Showing papers on "Mass segregation published in 2003"

Embedded Clusters in Molecular Clouds

Abstract: ▪ Abstract Stellar clusters are born embedded within giant molecular clouds (GMCs) and during their formation and early evolution are often only visible at infrared wavelengths, being heavily obscured by dust. Over the past 15 years advances in infrared detection capabilities have enabled the first systematic studies of embedded clusters in galactic molecular clouds. In this article we review the current state of empirical knowledge concerning these extremely young protocluster systems. From a survey of the literature we compile the first extensive catalog of galactic embedded clusters. We use the catalog to construct the mass function and estimate the birthrate for embedded clusters within ∼2 kpc of the sun. We find that the embedded cluster birthrate exceeds that of visible open clusters by an order of magnitude or more indicating a high infant mortality rate for protocluster systems. Less than 4–7% of embedded clusters survive emergence from molecular clouds to become bound clusters of Pleiades age. Th...

The Gravitational Million-Body Problem: A Multidisciplinary Approach to Star Cluster Dynamics

Abstract: The gravitational N-body problem is to describe the evolution of an isolated system of N point masses interacting only through Newtonian gravitational forces. For N =2 the solution is due to Newton. For N =3 there is no general analytic solution, but the problem has occupied generations of illustrious physicists and mathematicians including Laplace, Lagrange, Gauss and Poincare, and inspired the modern subjects of nonlinear dynamics and chaos theory. The general gravitational N-body problem remains one of the oldest unsolved problems in physics. Many-body problems can be simpler than few-body problems, and many physicists have attempted to apply the methods of classical equilibrium statistical mechanics to the gravitational N-body problem for N 1. These applications have had only limited success, partly because the gravitational force is too strong at both small scales (the interparticle potential energy diverges) and large scales (energy is not extensive). Nevertheless, we now understand a rich variety of behaviour in large-N gravitating systems. These include the negative heat capacity of isolated, gravitationally bound systems, which is the basic reason why nuclear burning in the Sun is stable; Antonov's discovery that an isothermal, self-gravitating gas in a container is located at a saddle point, rather than a maximum, of the entropy when the gas is sufficiently dense and hence is unstable (the 'gravothermal catastrophe'); the process of core collapse, in which relaxation induces a self-similar evolution of the central core of the system towards (formally) infinite density in a finite time; and the remarkable phenomenon of gravothermal oscillations, in which the central density undergoes periodic oscillations by factors of a thousand or more on the relaxation timescale - but only if N 104. The Gravitational Million-Body Problem is a monograph that describes our current understanding of the gravitational N-body problem. The authors have chosen to focus on N = 106 for two main reasons: first, direct numerical integrations of N-body systems are beginning to approach this threshold, and second, globular star clusters provide remarkably accurate physical instantiations of the idealized N-body problem with N = 105 – 106. The authors are distinguished contributors to the study of star-cluster dynamics and the gravitational N-body problem. The book contains lucid and concise descriptions of most of the important tools in the subject, with only a modest bias towards the authors' own interests. These tools include the two-body relaxation approximation, the Vlasov and Fokker-Planck equations, regularization of close encounters, conducting fluid models, Hill's approximation, Heggie's law for binary star evolution, symplectic integration algorithms, Liapunov exponents, and so on. The book also provides an up-to-date description of the principal processes that drive the evolution of idealized N-body systems - two-body relaxation, mass segregation, escape, core collapse and core bounce, binary star hardening, gravothermal oscillations - as well as additional processes such as stellar collisions and tidal shocks that affect real star clusters but not idealized N-body systems. In a relatively short (300 pages plus appendices) book such as this, many topics have to be omitted. The reader who is hoping to learn about the phenomenology of star clusters will be disappointed, as the description of their properties is limited to only a page of text; there is also almost no discussion of other, equally interesting N-body systems such as galaxies(N ≈ 106 – 1012), open clusters (N 102 – 104), planetary systems, or the star clusters surrounding black holes that are found in the centres of most galaxies. All of these omissions are defensible decisions. Less defensible is the uneven set of references in the text; for example, nowhere is the reader informed that the classic predecessor to this work was Spitzer's 1987 monograph, Dynamical Evolution of Globular Clusters, or that the standard reference on the observational properties of stellar systems is Binney and Merrifield's Galactic Astronomy. A minor irritation is that many concepts are discussed several times before they are defined, and the index provides no pointer to the primary discussion; thus, for example, there are ten index entries for 'phase mixing' and no indication that the fourth of these refers to the actual definition. The book is intended as a graduate textbook but more likely it will be used mainly in other contexts: by theoretical researchers, as an indispensable reference on the dynamics of gravitational N-body systems; by observational astronomers, as a readable summary of the theory of star cluster evolution; and by physicists seeking a well-written and accessible introduction to a simple problem that remains fascinating and incompletely understood after three centuries. Scott Tremaine

Kinematic masses of super-star clusters in m82 from high-resolution near-infrared spectroscopy

Abstract: Using high-resolution (R ~ 22,000) near-infrared (1.51-1.75 ?m) spectra from Keck Observatory, we measure the kinematic masses of two super-star clusters in M82. Cross-correlation of the spectra with template spectra of cool evolved stars gives stellar velocity dispersions of ?r = 15.9 ? 0.8 km s-1 for J0955505+694045 (MGG-9) and ?r = 11.4 ? 0.8 km s-1 for J0955502+694045 (MGG-11). The cluster spectra are dominated by the light of red supergiants and correlate most closely with template supergiants of spectral types M0 and M4.5. King model fits to the observed profiles of the clusters in archival Hubble Space Telescope/Near-Infrared Camera and Multi-Object Spectometer images give half-light radii of rhp = 2.6 ? 0.4 pc for MGG-9 and rhp = 1.2 ? 0.17 pc for MGG-11. Applying the virial theorem, we determine masses of 1.5 ? 0.3 ? 106 M? for MGG-9 and 3.5 ? 0.7 ? 105 M? for MGG-11 (where the quoted errors include ?r, rhp, and the distance). Population synthesis modeling suggests that MGG-9 is consistent with a standard initial mass function (IMF), whereas MGG-11 appears to be deficient in low-mass stars relative to a standard IMF. There is, however, evidence of mass segregation in the clusters, in which case the virial mass estimates would represent lower limits.

On the Central Structure of M15

Abstract: We present a detailed comparison between the latest observational data on the kinematical structure of the core of M15, obtained with the Hubble Space Telescope Space Telescope Imaging Spectrograph and Wide Field Planetary Camera 2 instruments, and the results of dynamical simulations carried out using the special purpose GRAPE-6 computer. The observations imply the presence of a significant amount of dark matter in the cluster core. In our dynamical simulations, neutron stars and/or massive white dwarfs concentrate to the center through mass segregation, resulting in a sharp increase in toward the center. While consistent with the presence of M/L a central black hole, the Hubble Space Telescope data can also be explained by this central concentration of stellar mass compact objects. The latter interpretation is more conservative, since such remnants result naturally from stellar evolution, although runaway merging leading to the formation of a black hole may also occur for some range of initial conditions. We conclude that no central massive object is required to explain the observational data, although we cannot conclusively exclude such an object at the level of. Our findings are similar to500-1000 M-circle dot. Our findings are unchanged when we reduce the assumed neutron star retention fraction in our simulations from 100% to 0%.

The CFHT Open Star Cluster Survey. IV. Two Rich, Young Open Star Clusters: NGC 2168 (M35) and NGC 2323 (M50)

Abstract: We continue our study of rich Galactic clusters by presenting deep CCD observations of both NGC 2168 (M35) and NGC 2323 (M50). Both clusters are found to be rich (NGC 2168 contains at least 1000 stars brighter than V = 22, and NGC 2323 contains ~2100 stars brighter than our photometric limit of V ~ 23) and young (NGC 2168 age = 180 Myr and for NGC 2323 age = 130 Myr). The color-magnitude diagrams for the clusters exhibit clear main sequences stretching over 14 mag in the (V, B-V)-plane. Comparing these long main sequences with those of earlier clusters in the survey, as well as with the Hyades, has allowed for accurate distances to be established for each cluster (for NGC 2168 d = 912 pc and for NGC 2323 d = 1000 pc). Analysis of the luminosity and mass functions suggests that, despite their young ages, both clusters are somewhat dynamically relaxed, exhibiting signs of mass segregation. This is especially interesting in the case of NGC 2323, which has an age of only 1.3 times the dynamical relaxation time. The present photometry is also deep enough to detect all of the white dwarfs in both clusters. We discuss some interesting candidates that may be the remnants of quite massive (M ≥ 5 M⊙) progenitor stars. The white dwarf cooling age of NGC 2168 is found to be in good agreement with the main-sequence turnoff age. These objects are potentially very important for setting constraints on the white dwarf initial-final mass relationship and the upper mass limit for white dwarf production.

The Mass Function of the Arches Cluster from Gemini Adaptive Optics Data

Abstract: We have analysed high resolution adaptive optics (AO) science demonstration data of the young, massive stellar cluster Arches near the Galactic Center, obtained with the Gemini North telescope in combination with the University of Hawai'i AO system Hokupa'a. The AO H and K 0 photometry is calibrated using HST/NICMOS observations in the equivalent filters F160W and F205W obtained by Figer et al. (1999). The calibration procedure allows a detailed comparison of the ground-based adaptive optics observations against diraction limited space-based photometry. The spatial resolution as well as the overall signal-to-noise ratio of the Gemini/Hokupa'a data is comparable to the HST/NICMOS data. The low Strehl ratio of only a few percent is the dominant limiting factor in the Gemini AO science demonstration data as opposed to space-based observations. After a thorough technical comparison, the Gemini and HST data are used in combination to study the spatial distribution of stellar masses in the Arches cluster. Arches is one of the densest young clusters known in the Milky Way, with a central density of3 10 5 M pc 3 and a total mass of about 10 4 M. A strong colour gradient is observed over the cluster field. The visual extinction increases byAV 10 mag over a distance of 15 00 from the cluster core. Extinction maps reveal a low-extinction cavity in the densest parts of Arches (R 5 00 ), indicating the depletion of dust due to stellar winds or photo-evaporation. We correct for the change in extinction over the field and show that the slope of the mass function is strongly influenced by the eects of dierential extinction. We obtain present-day mass function slopes of 0:8 0: 2i n the mass range 6 10 00 , in accordance with a Salpeter slope ( = 1:35). The bias in the mass function towards high-mass stars in the Arches center is a strong indication for mass segregation. The dynamical and relaxation timescales for Arches are estimated, and possible mass segregation eects are discussed with respect to cluster formation models.

Mass Segregation in Young Magellanic Clouds Star Clusters: Four Clusters observed with HST

Abstract: We present the results of our investigation on the phenomenon of mass segregation in young star clusters in the Magellanic Clouds. HST/WFPC2 observations on NGC 1818, NGC 2004 & NGC 2100 in the Large Magellanic Cloud and NGC 330 in the Small Magellanic Cloud have been used for the application of diagnostic tools for mass segregation: i) the radial density profiles of the clusters for various mass groups and ii) their mass functions (MFs) at various radii around their centres. All four clusters are found to be mass segregated, but each one in a different manner. Specifically not all the clusters in the sample show the same dependence of their density profiles on the selected magnitude range, with NGC 1818 giving evidence of a strong such relation and NGC 330 showing only a hint of the phenomenon. NGC 2004 did not also show any significant signature of mass segregation in its density profiles. The MFs radial dependence provides clear proof of the phenomenon for NGC 1818, NGC 2100 and NGC 2004, while for NGC 330 it gives only indications. An investigation on the constraints introduced by the application of both diagnostic tools is presented. We also discuss the problems related to the construction of a reliable MF for a cluster and their impact on the investigation of the phenomenon of mass segregation. We find that the MFs of these clusters as they were constructed with two methods, are comparable to Salpeter's IMF. A discussion is given on the dynamical status of the clusters and a test is applied on the equipartition among several mass groups in them. Both showed that the observed mass segregation in the clusters is of primordial nature.

Mass segregation in M 67 with 2MASS

Abstract: We make use of the homogeneity, depth and sky coverage of the 2MASS catalogue to study spatial variations of the stellar luminosity function in the intermediate age cluster M 67 (NGC 2682). We find that the central region ( R< 3:3 0 )h as a depletion of G 0 and G 5 stars with respect to the intermediate annulus (3:3 0 < R< 7:4 0 ) and the halo (12 0 < R< 24 0 ). In the non-eroded part of the mass function the slope is steeper in the intermediate annulus than in the central region, suggesting an enhancement of F 0 stars with respect to turno and A 5 stars in the latter zone. In the halo, the G 0-G 5 stars are not depleted as compared to the central region, but the overall mass function slope is rather shallow. This suggests that the halo is enriched in low mass stars, transferred there from the inner parts as a consequence of the internal dynamical evolution, but tidal losses to the Galactic field have also been important.

A stochastic Monte Carlo approach to modelling real star cluster evolution — III. Direct integration of three- and four-body interactions

Abstract: Spherically symmetric equal-mass star clusters containing a large number of primordial binaries are studied using a hybrid method, consisting of a gas dynamical model for single stars and a Monte Carlo treatment for relaxation of binaries and the setup of close resonant and fly-by encounters of single stars with binaries and binaries with each other (three- and four-body encounters). What differs from our previous work is that each encounter is being integrated using a highly accurate direct few-body integrator which uses regularized variables. Hence we can study the systematic evolution of individual binary orbital parameters (eccentricity, semi-major axis) and differential and total cross-sections for hardening, dissolution or merging of binaries (minimum distance) from a sampling of several tens of thousands of scattering events as they occur in real cluster evolution, including mass segregation of binaries, gravothermal collapse and re-expansion, a binary burning phase and ultimately gravothermal oscillations. For the first time we are able to present empirical cross-sections for eccentricity variation of binaries in close three- and four-body encounters. It is found that a large fraction of three-and four-body encounters result in merging. Eccentricities are generally increased in strong three- and four-body encounters and there is a characteristic scaling law α exp (4e f i n ) of the differential cross-section for eccentricity changes, where e f i n is the final eccentricity of the binary, or harder binary for four-body encounters. Despite these findings the overall eccentricity distribution remains thermal for all binding energies of binaries, which is understood from the dominant influence of resonant encounters. Previous cross-sections obtained by Spitzer and Gao for strong encounters can be reproduced, while for weak encounters non-standard processes such as the formation of hierarchical triples occur.

White Dwarf Sequences in Dense Star Clusters

Abstract: We use the results of realistic N-body simulations to investigate the appearance of the white dwarf population in dense star clusters. We show that the presence of a substantial binary population in a star cluster, and the interaction of this population with the cluster environment, has serious consequences for the morphology of the observed white dwarf sequence and the derived white dwarf cooling age of the cluster. We find that over time the dynamical evolution of the cluster?mass segregation, stellar interactions, and tidal stripping?hampers the use of white dwarfs as tracers of the initial mass function and also leads to a significant enhancement of the white dwarf mass fraction. Future observations of star clusters should be conducted slightly interior to the half-mass radius of the cluster in order to best obtain information about the cluster age and initial mass function from the white dwarf luminosity function. The evolution of binary stars and the cluster environment must necessarily be accounted for when the white dwarf populations of dynamically evolved star clusters are studied.

A stochastic Monte Carlo approach to model real star cluster evolution, III. Direct integrations of three- and four-body interactions

Abstract: Spherically symmetric equal mass star clusters containing a large amount of primordial binaries are studied using a hybrid method, consisting of a gas dynamical model for single stars and a Monte Carlo treatment for relaxation of binaries and the setup of close resonant and fly-by encounters of single stars with binaries and binaries with each other (three- and four-body encounters). What differs from our previous work is that each encounter is being integrated using a highly accurate direct few-body integrator which uses regularized variables. Hence we can study the systematic evolution of individual binary orbital parameters (eccentricity, semi-major axis) and differential and total cross sections for hardening, dissolution or merging of binaries (minimum distance) from a sampling of several ten thousands of scattering events as they occur in real cluster evolution including mass segregation of binaries, gravothermal collapse and reexpansion, binary burning phase and ultimately gravothermal oscillations. For the first time we are able to present empirical cross sections for eccentricity variation of binaries in close three- and four-body encounters. It is found that a large fraction of three-body and four-body encounters results in merging. Previous cross sections obtained by Spitzer and Gao for strong encounters can be reproduced, while for weak encounters non-standard processes like formation of hierarchical triples occur.

Population and dynamical state of the eta Chamaeleontis sparse young open cluster

Abstract: We report new results in our continuing study of the unique compact (1 pc extent), nearby (d = 97 pc), young (t = 9 Myr) stellar cluster dominated by the B9 star eta Chamaeleontis. An optical photometric survey spanning 1.3 x 1.3 pc adds two M5-M5.5 weak-lined T Tauri (WTT) stars to the cluster inventory which is likely to be significantly complete for primaries with masses > 0.15 M_sun. The cluster now consists of 17 primaries and approximately 9 secondaries lying within 100 AU of their primaries. The apparent distribution of 9:7:1 single:binary:triple systems shows 2-4 x higher multiplicity than in the field main sequence stars, and is comparable to that seen in other pre-main sequence (PMS) populations. The initial mass function (IMF) is consistent with that of rich young clusters and field stars. By extending the cluster IMF to lower masses, we predict 10-14 additional low mass stars with 0.08 50 percent of the stellar mass residing in the inner 6 arcmin (0.17 pc). Considering that the eta Cha cluster is sparse, diffuse and young, the cluster may be an ideal laboratory for distinguishing between mass segregation that is primordial in nature, or arising from dynamical interaction processes.

Wide-Field CCD Photometry of the Globular Cluster M92

Abstract: We present wide-field CCD photometry of the Galactic globular cluster M92 obtained in the V and I bands with the CFH12K mosaic CCD at the Canada-France-Hawaii Telescope. A well-defined color-magnitude diagram is derived down to 5 mag fainter than the cluster main-sequence turnoff. After removing the background contribution, we obtain luminosity and mass functions, surface density profiles, and the surface number density maps of the stars belonging to the cluster. The surface density profile of all stars shows that the cluster's halo extends at least out to ~30' from the cluster center, in agreement with previous study, but the profile of faint stars at the very outer region of the cluster shows a different gradient compared with that of bright stars. For a mass function of the form Φ(M) ∝ M-(1+x), we find that the inner region (5' < r < 9') of the cluster has x 1.2 ± 0.2, whereas the outer region (9' < r < 15') has x 1.8 ± 0.3, clearly indicating a mass segregation of the cluster. An estimate of the photometric mass of the cluster implies that the remnant populations (white dwarfs and neutron stars) contribute at least 25% of the total cluster mass. The surface density map of M92 shows some evidence that the tidal tail of M92 may be oriented perpendicular to the direction toward the Galactic center.

Radio and far-infrared emission as tracers of star formation and agn in nearby cluster galaxies

Abstract: We have studied the radio and far-infrared (FIR) emission from 114 galaxies in the 7 nearest clusters (< 100 Mpc) with prominent X-ray emission to investigate the impact of the cluster environment on the star formation and AGN activity in the member galaxies. The X-ray selection criterion is adopted to focus on the most massive and dynamically relaxed clusters. A large majority of cluster galaxies show an excess in radio emission over that predicted from the radio-FIR correlation, the fraction of sources with radio excess increases toward cluster cores, and the radial gradient in the FIR/radio flux ratio is a result of radio enhancement. Of the radio-excess sources, 70% are early-type galaxies and the same fraction host an AGN. The galaxy density drops by a factor of 10 from the composite cluster center out to 1.5 Mpc, yet galaxies show no change in FIR properties over this region, and show no indication of mass segregation. We have examined in detail the physical mechanisms that might impact the FIR and radio emission of cluster galaxies. While collisional heating of dust may be important for galaxies in cluster centers, it appears to have a negligible effect on the observed FIR emission for our sample galaxies. The correlations between radio and FIR luminosity and radius could be explained by magnetic compression from thermal ICM pressure. We also find that simple delayed harassment cannot fully account for the observed radio, FIR, and mid-IR properties of cluster galaxies.

Mass segregation in the young open cluster NGC 2547

Abstract: We present a study of mass segregation of the young (20-35 Myr isochronal age) open cluster NGC 2547. We find good evidence that mass segregation exists in NGC 2547 down to 3 M O ., and weak evidence for mass segregationdown to 1 M O .. Theoretical models of an initially unsegregated model of NGC 2547 using the NBODY2 code show weaker mass segregation, implying that at least some of the observed mass segregation has a primordial origin. We also report the discovery of three possible escaped cluster members, which share the proper motion and colours of the cluster, but lie nearly a degree from the cluster centre.

Kinematics and binaries in young stellar aggregates. II. NGC 6913 = M29

Abstract: Between 1996 and 2003 we have obtained 226 high resolution spectra of 16 stars in the field of the young open cluster NGC 6913, to the aim of constraining its main properties and study its internal kinematics. Twelve of the program stars turned out to be members, one of them probably unbound. Nine are binaries (one eclipsing and another double lined) and for seven of them the observations allowed to derive the orbital elements. All but two of the nine discovered binaries are cluster members. In spite of the young age (a few Myr), the cluster already shows signs that could be interpreted as evidence of dynamical relaxation and mass segregation. However, they may be also the result of an unconventional formation scenario. The dynamical (virial) mass as estimated from the radial velocity dispersion is larger than the cluster luminous mass, which may be explained by a combination of the optically thick interstellar cloud that occults part of the cluster, the unbound state or undetected very wide binary orbit of some of the members that inflate the velocity dispersion and a high inclination for the axis of a possible cluster angular momentum. All discovered binaries are hard enough to survive average close encounters within the cluster and do not yet show sign of relaxation of the orbital elements to values typical of field binaries.

Mass Functions and Mass Segregation in Young Starburst Clusters

Abstract: The Milky Way starburst clusters Arches in the Galactic Center region and NGC 3603 in the Carina spiral arm are studied with the aim to gain deeper insight into the stellar mass distribution in starburst clusters. The dense stellar population in both clusters is resolved in unprecedented detail with high angular resolution, near-infrared instruments. In the case of the Arches cluster, diffraction-limited, adaptive optics observations are analysed, and the achievements and limitations of ground-based vs. space-based diffraction-limited imaging are revealed by comparison with HST/NICMOS data. In the case of NGC 3603, seeing-limited JHKL photometry is used to derive colour-excess fractions, and is complemented by space-based Halpha data, both serving as tracers for circumstellar disks. Disk survival in starburst clusters is discussed. The present-day mass function (MF) of both clusters is deduced from colour-magnitude diagrams. Radial variations in the MFs reveal a heavily mass-segregated core in both starburst clusters. Dynamical timescales are estimated and interpreted with respect to primordial and dynamical segregation. The implications for massive star and cluster formation scenarios are discussed. Evidence for a low-mass cut-off is observed in the Arches MF, but not in NGC 3603, indicating a reduced formation efficiency for M < 10 Msun stars in the Galactic Center. This environmental difference has strong implications for the formation of stellar populations in galactic nuclei and starburst galaxies.

Collisional Dynamics of Black Holes, Star Clusters and Galactic Nuclei

Abstract: We use high-precision direct N-body integration to study questions of the thermodynamic behaviour of dense stellar systems The processes examined include mass segregation and equipartition processes, the study of planetary orbits in dense star clusters, and stellar orbits in galactic nuclei with thick accretion disks

The luminosity function of Palomar 5 and its tidal tails

Abstract: We present the main sequence luminosity function of the tidally disrupted globular cluster Palomar 5 and its tidal tails. For this work we analyzed imaging data obtained with the Wide Field Camera at the INT (La Palma) and data from the Wide Field Imager at the MPG/ESO 2.2 m telescope at La Silla down to a limiting magnitude of approximately 24.5 mag in B. Our results indicate that preferentially fainter stars were removed from the cluster so that the LF of the cluster's main body exhibits a significant degree of flattening compared to other GCs. This is attributed to its advanced dynamical evolution. The LF of the tails is, in turn, enhanced with faint, low-mass stars, which we interpret as a consequence of mass segregation in the cluster.

The evolution of the initial mass function: from globular to young clusters *

Abstract: The main sequence mass functions (MF) of a large sample of Galactic clusters (young and old) can be well reproduced with a tapered power law distribution function with an exponential truncation of the form d N /d M ∝ m -α [1 - exp(- m / m p ) β ]. The average value of the power-law index α is very close to Salpeter (~2.3), whereas the peak mass m p is in the range 0.1-0.6 M ⊙ and does not seem to vary in a systematic way with the present cluster parameters such as metal abundance and central concentration. A remarkable correlation with age, however, is seen in that older disc clusters have higher m p , although this trend does not extend to globular clusters, whose value of m p is lower than that of old open clusters. This trend most likely results from the onset of mass segregation following early dynamical interactions in the loose cluster cores. Differences between globular and younger clusters may depend on the initial environment of star formation, which in turn affects their total mass. Mass functions of field populations such as the solar neighbourhood and bulge are consistent with the hypothesis that they were built up over time by contributions from many functions of this type with different peak masses.

The Stellar mass function of Galactic clusters and its evolution

Abstract: The stellar mass function of a large sample of Galactic clusters (young and old) can be well reproduced with a tapered power law distribution function with an exponential truncation of the form dN/dM α m −α [1−exp(−m/m p)β]. The average value of the power-law index α is very close to Salpeter (∼ 2.3), whereas the peak mass m p is in the range 0.1–0.6 M⊙ and does not seem to vary in a systematic way with the present cluster parameters such as metal abundance and central concentration. A remarkable correlation with age, however, is seen in that older disc clusters have highermp, m p although this trend does not extend to globular clusters, whose value of m p is lower than that of old open clusters. This trend most likely results from the onset of mass segregation following early dynamical interactions in the loose cluster cores. Dierences between globular and younger clusters may depend on the initial environment of star formation.

Is mass segregation in star clusters primordial? Simulations of spherical star clusters

Abstract: We have written a program in C to investigate the primordiality of mass segregation in star clusters. As integration method we use the time-symmetric leapfrog. From the simulation data we calculate ln⁄ (the coulomb logarithm). The method we use for this is not very precise, but when we compare our result with the ln⁄ Bonnell and Davies have found in a similar study in 1998 [1], we flnd that they are almost exactly the same. From our results we can conclude that mass segregation takes place and from the comparison with Bonnell and Davies we flnd that it may be primordial.

Massive Stars: Their Birth Sites and Distribution

Abstract: The stellar IMF has been found to be an invariant Salpeter power-law (alpha=2.35) above about 1 Msun, but at the same time a massive star typically has more than one companion. This constrains the possible formation scenarios of massive stars, but also implies that the true, binary-star corrected stellar IMF could be significantly steeper than Salpeter, alpha>2.7. A significant fraction of all OB stars are found relatively far from potential birth sites which is most probably a result of dynamical ejections from cores of binary-rich star clusters. Such cores form rapidly due to dynamical mass segregation, or they are primordial. Probably all OB stars thus form in stellar clusters together with low-mass stars, and they have a rather devastating effect on the embedded cluster by rapidly driving out the remaining gas leaving expanding OB associations and bound star clusters. The distributed population of OB stars has a measured IMF with alpha about 4, which however, does not necessarily constitute a different physical mode for isolated star formation. A steep field-star IMF is obtained naturally because stars form in clusters which are distributed according to a power-law cluster mass function.

Mass Segregation in Young LMC Clusters

Abstract: We present the detailed analysis of Hubble Space Telescope observations of the spatial distributions of different stellar species in two young compact star clusters in the Large Magellanic Cloud (LMC), NGC 1805 and NGC 1818. Based on a comparison of the characteristic relaxation times in their cores and at their half-mass radii with the observed degree of mass segregation, it is most likely that significant primordial mass segregation was present in both clusters, particularly in NGC 1805. Both clusters were likely formed with very similar initial mass functions (IMFs). In fact, we provide strong support for the universality of the IMF in LMC clusters for stellar masses m >= 0.8 M_sun.