidates are likely to be true members of M35. These stars approximately double the number of stars identified as candidate members of this cluster (~2700). We provide the photometry and accurate positions of these stars. The deep photometry has allowed us to study the mass segregation within the cluster, the luminosity function, and mass function. We show that in the magnitude range 13 ≤ Ic ≤ 22 there is a reduced mass segregation, in opposition to what happens to higher mass stars, where the mass segregation is stronger. The luminosity function behaves essentially as the one characteristic of the Pleiades, presenting a peak at Ic ~ 19 mag (MI ~ 9). Combining our photometry with previous data corresponding to more massive stars, we find that the mass function increases monotonically, when plotted in a log-log form, until it reaches ~0.8 M☉ (α = 2.59). It remains shallower for less massive stars (α = 0.81 for 0.8-0.2 M☉), whereas a decrease is observed for stars close to the substellar regime. These different behaviors suggest that at least three mechanisms play a role in the formation of stellar and substellar objects. The total mass of the cluster is ~1600 M☉ in the area covered by this study.

Journal ArticleDOI
TL;DR: In this paper, the authors used multislit spectroscopy of 180 stars in the ionising cluster of 30 Doradus to obtain reliable radial velocities for 55 stars.
Abstract: On the basis of multislit spectroscopy of 180 stars in the ionising cluster of 30 Doradus we present reliable radial velocities for 55 stars. We calculate a radial velocity dispersion of35 km s 1 for the cluster and we analyse the possible influence of spectroscopic binaries in this rather large velocity dispersion. We use numerical simulations to show that the observations are consistent with the hypothesis that all the stars in the cluster are binaries, and the total mass of the cluster is5 10 5 M. A simple test shows only marginal evidence for dynamical mass segregation which if present is most likely not due to dynamical relaxation.

Journal ArticleDOI
TL;DR: In this paper, the authors used UBV IC photometry in a wide field around the open cluster NGC 7654 for 17 860 stars, down to V 20, and found that the star formation was a gradual process that proceeded sequentially in mass and terminated with the formation of most massive stars.
Abstract: CCD UBV IC photometry in a wide eld around the open cluster NGC 7654 has been carried out for 17 860 stars, down to V 20. The reddening across the cluster region is found to be variable with E(B V )min = 0:46 to E(B V )max =0 :80. The cluster is situated at a distance of 1380 70 pc. The colour magnitude diagrams show a large age spread in the ages. Star formation was biased towards relatively higher masses during the early phase of star formation whereas most of the low mass stars of the cluster were formed during the later phase. The star formation seems to have been a gradual process that proceeded sequentially in mass and terminated with the formation of most massive stars. The present data do not support a uniform mass function (MF) for dierent regions in the cluster. Although for the whole cluster region, the MF in the observed mass range 0:8 M 4:5, can be represented by a single power law with a slope = 1:400:07, however it indicates various features when examined carefully. In three subregions of the cluster the slope , for the mass range 1:5

Journal ArticleDOI
TL;DR: In this paper, the main-sequence luminosity functions of the Palomar 5 globular cluster were determined using deep WFPC2 F555W and F814W photometry.
Abstract: A low mass, large core radius, low central concentration, and strong tidal tails suggest that the globular cluster Palomar 5 has lost a large fraction of its initial mass over time. If the dynamical evolution of Pal 5 has been dominated by the effects of mass loss, then theoretical arguments suggest that the luminosity function should be deficient in low-mass stars. Using deep WFPC2 F555W and F814W photometry, we determine the main-sequence luminosity functions both near the cluster center and in a field near the half-light radius. A comparison of these luminosity functions yields no compelling evidence of mass segregation within the cluster, in accord with expectations for low-concentration clusters. On the other hand, a comparison of the global mass function of Pal 5 with that of ω Cen and M55 indicates an increasing deficiency of stars with progressively lower masses. A fit of the observed luminosity function to theoretical models indicates a mass function for Pal 5 of dN/dm ∝ m-0.5, which is notably more deficient in low-mass stars than other globular clusters that have been studied with the Hubble Space Telescope. The flatness of the mass function is consistent with models of the dynamical evolution of globular clusters that have lost ~90% of their original stellar mass. We suggest that, like NGC 6712, Pal 5 has lost a large percentage of its original stellar content as a result of tidal shocking.

Journal ArticleDOI
TL;DR: In this paper, the authors used deep WFPC2 F555W and F814W photometry to determine the main sequence luminosity functions both near the cluster center and in a field near the half-light radius.
Abstract: A low mass, large core radius, and strong tidal tails suggest that the globular cluster Palomar 5 has lost a large fraction of its initial mass over time. If the dynamical evolution of Palomar 5 has been dominated by the effects of mass loss, then the luminosity function should be deficient in low-mass stars. Using deep WFPC2 F555W and F814W photometry, we determine the main sequence luminosity functions both near the cluster center and in a field near the half-light radius. There is no compelling evidence for mass segregation within the cluster, but a comparison of the global mass function of Palomar 5 with those of Omega Cen and M55 indicates an increasing deficiency of stars with progressively lower masses. A fit of the observed luminosity function to theoretical models yields a power-law mass function with an exponent of -0.5. The flatness of the mass function is consistent with models of the dynamical evolution of globular clusters that have lost on the order of 90% of their original stellar mass. We suggest that Palomar 5 has lost a large percentage of its original stellar content as a result of tidal shocking.

Journal ArticleDOI
TL;DR: In this paper, CCD optical (B and V passbands) and near IR (J and K bands) observations in the region of the old open cluster NGC 2141 were reported.
Abstract: We report CCD optical (B and V passbands) and near IR (J and K bands) observations in the region of the old open cluster NGC 2141. By combining the two sets of photometry (500 stars in common) we derive new estimates of the cluster's fundamental parameters. We conrm that the cluster is 2.5 Gyrs old, but, with respect to previous investigations, we obtain a slightly larger reddening (E(B V )=0 :40), and a slightly shorter distance (3.8 kpc) from the Sun. Finally, we present the Luminosity Function (LF) in the V band, which is another age indicator. We provide a good t for the age range inferred from isochrones by assuming the Kroupa et al. (1993) IMF up to MV = 5.0. We interpret the disagreement at fainter magnitudes as evidence of mass segregation.

Journal ArticleDOI
TL;DR: In this paper, deep V and I photometry for the open cluster NGC 4815 and four surrounding Galactic elds down to a limiting magnitude V 25 was used to study cluster spatial extension by means of star counts, and derive the luminosity (LF) and mass function (MF).
Abstract: We present deep V and I photometry for the open cluster NGC 4815 and four surrounding Galactic elds down to a limiting magnitude V 25. These data are used to study cluster spatial extension by means of star counts, and to derive the luminosity (LF) and mass function (MF). The radius turns out to be 3:60:3 arcmin at V =1 9:0, whereas the mass is 880 230 m down to V =2 0:8. From the color-magnitude diagram, we obtain the LFs in the V and I bands, using both the standard histogram and an adaptive kernel. After correction for incompleteness and eld star contamination, the LFs were transformed into the present day mass functions (PDMF). The PDMFs from the V and I photometry can be represented as a power-law with a slope =3 :1 0:3 and =2 :9 0:3 (the (Salpeter 1955) MF in this notation has a slope =2 :35) respectively, in the mass range 2:5 m m 0:8. Below this mass, the MF cannot be considered as representative of the cluster IMF, as it is the result of the combined eect of strong irregularities in the stellar background, probable internal dynamical evolution of the cluster and/or interaction of the cluster with the dense Galactic eld. Unresolved binaries and mass segregation can only flatten the apparent derived IMF, so we expect that the real IMF must be steeper than the quoted slope by an unknown amount.

Journal ArticleDOI
Stefano Andreon1
TL;DR: In this article, the authors detect the galaxy cluster A118 from the center to half the Abell radius (1.5 Mpc, Ho=50 km/s/Mpc) and possibly to 2.0 Mpc.
Abstract: Using new observations of the galaxy cluster A118 at intermediate redshift (z=0.31) in the Ks band, we were able to detect the cluster from the center to half the Abell radius (1.5 Mpc, Ho=50 km/s/Mpc) and possibly to 2.0 Mpc. The analysis of both the spatial distribution of galaxies of various luminosities and of the luminosity function (LF) of galaxies in different cluster locations strongly confirms and extends to larger clustercentric radii the luminosity segregation found in a previous analysis of this cluster restricted to a smaller cluster area: there is an excess of bright galaxies in the cluster core (inside 250 Kpc) or a deficit of dwarfs in the remain part of the cluster. Outside the cluster core and as far as 1.5 or even 2 Mpc, the giant-to-dwarf ratio is constant. Because of the luminosity segregation, the LF of the AC118 shows a larger number of bright galaxies per unit dwarf in the core than in other cluster locations. All non-core LFs, computed at several cluster locations, are compatible each other. These results hold both including or excluding the galaxies located in an overdensity found in the far South of AC118 and in the second clump in galaxy density at the cluster North-West. Since the near-infrared emission is a good tracer of the stellar mass, we interpret the segregation found as a mass segregation.

Journal ArticleDOI
TL;DR: In this article, the results of deep V and I HST photometry of 6 rich star clusters in the Large Magellanic Cloud with different ages and metallicities are presented.
Abstract: We show the results of deep V and I HST photometry of 6 rich star clusters in the Large Magellanic Cloud with different ages and metallicities. The number of stars with measured magnitudes in each cluster varies from about 3000 to 10 000 stars. We build stellar density and surface brightness profiles for the clusters and extract half-light radii and other structural parameters for each. We also obtain luminosity functions, , down to , and investigate their dependence with distance from the cluster centre well beyond their half-light radius. In all clusters we find a systematic increase in the luminosity functions slope, , with radial distance from the centre. Among the clusters displaying significant mass segregation are the 2 youngest in the sample: NGC 1805, NGC 1818. For these two clusters we obtain present day mass functions. The NGC 1818 mass function is in excellent agreement with that derived by other authors, also using HST data. The young cluster mass function slopes differ, that of NGC 1805 being systematically steeper than NGC 1818. Since these are very young stellar systems ( Myrs), these variations may reflect the initial conditions rather than evolution due to internal dynamics.

Journal ArticleDOI
TL;DR: In this article, the UBVRI data of ve northern open star clusters are used to study the mass function (MF), structure and dynamical state of these clusters, which indicates that all these clusters are dynamically relaxed.
Abstract: The UBVRI data of ve northern open star clusters are used to study the mass function (MF), structure and dynamical state of these clusters. The clusters under discussion have ages ranging from 0.6 to 5 Gyr. A comparison of the density proles indicates that the evolution of the core of these clusters is almost the same, whereas the corona of the clusters are probably aected by the external environment and dynamical evolution. For the entire cluster region, the slope of the MF of three clusters (Be 64, Be 69 and King 5) has a value that agrees within the error with the Salpeter value, whereas King 7 and Be 20 show a steeper ( = 2:02 0:24) and almost a flat MF respectively. We nd that the slope of the MF of two clusters (King 5 and King 7) changes signicantly from the inner region to the outer region, becoming steeper at larger radii. The dynamical relaxation time of the clusters under discussion is less than the age of the clusters, which indicates that all of these clusters are dynamically relaxed. Thus the observed mass segregation in three clusters can be attributed to the dynamical evolution of the clusters. The ratio of the clusters' present radius to the limiting radius (determined from the relation given by King 1962) can be representated by an exponent law.

Journal ArticleDOI
TL;DR: In this article, color-Magnitude diagrams (CMDs) extending from the red-giant tip to about 5 magnitudes below the main-sequence turnoff MSTO have been constructed.
Abstract: We have used ESO telescopes at La Silla and the Hubble Space Telescope (HST) in order to obtain accurate B , V , I CCD photometry for the stars located within 200″ ($\simeq$2 half-mass radii, $r_{\rm h} = 1.71\arcmin$) from the center of the cluster NGC 6101. Color-Magnitude Diagrams (CMDs) extending from the red-giant tip to about 5 magnitudes below the main-sequence turnoff MSTO ($V = 20.05 \pm 0.05$) have been constructed. The following results have been obtained from the analysis of the CMDs: a) The overall morphology of the main branches confirms previous results from the literature, in particular the existence of a sizeable population of 73 "blue stragglers" (BSS), which had been already partly detected (27). They are considerably more concentrated than either the subgiant branch (SGB) or the main sequence (MS) stars, and have the same spatial distribution as the horizontal branch (HB) stars (84% probability from K-S test). An hypothesis on the possible BSS progeny is also presented. b) The HB is narrow and the bulk of stars is blue, as expected for a typical metal-poor globular cluster. c) The derived magnitudes for the HB and the MSTO, $V_{\rm ZAHB} = 16.59 \pm 0.10$, $V_{\rm TO} = 20.05 \pm 0.05$, coupled with the values $E(B-V) = 0.1$, ${\rm [Fe/H]} = -1.80$, $Y = 0.23$ yield a distance modulus $(m-M)_{V} = 16.23$ and an age similar to other "old" metal-poor globular clusters. In particular, from the comparison with theoretical isochrones, we derive for this cluster an age of 13 Gyrs. d) By using the large statistical sample of Red Giant Branch (RGB) stars, we detected with high accuracy the position of the bump in the RGB luminosity function. This observational feature has been compared with theoretical prescriptions, yielding a good agreement within the current theoretical and observational uncertainties.

Journal ArticleDOI
G. Marconi, Gloria Andreuzzi1, L. Pulone1, S. Cassisi1, Vincenzo Testa1, R. Buonanno1 
TL;DR: In this article, color-Magnitude Diagrams extending from the red-giant tip to about 5 magnitudes below the main-sequence turnoff MSTO (V = 20.05 +- 0.05) have been constructed.
Abstract: We have used ESO telescopes at La Silla and the Hubble Space Telescope (HST) in order to obtain accurate B,V,I CCD photometry for the stars located within 200" (~= 2 half-mass radii, r_h = 1.71') from the center of the cluster NGC 6101. Color-Magnitude Diagrams extending from the red-giant tip to about 5 magnitudes below the main-sequence turnoff MSTO (V = 20.05 +- 0.05) have been constructed. The following results have been obtained from the analysis of the CMDs: a) The overall morphology of the main branches confirms previous results from the literature, in particular the existence of a sizeable population of 73 "blue stragglers", which had been already partly detected (27).They are considerably more concentrated than either the subgiant branch or the main sequence stars, and have the same spatial distribution as the horizontal branch stars (84% prob. from K-S test). An hypothesis on the possible BSS progeny is also presented. b) The HB is narrow and the bulk of stars is blue, as expected for a typical metal-poor globular cluster. c) The derived magnitudes for the HB and the MSTO, $V(ZAHB) = 16.59+-0.10, V(TO) = 20.05+-0.05, coupled with the values E(B-V) = 0.1, [Fe/H] = -1.80, Y = 0.23 yield a distance modulus (m-M)_V = 16.23 and an age similar to other ``old'' metal-poor globular clusters. In particular, from the comparison with theoretical isochrones, we derive for this cluster an age of 13 Gyrs. d) By using the large statistical sample of Red Giant Branch (RGB) stars, we detected with high accuracy the position of the bump in the RGB luminosity function. This observational feature has been compared with theoretical prescriptions, yielding a good agreement within the current theoretical and observational uncertainties.

Journal ArticleDOI
TL;DR: In this article, the positions and local projected densities of the protostellar continuum sources in Ophiuchus and Serpens are investigated as a function of their masses in order to search for possible environmental effects during star formation.
Abstract: The positions and local projected densities of the protostellar continuum sources in Ophiuchus and Serpens are investigated as a function of their masses in order to search for possible environmental effects during star formation. There is a slight trend toward mass segregation for the 850 μm sources found by Johnstone and coworkers in Ophiuchus, but there is no obvious mass segregation for the 1.3 mm sources in Ophiuchus or Serpens. There is also no systematic change in the local projected source density with source mass, as might be expected if clump collisions or gas clearing are involved.

Journal ArticleDOI
TL;DR: In this article, the authors present an analysis of deep CCD photometry for the very rich, old open star cluster NGC 6819, which represents the first of nineteen open star clusters which were imaged as a part of the CFHT Open Star Cluster Survey.
Abstract: We present analysis of deep CCD photometry for the very rich, old open star cluster NGC 6819. These CFH12K data results represent the first of nineteen open star clusters which were imaged as a part of the CFHT Open Star Cluster Survey. We find a tight, very rich, main-sequence and turn-off consisting of over 2900 cluster stars in the V, B-V color-magnitude diagram (CMD). Main-sequence fitting of the un-evolved cluster stars with the Hyades star cluster yields a distance modulus of (m-M)v = 12.30 +/- 0.12, for a reddening of E(B-V) = 0.10. These values are consistent with a newly calculated theoretical stellar isochrone of age 2.5 Gyrs, which we take to be the age of the cluster. Detailed star counts indicate a much larger cluster extent (R = 9.5' +/- 1.0'), by a factor of ~2 over some previous estimates. Incompleteness tests confirm a slightly negatively sloped luminosity function extending to faint (V ~ 23) magnitudes which is indicative of a dynamically evolved cluster. Further luminosity function and mass segregation tests indicate that low mass objects (M < 0.65Mo) predominate in the outer regions of the cluster, 3.5 < R < 9.5. The estimation of the number of white dwarfs in NGC 6819 are in good agreement with the observed number. For those white dwarf candidates which pass both a statistical and image classification tests, we show comparisons to white dwarf isochrones and cooling models which suggest the need for spectroscopy to confirm the white dwarf nature of the brighter objects.

Journal ArticleDOI
TL;DR: In this paper, a simple test for the existence of a cluster of black hole remnants around Sgr A* was proposed based on a small sample of any type of Galactic Center objects, provided they are substantially less massive than the black holes and constitute part of an old (> 1 Gyr) population.
Abstract: We propose a simple test for the existence of a cluster of black hole remnants around Sgr A* that is based on a small sample of any type of Galactic Center objects, provided they are substantially less massive than the black holes and constitute part of an old (> 1 Gyr) population. The test relies on the fact that, under the presence of such a cluster of heavy remnants and because of energy equipartition, lower mass objects would be expelled from the central regions and settle into a distribution very different than the cusp expected to be induced by the supermassive black hole alone. We show that with a sample of just 50 objects and using only their angular positions on the sky relative to Sgr A* it is possible to clearly differentiate between a distribution consistent with the presence of the cluster of black holes and a power-law cusp distribution. We argue that millisecond pulsars might currently be the best candidate to perform this test, because of the large uncertainties involved in the age determination of less exotic objects. In addition, by measuring their first and second period derivatives, millisecond pulsars offer the rare opportunity of determining the complete phase space information of the objects. We show that this extra information improves the detection of mass segregation by about 30%.

Journal ArticleDOI
TL;DR: In this paper, the authors present the results of a new study of mass segregation in two-component star clusters, based on a large number of numerical N-body simulations using dynamical Monte Carlo code.
Abstract: We present the results of a new study of mass segregation in two-component star clusters, based on a large number of numerical N-body simulations using our recently developed dynamical Monte Carlo code. Specifically, we follow the dynamical evolution of clusters containing stars with individual masses m_1 as well as a tracer population of objects with individual masses m_2=\mu m_1, using N=10^5 total stars. For heavy tracers, which could represent stellar remnants such as neutron stars or black holes in a globular cluster, we characterize in a variety of ways the tendency for these objects to concentrate in or near the cluster core. In agreement with simple theoretical arguments, we find that the characteristic time for this mass segregation process varies as 1/\mu. For models with very light tracers (\mu <~ 10^-2), which could represent free-floating planets or brown dwarfs, we find the expected depletion of light objects in the cluster core, but also sometimes a significant enhancement in the halo. Using these results we estimate the optical depth to gravitational microlensing by planetary mass objects or brown dwarfs in typical globular clusters. For some initial conditions, the optical depth in the halo due to very low-mass objects could be much greater than that of luminous stars. If we apply our results to M22, using the recent null detection of Sahu, Anderson, & King (2001), we find an upper limit of ~25% at the 63% confidence level for the current mass fraction of M22 in the form of very low-mass objects.

Journal ArticleDOI
TL;DR: In this paper, the authors present deep CCD photometry of the very rich, intermediate aged (similar to the Hyades) open star cluster NGC 2099 (M37), showing an extremely well populated and very tightly constrained main-sequence extending over 12 magnitudes from the turn-off.
Abstract: We present deep CCD photometry of the very rich, intermediate aged (similar to the Hyades) open star cluster NGC 2099 (M37). The V, B-V color-magnitude diagram (CMD) for the cluster shows an extremely well populated and very tightly constrained main-sequence extending over 12 magnitudes from the turn-off. The CFH12K photometry for this cluster is faint enough (V ~ 24.5) to detect the remnants of the most massive progenitor cluster stars under the Type I SNe limit. Therefore, the CMD of the cluster also exhibits a well defined white dwarf `clump' caused by the decreased rate of cooling of these stars as they age, and a subsequent gap with very few objects. The termination point (Mv = 11.95 +/- 0.30) of the white dwarf luminosity function gives a white dwarf cooling age of 566 +/- 154/176 Myrs which is in excellent agreement with the main-sequence turn-off isochrone age (520 Myrs). By carefully accounting for errors, we show that the cooling age confirms that models including convective core overshooting are preferred for young-intermediate aged clusters. We also derive the reddening (E(B-V) = 0.21 +/- 0.03) and distance ((m-M)v = 11.55 +/- 0.13) to NGC 2099 by matching main-sequence features in the cluster to a new fiducial main-sequence for the Hyades. As a continuing part of the goals of the CFHT Open Star Cluster Survey to better understand dynamical processes of open clusters, we also fit a King model to the cluster density distribution and investigate the cluster main-sequence luminosity and mass functions in increasing concentric annuli. We find some evidence for mass segregation within the boundary of NGC 2099 as expected given the cluster's age relative to the dynamical age. The present global mass function for the cluster is found to be shallower than a Salpeter IMF.

Journal ArticleDOI
TL;DR: In this paper, a detailed analysis of HST/WFPC2 and STIS imaging observations, and of supplementary wide-field ground-based observations obtained with the NTT of two young compact star clusters in the LMC, NGC 1805 and NGC1818, was performed.
Abstract: We have undertaken a detailed analysis of HST/WFPC2 and STIS imaging observations, and of supplementary wide-field ground-based observations obtained with the NTT of two young ~10-25 Myr) compact star clusters in the LMC, NGC 1805 and NGC 1818. The ultimate goal of our work is to improve our understanding of the degree of primordial mass segregation in star clusters. This is crucial for the interpretation of observational luminosity functions (LFs) in terms of the initial mass function (IMF), and for constraining the universality of the IMF. We present evidence for strong luminosity segregation in both clusters. The LF slopes steepen with cluster radius; in both NGC 1805 and NGC 1818 the LF slopes reach a stable level well beyond the clusters' core or half-light radii. In addition, the brightest cluster stars are strongly concentrated within the inner ~4 R_hl. The global cluster LF, although strongly nonlinear, is fairly well approximated by the core or half-light LF; the (annular) LFs at these radii are dominated by the segregated high-luminosity stars, however. We present tentative evidence for the presence of an excess number of bright stars surrounding NGC 1818, for which we argue that they are most likely massive stars that have been collisionally ejected from the cluster core. We therefore suggest that the cores of massive young stars clusters undergo significant dynamical evolution, even on time-scales as short as ~25 Myr.

Posted Content
TL;DR: In this article, the authors present the case for how massive stars may form through stellar collisions, which requires very high stellar densities, up to 4 orders of magnitude higher than are observed in the cores of dense young clusters.
Abstract: In this review, I present the case for how massive stars may form through stellar collisions. This mechanism requires very high stellar densities, up to 4 orders of magnitude higher than are observed in the cores of dense young clusters. In this model, the required stellar densities arise due to gas accretion onto stars in the cluster core, including the precursers of the massive stars. This forces the core to contract until the stellar densities are sufficiently high for collisions to occur. Gas accretion is also likely to play a major role in determining the distribution of stellar masses in the cluster as well as the observed mass segregation. One of the main advantages of this mechanism is that it explicitly relies on the cluster environment in order to produce the massive stars. It is thus in a position to explain the relation between clustered and massive star formation which is not an obvious outcome of an isolated accretion mechanism. A recent numerical simulation supports this model as the cluster core increases its density by $10^5$ during gas accretion. Approximately 15 stellar collisions occur (with $R_{coll}=1$ au) in the cluster core, making a significant contribution to the mass of the most massive star.

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TL;DR: In this article, the theoretical definition of bound or unbound stellar systems, based on their central density and corresponding observational constraints regarding their classification (in stellar associations and open or globular clusters) are discussed.
Abstract: The central density ϱO in M [odot] pc−3 of a stellar system is known to be one of the principal parameters determining its dynamical history and disruption time. The central density of young populous clusters seems also to be a predominant parameter for mass segregation effects. The theoretical definition of bound or unbound stellar systems, based on their central density and the corresponding observational constraints regarding their classification (in stellar associations and open or globular clusters) are discussed. We also present our results on the spatial distribution of the star clusters in the LMC and SMC according to their central density, and the possible explanations for cluster formation in galaxies, which can be derived from these distributions.