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Showing papers on "Mass segregation published in 2005"


Journal ArticleDOI
TL;DR: In this paper, the authors detect 300 X-ray sources within the half-mass radius (279) of the globular cluster 47 Tucanae in a deep (281 ks) Chandra exposure.
Abstract: We have detected 300 X-ray sources within the half-mass radius (279) of the globular cluster 47 Tucanae in a deep (281 ks) Chandra exposure. We perform photometry and simple spectral fitting for our detected sources and construct luminosity functions, X-ray color-magnitude, and color-color diagrams. Eighty-seven X-ray sources show variability on timescales from hours to years. Thirty-one of the new X-ray sources are identified with chromospherically active binaries from the catalogs of Albrow and coworkers. The radial distributions of detected sources imply that roughly 70 are background sources of some kind. The radial distribution of the known millisecond pulsar (MSP) systems is consistent with that expected from mass segregation, if the average neutron star mass is 1.39 ? 0.19 M?. Most source spectra are well fitted by thermal plasma models, except for quiescent low-mass X-ray binaries (qLMXBs; containing accreting neutron stars) and MSPs. We identify three new candidate qLMXBs with relatively low X-ray luminosities. One of the brightest cataclysmic variables (CVs; X10) shows evidence (a 4.7 hr period pulsation and strong soft X-ray emission) for a magnetically dominated accretion flow as in AM Her systems. Most of the bright CVs require intrinsic NH columns of order 1021 cm-2, suggesting a possible DQ Her nature. A group of X-ray sources associated with bright (sub)giant stars also requires intrinsic absorption. By comparing the X-ray colors, luminosities, variability, and quality of spectral fits of the detected MSPs to those of unidentified sources, we estimate that a total of ~25 MSPs exist in 47 Tuc (<60 at 95% confidence), regardless of their radio beaming fraction. We estimate that the total number of neutron stars in 47 Tuc is of order 300, reducing the discrepancy between theoretical neutron star retention rates and observed neutron star populations in globular clusters. Comprehensive tables of source properties and simple spectral fits are provided electronically.

223 citations


Journal ArticleDOI
TL;DR: In this paper, photometric, structural and dynamical evolution-related parameters of 11 nearby open clus- ters with ages in the range 70Myr to 7Gyr and masses of approximately 400M to 5300M were derived from the background-subtracted radial density profiles.
Abstract: Received -; accepted - Abstract. We derive photometric, structural and dynamical evolution-related parameters of 11 nearby open clus- ters with ages in the range 70Myr to 7Gyr and masses in the range � 400M⊙ to � 5300M⊙. The clusters are homogeneously analysed by means of J, H and KS 2MASS photometry, which provides spatial coverage wide enough to properly take into account the contamination of the cluster field by Galaxy stars. Structural parameters such as core and limiting radii are derived from the background-subtracted radial density profiles. Luminosity and mass functions (MFs) are built for stars later than the turnoff and brighter than the 2MASS PSC 99.9% completeness limit. The total mass locked up in stars in the core and the whole cluster, as well as the correspond- ing mass densities, are calculated by taking into account the observed stars (evolved and main sequence) and extrapolating the MFs down to the H-burning mass limit, 0.08M⊙. We illustrate the methods by analysing for the first time in the near-infrared the populous open clusters NGC2477 and NGC2516. For NGC2477 we derive an age of 1.1±0.1Gyr, distance from the Sun d⊙ = 1.2±0.1kpc, core radius Rcore = 1.4±0.1pc, limiting radius Rlim = 11.6 ± 0.7pc and total mass mtot � (5.3 ± 1.6) × 10 3 M⊙. Large-scale mass segregation in NGC2477 is reflected in the significant variation of the MF slopes in different spatial regions of the cluster, and in the large number-density of giant stars in the core with respect to the cluster as a whole. For NGC2516 we derive an age of 160 ± 10Myr, d⊙ = 0.44 ± 0.02 kpc, Rcore = 0.6 ± 0.1pc, Rlim = 6.2 ± 0.2pc and mtot � (1.3 ± 0.2) × 10 3 M⊙. Mass-segregation in NGC2516 shows up in the MFs. Six of the 11 clusters present a slope break in the MF occurring at essentially the same mass as that found for the field stars in Kroupa's universal IMF. The MF break is not associated to cluster mass, at least in the clusters in this paper. In two clusters the low-mass end of the MF occurs above the MF break. However, in three clusters the MF break does not occur, at least for the mass range m � 0.7M⊙. One possibility is dynamical evolution affecting the MF slope distribution. We also search for relations of structural and evolutionary parameters with age and Galactocentric distance. The main results for the present sample are: (i) cluster size correlates both with age and Galactocentric distance; (ii) because of size and mass scaling, core and limiting radii, and core and overall mass correlate; (iii) massive (m � 1000M⊙) and less-massive clusters follow separate correlation paths on the plane core radius and overall mass; (iv) MF slopes of massive clusters are restricted to a narrow range, while those of the less-massive ones distribute over a wider range. Core and overall MF flattening is related to the ratio (�) of age to relaxation time. For large values ofthe effects of large-scale mass segregation and low-mass stars evaporation can be observed in the MFs. In this sense, � appears to characterize the evolutionary state of the clusters. We conclude that appreciable slope flattenings in the overall MFs of the less-massive clusters take � 6 times longer to occur than in the core, while in the massive clusters they take a time � 13 times longer. We investigate cluster parameters equivalent to those determining the fundamental plane of ellipticals. These parameters are: overall mass, projected mass density and core radius. We conclude that in the present sample there is evidence of a fundamental plane. Larger samples are necessary to pin down this issue.

108 citations


Journal ArticleDOI
TL;DR: In this article, the authors used 2MASS photometry to study colour-magnitude and colour-colour diagrams, structure and mass distribution in the ionizing open cluster NGC\,6611.
Abstract: We use 2MASS photometry to study colour-magnitude and colour-colour diagrams, structure and mass distribution in the ionizing open cluster NGC\,6611. Reddening variation throughout the cluster region is taken into account followed by field-star decontamination of the CMDs. The field-star decontamination showed that the lower limit of the main sequence (MS) occurs at $\rm\approx5\,\ms$. Based on the fraction of Ks excess stars in the colour-colour diagram we estimate an age of $1.3\pm0.3$\,Myr which is consistent with the presence of a large number of pre-main sequence (PMS) stars. The radial density distribution including MS and PMS stars is fitted by a King profile with a core radius $0.70\pm0.08\,pc$. The cluster density profile merges into the background at a limiting radius $6.5\pm0.5\,pc$. In the halo and through the whole cluster the MFs have slopes $\rm\chi=1.52\pm0.13$ and $\rm\chi=1.45\pm0.12$, respectively, thus slightly steeper than Salpeter's IMF. In the core the MF is flat, $\rm\chi=0.62\pm0.16$, indicating some degree of mass segregation since the cluster age is a factor $\sim2$ larger than the relaxation time. Because of the very young age of NGC\,6611, part of this effect appears to be related to the molecular cloud-fragmentation process itself. We detect $362\pm120$ PMS stars. The total observed mass including detected MS (in the range $\rm5-85\,\ms$) and PMS stars amounts to $\sim1\,600\,\ms$, thus more massive than the Trapezium cluster. Compared to older open clusters of different masses, the overall NGC\,6611 fits in the relations involving structural and dynamical parameters. However, the core is atypical in the sense that it looks like an old/dynamically evolved core. Again, part of this effect must be linked to formation processes.

88 citations


Journal ArticleDOI
TL;DR: In this article, luminosity and mass functions in the old open cluster NGC 188 are analyzed by means of J and H 2MASS pho-tometry, which provides uniformity and spatial coverage for a proper background subtraction.
Abstract: Luminosity and mass functions in the old open cluster NGC 188 are analysed by means of J and H 2MASS pho- tometry, which provides uniformity and spatial coverage for a proper background subtraction. With an age of about 6-8 Gyr, NGC 188 is expected to be suffering the effects of advanced dynamical evolution. Indeed, previous works in optical bands have suggested the presence of mass segregation. Within the uncertainties, the observed projected radial density profile of NGC 188 departs from the two-parameter King model in two inner regions, which reflects the non-virialized dynamical state and possibly, some degree of non-sphericity in the spatial shape of this old open cluster. Fits with two and three-parameter King models to the radial distribution of stars resulted in a core radius Rcore = 1.3 ± 0.1 pc and a tidal radius Rtidal = 21 ± 4 pc, about twice as large as the visual limiting radius. The concentration parameter c = 1.2 ± 0.1 of NGC 188 makes this open cluster structurally comparable to the loose globular clusters. The present 2MASS analysis resulted in significant slope variations with distance in the mass function φ(m) ∝ m −(1+χ) , being flat in the central parts (χ = 0.6 ± 0.7) and steep in the cluster outskirts (χ = 7.2 ± 0.6). The overall mass function has a slope χ = 1.9 ± 0.7, slightly steeper than a standard Salpeter mass function. In this context, NGC 188 is similar to the 3.2 Gyr, dynamically evolved open cluster M 67. Solar metallicity Padova isochrone fits to the near- infrared colour-magnitude diagram of NGC 188 resulted in an age of 7.0 ± 1.0 Gyr. The best fit, obtained with the 7.1 Gyr isochrone, produced a distance modulus (m − M)0 = 11.1 ± 0.1, E(B − V) = 0.0, and a distance to the Sun d� = 1.66 ± 0.08 kpc. The observed stellar mass (in the range 0.98 M� −1.08 M� ) in NGC 188 is mobs = 380 ± 12 M� . A simple extrapolation of the observed overall mass function to stars with 0.08 Mresulted in a total present mass of mtot ∼ (1.8 ± 0.7) × 10 4 M� .O n the other hand, for a more realistic initial mass function which flattens in the low-mass range, the total mass in NGC 188 drops to mtot ∼ (3.8 ± 1.6) × 10 3 M� . Since mass-loss processes such as evaporation and tidal stripping have been occurring in this old open cluster for about 7 Gyr, the primordial mass in NGC 188 must have been significantly larger than ∼4 × 10 3 M� .W e also examined the consequences of the presence of unresolved binaries and concluded that, even if dominant in the CMD, binaries alone are not responsible for the flat central mass function, which supports the mass-segregation scenario.

87 citations


Journal ArticleDOI
TL;DR: In this paper, the initial mass function and mass segregation in super star cluster M82-F with high-resolution Keck NIRSPEC echelle spectroscopy were investigated.
Abstract: We investigate the initial mass function and mass segregation in super star cluster M82-F with high-resolution Keck NIRSPEC echelle spectroscopy. Cross-correlation with template supergiant spectra provides the velocity dispersion of the cluster, enabling measurement of the kinematic (virial) mass of the cluster when combined with sizes from NICMOS and Advanced Camera for Surveys (ACS) images. We find a mass of 6.6 ± 0.9 × 105 M☉ based on near-IR light and 7.0 ± 1.2 × 105 M☉ based on optical light. Using PSF-fitting photometry, we derive the cluster's light-to-mass (L/M) ratio in both near-IR and optical light and compare to population-synthesis models. The ratios are inconsistent with a normal stellar initial mass function for the adopted age of 40-60 Myr, suggesting a deficiency of low-mass stars within the volume sampled. King model light profile fits to new Hubble Space Telescope ACS images of M82-F, in combination with fits to archival near-IR images, indicate mass segregation in the cluster. As a result, the virial mass represents a lower limit on the mass of the cluster.

73 citations


Journal ArticleDOI
TL;DR: In this paper, a Monte Carlo code is used to simulate the stellar dynamics of spherical star clusters using a very large number of particles (a few 1e5 to several 1e6).
Abstract: We present the methods and preparatory work for our study of the collisional runaway scenario to form a very massive star (VMS, M > 400 Msun) at the centre of a young, compact stellar cluster. In the first phase of the process, a very dense central core of massive stars (M =~ 30-120 Msun) forms through mass segregation and gravothermal collapse. This leads to a collisional stage, likely to result in the formation of a VMS (itself a possible progenitor for an intermediate-mass black hole) through a runaway sequence of mergers between the massive stars. In this paper we present the runaway scenario in a general astrophysical context. We then explain the numerical method used to investigate it. Our approach is based on a Monte Carlo code to simulate the stellar dynamics of spherical star clusters using a very large number of particles (a few 1e5 to several 1e6). Finally, we report on test computations carried out to ensure that our implementation of the important physics is sound. In a second paper, we present results from more than 100 cluster simulations realized to determine the conditions leading to the collisional formation of a VMS and the characteristics of the runaway sequences.

64 citations


Journal ArticleDOI
TL;DR: In this paper, the authors derived photometric, structural and dynamical evolution-related parameters of five as yet unstudied low-contrast open clusters located in the third quadrant using 2MASS data.
Abstract: We derive photometric, structural and dynamical evolution-related parameters of five as yet unstudied low-contrast open clusters located in the third quadrant using 2MASS data. The target clusters are Czernik 31, Czernik 32, Haffner 9, Haffner 11 and Trumpler 13. We apply a statistical field-star decontamination procedure to infer on the intrinsic colour–magnitude diagram (CMD) morphology which is critical for such low-contrast objects. Consequently, it became possible to derive accurate reddening, age, distance from the Sun and Galactocentric distance for the five clusters. In the structural and luminosity/mass-function analyses we apply a colour-magnitude filter which encompasses the cluster evolutionary CMD sequences and excludes stars with discrepant colours. Using this procedure we derive core and limiting radii, mass function (MF) slope, total mass, mass density and relaxation time. We derive ages in the range 140–1100 Myr, Galactocentric distances within 7.7–11.4 kpc, and total masses within 360–2900 $M_\odot$. Reflecting large-scale mass segregation, the MF slope in the core is significantly flatter than that in the halo of the five clusters. Although some of the present clusters are relatively younger than the Gyr-old clusters, they present evidence of advanced dynamical evolution. This kind of study has become possible because of the photometric uniformity and spatial coverage of 2MASS which allows a proper subtraction of the field-star contamination on the target CMDs. The present study indicates that low-contrast clusters can be studied with 2MASS, particularly after field-star subtraction, which is important since most of the unstudied open clusters belong to this class.

42 citations


Journal ArticleDOI
TL;DR: In this article, a wide-angle 2MASS spatial coverage was used to derive photometric and structural parameters and discuss the spatial dependance of the luminosity and mass functions.
Abstract: An overall analysis of the structure and stellar content of M 11 is presented, thanks to the wide-angle 2MASS spatial coverage. We derive photometric and structural parameters and discuss the spatial dependance of the luminosity and mass functions. Photometric parameters basically agree with previous ones mostly based on the optical. We obtained a core radius of 1.23 pc and a tidal radius of 29 pc. In particular, the cluster is populous enough so that the tidal radius could be obtained by fitting the three-parameter King profile to the radial distribution of stars. We analyzed the spatial distribution of mass functions, finding that the slope changes from -0.73 in the core to +2.88 in the outer halo. The spatial distribution of mass function slopes derived from 2MASS agrees with that derived from optical CCD data, which further confirms the reliability of 2MASS data for future analyses of this kind at comparable observational limits. We detect mass segregation up to distances from the center of ≈ $20\arcmin$. We emphasize that the mass function slope in the core is flatter than anywhere else as a consequence of mass segregation. The derived total cluster mass is ≈ $11\,000\,\mbox{$M_\odot$}$.

36 citations


Journal ArticleDOI
TL;DR: In this article, an automatic method for deriving spectral types and comparing classifications for observations made at two different facilities is described. But, there appears to be a correlation between the spectral types of the stars used and the resulting value of the extinction.
Abstract: We use V magnitudes and spectral types to examine the density structure of h and χ Per. We describe an automatic method for deriving spectral types and compare classifications for observations made at two different facilities. With these data, we measure an extinction to the clusters of E(B - V) = 0.52 ± 0.07, consistent with other authors. However, there appears to be a correlation between the spectral types of the stars used and the resulting value of the extinction. We compare extinction values measured by different authors using different numbers of stars and reproduce their values by imposing different cuts in the V magnitude. This variation in color excess versus spectral type suggests that the standard intrinsic colors for the earliest type stars are bluer than the stars in h and χ Per. We measure centers for h and χ Per at α(2000) = 2h18m564 ± 30, δ(2000) = 57°8'25'' ± 23'' and α(2000) = 2h22m43 ± 29, δ(2000) = 57°8'35'' ± 25'', respectively. We fit the density structure of the clusters and find core radii of 1.9 and 2.4 pc, respectively. Integration of the Miller-Scalo initial mass function suggests overall cluster masses of 5500 and 4300 M⊙ and central densities of 27 and 50 M⊙ pc-3, respectively. We find strong evidence of mass segregation in h Per but not in χ Per. Examination of the dynamical timescales, as well as comparisons between the two clusters, suggest that the mass segregation is partly primordial.

32 citations


Journal ArticleDOI
TL;DR: In this article, a reanalysis of far-ultraviolet (FUV) observations of the globular cluster NGC 2808 obtained with the Hubble Space Telescope is presented, focusing on the population of fainter FUV sources, which include white dwarfs, blue stragglers (BSs), and cataclysmic variables (CVs).
Abstract: We present a reanalysis of far-ultraviolet (FUV) observations of the globular cluster NGC 2808 obtained with the Hubble Space Telescope. These data were first analyzed by Brown and coworkers, with an emphasis on the bright, blue horizontal-branch (HB) stars in this cluster. Here, our focus is on the population of fainter FUV sources, which include white dwarfs (WDs), blue stragglers (BSs), and cataclysmic variables (CVs). We have therefore constructed the deepest FUV - NUV color-magnitude diagram of NGC 2808 and searched for variability among our FUV sources. Overall, we have found ≈40 WD, ≈60 BS, and ≈60 CV candidates; three of the BSs and two of the CV candidates are variable. We have also recovered a known RR Lyrae star in the core of NGC 2808, which exhibits massive (≈4 mag) FUV variability. We have investigated the radial distribution and found that our CV and BS candidates are more centrally concentrated than the HB stars and WD candidates. This might be an effect of mass segregation but could as well be due to the preferential formation of such dynamically formed objects in the dense cluster core. For one of our CV candidates we found a counterpart in WFPC2 optical data published by Piotto and coworkers.

31 citations


Journal ArticleDOI
TL;DR: In this article, an overall analysis of the structure and stellar content of M11 is presented, thanks to the wide-angle 2MASS spatial coverage, where photometric and structural parameters and discuss the spatial dependance of the luminosity and mass functions are derived.
Abstract: An overall analysis of the structure and stellar content of M11 is presented, thanks to the wide-angle 2MASS spatial coverage. We derive photometric and structural parameters and discuss the spatial dependance of the luminosity and mass functions. Photometric parameters basically agree with previous ones mostly based on the optical. We obtained a core radius of 1.23pc and a tidal radius of 29pc. In particular, the cluster is populous enough so that the tidal radius could be obtained by fitting the three-parameter King profile to the radial distribution of stars. We analyzed the spatial distribution of mass functions, finding that the the slope changes from -0.73 in the core to +2.88 in the outer halo. The spatial distribution of mass function slopes derived from 2MASS agrees with that derived from optical CCD data, which further confirms the reliability of 2MASS data for future analyses of this kind at comparable observational limits. We detect mass segregation up to distances from the center of ~20arcmin. We emphasize that the mass function slope in the core is flatter than anywhere else as a consequence of mass segregation. The derived total cluster mass is ~11000 solar masses.

Journal ArticleDOI
TL;DR: In this article, N-body simulations of realistic globular clusters containing initial mass function in the galaxy were presented to study effects of tidal field systematically on the properties of outer parts of globular cluster.
Abstract: We present N-body simulations of realistic globular clusters containing initial mass function in the galaxy to study effects of tidal field systematically on the properties of outer parts of globular clusters. Using NBODY6 which takes into account the two-body relaxation correctly, we investigate general evolution of globular clusters in galactic tidal field. For simplicity, we have employed only spherical components (bulge and halo) of the galaxy. Total number of stars in our simulations was about 20,000. All simulations were done for several orbital periods in order to understand the development of the tidal tails. In our scaled down models, the relaxation time is sufficiently short to show the mass segregation effect, but we did not go far enough to see the core-collapse, and the fraction of stars lost from the cluster at the end of simulation is only about 10%. The radial distribution of extra-tidal stars can be described by a power law with a slope around -3. The direction of tidal tails are determined by the orbits and locations of the clusters. We find that the length of tidal tails increases towards the apogalacticon and decreases towards the perigalacticon. This is an anti-correlation with the strength of the tidal field, caused by the fact that the the time-scale for the stars to respond to the potential is similar to the orbital time-scale of the cluster. When the length of tidal tails decreases some of the stars in the tidal tails are recaptured by the host cluster. From the investigation of velocity anisotropy of the model clusters, we find that in the early stages of globular cluster evolution the clusters have radial anisotropy in the outermost parts, while clusters are nearly isotropic in the cental region. The radial anisotropy decreases with time.

Book ChapterDOI
01 Jan 2005
TL;DR: In this paper, the authors obtained a good fit to the measured main sequence mass function (MF) of a large sample of Galactic clusters (young and old) with a tapered Salpeter power law distribution function with an exponential truncation.
Abstract: We show that one can obtain a good fit to the measured main sequence mass function (MF) of a large sample of Galactic clusters (young and old) with a tapered Salpeter power law distribution function with an exponential truncation. The average value of the power law index is very close to Salpeter (~2.3), whereas the characteristic mass is in the range 0.1 - 0.5 Msolar and does not seem to vary in a systematic way with the present cluster parameters such as metal abundance and central concentration. However, a remarkable correlation with age is seen, in that the peak mass of young clusters increases with it. This trend does not extend to globular clusters, whose peak mass is firmly at ~0.35 Msolar. This correlation is due to 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.

Journal ArticleDOI
TL;DR: In this article, the authors used the Log N-Log S relationship of extragalactic sources derived from XMM-Newton Lockman Hole observations, to estimate the background source population, and found that very few of the sources (1.5+/-1.0) in the field of view of M 55 actually belong to the cluster.
Abstract: We have observed two low concentration Galactic globular clusters with the X-ray observatory XMM-Newton. We detect 47 faint X-ray sources in the direction of M 55 and 62 in the field of view of NGC 3201. Using the statistical Log N-Log S relationship of extragalactic sources derived from XMM-Newton Lockman Hole observations, to estimate the background source population, we estimate that very few of the sources (1.5+/-1.0) in the field of view of M 55 actually belong to the cluster. These sources are located in the centre of the cluster as we expect if the cluster has undergone mass segregation. NGC 3201 has approximately 15 related sources, which are centrally located but are not constrained to lie within the half mass radius. The sources belonging to this cluster can lie up to 5 core radii from the centre of the cluster which could imply that this cluster has been perturbed. Using X-ray (and optical, in the case of M 55) colours, spectral and timing analysis (where possible) and comparing these observations to previous X-ray observations, we find evidence for sources in each cluster that could be cataclysmic variables, active binaries, millisecond pulsars and possible evidence for a quiescent low mass X-ray binary with a neutron star primary, even though we do not expect any such objects in either of the clusters, due to their low central concentrations. The majority of the other sources are background sources, such as AGN.

Journal ArticleDOI
TL;DR: In this paper, a reanalysis of far-ultraviolet (FUV) observations of the globular cluster NGC 2808 obtained with the Hubble Space Telescope is presented, with an emphasis on the bright, blue horizontal branch (HB) stars in this cluster.
Abstract: We present a reanalysis of far-ultraviolet (FUV) observations of the globular cluster NGC 2808 obtained with the Hubble Space Telescope. These data were first analyzed by Brown and coworkers, with an emphasis on the bright, blue horizontal branch (HB) stars in this cluster. Here, our focus is on the population of fainter FUV sources, which include white dwarfs (WDs), blue stragglers (BSs) and cataclysmic variables (CVs). We have therefore constructed the deepest FUV-NUV colour-magnitude diagram of NGC 2808 and also searched for variability among our FUV sources. Overall, we have found approx. 40 WD, approx. 60 BS and approx. 60 CV candidates; three of the BSs and two of the CV candidates are variable. We have also recovered a known RR Lyrae star in the core of NGC 2808, which exhibits massive (approx. 4 mag) FUV variability. We have investigated the radial distribution and found that our CV and BS candidates are more centrally concentrated than the HBs and WD candidates. This might be an effect of mass segregation, but could as well be due to the preferential formation of such dynamically-formed objects in the dense cluster core. For one of our CV candidates we found a counterpart in WFPC2 optical data published by Piotto and coworkers.

Journal ArticleDOI
TL;DR: In this paper, a collisional runaway scenario to form an intermediate-mass black hole (IMBH, MBH > 100 Msun) at the center of a young, compact stellar cluster was studied.
Abstract: We present a new study of the collisional runaway scenario to form an intermediate-mass black hole (IMBH, MBH > 100 Msun) at the centre of a young, compact stellar cluster. The first phase is the formation of a very dense central core of massive stars (Mstar =~ 30-120 Msun) through mass segregation and gravothermal collapse. Previous work established the conditions for this to happen before the massive stars evolve off the main sequence (MS). In this and a companion paper, we investigate the next stage by implementing direct collisions between stars. Using a Monte Carlo stellar dynamics code, we follow the core collapse and subsequent collisional phase in more than 100 models with varying cluster mass, size, and initial concentration. Collisions are treated either as ideal, ``sticky-sphere'' mergers or using realistic prescriptions derived from 3-D hydrodynamics computations. In all cases for which the core collapse happens in less than the MS lifetime of massive stars (~3 Myr), we obtain the growth of a single very massive star (VMS, Mstar =~ 400-4000 Msun) through a runaway sequence of mergers. Mass loss from collisions, even for velocity dispersions as high as sigma1D ~ 1000 km/s, does not prevent the runaway. The region of cluster parameter space leading to runaway is even more extended than predicted in previous work because, in clusters with sigma1D > 300 km/s, collisions accelerate (and, in extreme cases, drive) core collapse. Although the VMS grows rapidly to > 1000 Msun in models exhibiting runaway, we cannot predict accurately its final mass. This is because the termination of the runaway process must eventually be determined by a complex interplay between stellar dynamics, hydrodynamics, and the stellar evolution of the VMS. [abridged]

Journal ArticleDOI
TL;DR: In this paper, the authors explore loose congregations of medium-to high-mass protoclusters identified to the southeast of NGC 2244 and investigate the properties and fine structures of the most prominent cluster embedded in the densest rim of the Rosette Molecular Cloud.
Abstract: We explore loose congregations of medium- to high-mass protoclusters identified to the southeast of NGC 2244. On using data from the spatially complete Two Micron All Sky Survey, the true extent of the burst of cluster formation along the ridge of the Rosette Molecular Cloud is revealed. Here we investigate the properties and fine structures of the most prominent cluster embedded in the densest rim of the cloud. This protocluster is resolved into two compact subclusters aligned along the major axis of the entire complex, in line with NGC 2244. The subclusters are found to have a physical scale of around 1 pc, typical of known embedded clusters. The K-band luminosity function also suggests a young age. However, near-infrared excess emission is found in approximately one-sixth of the reddened objects. This is still commensurate with an age estimate of <1 Myr, provided that the massive stars have rapidly stripped the circumstellar material from their neighbors. The well-known massive young binary associated with AFGL 961, however, is situated to the south of the major components of the cluster, where the stellar density is comparatively low. This is inconsistent with mass segregation and signifies a different formation process for these high-mass protostellar objects.

Posted Content
TL;DR: In this paper, a binary black hole simulation with two massive members is presented, where three-body interactions often lead to energetic ejections, with one or more of the components attaining relatively large terminal velocities which can be observed in principle.
Abstract: (Abridged) In this celebratory contribution, we present some new data from standard star cluster modelling containing primordial binaries and triples, as well as results from a binary black hole simulation with two massive members. In the star cluster case, the process of mass loss from evolving stars, together with general mass segregation, promotes favourable interactions involving compact subsystems of binaries and triples in the central region. Three-body interactions often lead to energetic ejections, with one or more of the components attaining relatively large terminal velocities which can be observed in principle. The second type of stellar system to be discussed is based on the scenario of two approaching galactic cores with density cusps, each having a massive black hole. After the subsystems become well mixed, the two massive components soon form a hard binary which gains energy by ejecting other members. Such a massive binary has a large cross section and can be very effective in depleting bound stars from the core. Again high-velocity escapers are produced, with their terminal speeds related to the shrinking binary size which eventually leads to coalescence.

Journal ArticleDOI
TL;DR: In this article, a deep stellar analysis is introduced for the poorly studied open cluster NGC 2509, where the Near-IR database of the digital Two Micron All Sky Survey (2MASS) has been used to re-estimate and refine the fundamental parameters of the cluster, i.e. age, reddening, distance, and diameter.
Abstract: A deep stellar analysis is introduced for the poorly studied open cluster NGC 2509. The Near-IR database of the digital Two Micron All Sky Survey (2MASS) has been used to re-estimate and refine the fundamental parameters of the cluster, i.e. age, reddening, distance, and diameter. As well as, luminosity function, mass function, total mass, relaxation time, and mass segregation of NGC 2509 have been estimated here for the first time..

Posted Content
TL;DR: In this article, the authors derived photometric, structural and dynamical evolution-related parameters of five low-contrast open clusters located in the third quadrant using 2MASS data.
Abstract: We derive photometric, structural and dynamical evolution-related parameters of five as yet unstudied low-contrast open clusters located in the third quadrant using 2MASS data. The target clusters are Czernik 31, Czernik 32, Haffner 9, Haffner 11 and Trumpler 13. We apply a statistical field-star decontamination procedure to infer on the intrinsic CMD morphology which is critical for such low-contrast objects. Consequently, it became possible to derive accurate reddening, age, distance from the Sun and Galactocentric distance for the five clusters. In the structural and luminosity/mass-function analyses we apply a colour-magnitude filter which encompasses the cluster evolutionary CMD sequences and excludes stars with discrepant colours. Using this procedure we derive core and limiting radii, mass function slope, total mass, mass density and relaxation time. We derive ages in the range 140 -- 1 100 Myr, Galactocentric distances within 7.7 -- 11.4 kpc, and total masses within 360--2 900 \ms. Reflecting large-scale mass segregation, the MF slope in the core is significantly flatter than that in the halo of the five clusters. Although some of the present clusters are relatively younger than the Gyr-old clusters, they present evidence of advanced dynamical evolution. This kind of study has become possible because of the photometric uniformity and spatial coverage of 2MASS which allows a proper subtraction of the field-star contamination on the target CMDs. The present study indicates that low-contrast clusters can be studied with 2MASS, particularly after field-star subtraction, which is important since most of the unstudied open clusters belong to this class.

Journal Article
TL;DR: In this article, a deep photometric analysis in the open star cluster NGC 1883 has been presented and added to the work of Carraro et al. (2003), which is the only previous work introduced for this cluster.
Abstract: A deep photometric analysis in the open star cluster NGC 1883 has been presented here and added to the work of Carraro et al. (2003), which is the only previous work introduced for this cluster. The radius and metallic- ity have been re-estimated in the present work. In addition, new parameters, namely luminosity function, mass function, total mass, mass segregation and the relaxation time of the cluster NGC 1883 have been estimated here for the flrst time.

Book ChapterDOI
01 Jan 2005
TL;DR: In this article, the authors derived the initial mass function (IMF) of NGC 3603, one of the most massive galactic star-forming regions, to answer a fundamental question in current astrophysics - is the IMF universal, or does it vary?
Abstract: We study the initial mass function (IMF) of NGC 3603, one of the most massive galactic star-forming regions, to answer a fundamental question in current astrophysics - is the IMF universal, or does it vary? Using our very deep high angular resolution images obtained with the NAOS-CONICA adaptive optics system at the VLT/ESO, we have successfully revealed the low-mass stellar population in the cluster core down to about 04 Msun (50 % completeness limit) Based on the JHKsL' color-magnitude and color-color diagrams, we first derive an average age 07 Myr for the pre-main sequence stars, and an upper limit of ~25 Myr for the main sequence stars We find an average foreground extinction of Av = 45 +- 05 mag, with a radial increase of Delta_Av ~ 20 mag towards larger radii (r < 50'') From the infrared excess emission identified in the Ks - L' vs J - H color-color diagram, we measure a disk fraction of ~25 % for stars with M > 09 Msun in the cluster center (r < 10'') Applying a field star rejection and correcting for incompleteness, we derive the Ks-band luminosity function (LF) for stars simultaneously detected in the JHKs-bands The LF follows a power-law with an index of alpha ~ 027, and shows no turnover or truncation within the detection limit The IMF for stars within r < 110'' is reasonably fitted by a single power-law with index Gamma ~ -074 in the mass range of $04 - 20 Msun This is substantially flatter than the Salpeter-like IMF (Gamma = -135) The IMF power-law index decreases from Gamma ~ -031 at r < 5'' to Gamma ~ -086 at 30'' < r < 110'' This radial steepening of the IMF mainly occurs in the inner r < 30'' field, indicating mass segregation at the very center of the starburst cluster Analyzing the radial mass density profile, we derive a cluster core radius of ~4''8 (~014 pc), and a lower limit of ~110'' (~32 pc) for the cluster size We also derive an upper limit of r ~ 1260'' (~37 pc) for the cluster size adopting an estimate of the tidal radius of the cluster Based on the de-projected stellar density distribution, we estimate the total mass and the half-mass radius of NGC 3603 to be about 10 - 16 x 10^4 Msun and 25'' - 50'' (~07 - 15 pc), respectively The derived core radius is > 6 x 10^4 Msun pc^-3 The estimate of the half-mass relaxation time for stars with a typical mass of 1 Msun is 10 - 40 Myr, suggesting that the intermediate- and low-mass stars have not yet been affected significantly by the dynamical relaxation in the cluster The relaxation time for the high-mass stars is expected to be much smaller, and is comparable to the age of the cluster We can thus not conclude if the mass segregation of the high-mass stars is primordial or caused by dynamical evolution Our observation covers at least ~67 % of intermediate- and low-mass stars in NGC 3603, and the stars residing outside the observed field can merely steepen the IMF by Delta_Gamma 30'', we are confident that our IMF adequately describes the whole NGC 3603 starburst cluster We also thoroughly analyze the systematic uncertainties in our IMF determination We conclude that the power-law index of NGC 3603 including the systematic uncertainties is Gamma = -074^{+062}_{-047} Our result thus supports the hypothesis of a top-heavy IMF in starbursts, especially in combination with other studies of similar clusters such as the Arches cluster and the Galactic Center cluster

Journal ArticleDOI
TL;DR: A massive black hole resides in the center of most, perhaps all galaxies as discussed by the authors, and it provides a uniquely accessible laboratory for studying in detail the connections and interactions between a massive Black Hole and the stellar system in which it grows; and for using stars to probe the central dark mass and probe post-Newtonian gravity in the weak and strong field limits.
Abstract: A massive black hole resides in the center of most, perhaps all galaxies. The one in the center of our home galaxy, the Milky Way, provides a uniquely accessible laboratory for studying in detail the connections and interactions between a massive black hole and the stellar system in which it grows; for investigating the effects of extreme density, velocity and tidal fields on stars; and for using stars to probe the central dark mass and probe post-Newtonian gravity in the weak- and strong-field limits. Recent results, open questions and future prospects are reviewed in the wider context of the theoretical framework and physical processes that underlie them. Contents: [1] Introduction (1.1) Astrophysical context (1.2) Science questions (1.3) Scope and connections to related topics [2] Observational overview: Stars in the Galactic center (2.1) The central 100 parsecs (2.2) The central parsec [3] Stellar dynamics at extreme densities (3.1) Physical processes and scales (3.2) The stellar cusp in the Galactic center (3.3) Mass segregation (3.4) Stellar Collisions [4] Probing the dark mass with stellar dynamics (4.1) Weighing and pinpointing the dark mass (4.2) Constraints on non-BH dark mass alternatives (4.3) Limits on MBH binarity (4.4) High-velocity runaway stars [5] Probing post-Newtonian gravity near the MBH (5.1) Relativistic orbital effects (5.2) Gravitational lensing [6] Strong star-MBH interactions (6.1) Tidal disruption (6.2) Dissipative interactions with the MBH [7] The riddle of the young stars (7.1) The difficulties of forming or importing stars near a MBH (7.2) Proposed solutions (7.3) Feeding the MBH with stellar winds [8] Outlook (8.1) Progress report (8.2) Future directions


Book ChapterDOI
01 Jan 2005
TL;DR: The mass segregation in very young rich Galactic starburst clusters like NGC 3603 and Arches may be due to their extremely short relaxation times and high densities, which appear to lead to rapid dissolution.
Abstract: The pronounced mass segregation in very young rich Galactic starburst clusters like NGC 3603 and Arches may be due to their extremely short relaxation times and high densities, which appear to lead to rapid dissolution. To what extent mass segregation in these systems is primordial as opposed to evolutionary can- not easily be distinguished. While flat in their centers, the total mass functions of these rich clusters seem close to a Salpeter slope, supporting a universal IMF.

Book ChapterDOI
01 May 2005
TL;DR: In this article, the authors used dynamical models that account for stellar evolution to compute the evolution of this relationship, depending on the wavelengths at which radii and velocity dispersions are observed.
Abstract: Dynamical masses of star clusters are important constraints on the slope or lower-mass cut-off of the stellar IMF. The measurements of dynamical masses rest on model-based relationships between mass, line-of-sight velocity dispersion and projected half-light radius. We have used dynamical models that account for stellar evolution to compute the evolution of this relationship, depending on the wavelengths at which radii and velocity dispersions are observed. The conversion factor varies significantly over a few × 107 yr. In reddened starburst clusters that are observed at near-IR wavelengths, red supergiants are the dominant sources of light. We report on progress in the synthesis of stellar populations with strong red supergiant contributions. These stars are initially among the most massive stars, but they lose mass rapidly and the resulting dynamical evolution is complex.

Book ChapterDOI
01 May 2005
TL;DR: In this paper, the authors used high-resolution near-infrared Keck/NIRSPEC echelle spectroscopy to measure the stellar velocity dispersions in a nuclear starburst in M82.
Abstract: The nuclear starburst in M82 is host to over 20 infrared-bright, dense, young super star clusters (SSCs). We use high-resolution near-infrared Keck/NIRSPEC echelle spectroscopy to measure the stellar velocity dispersions. The SSCs are resolved in Hubble Space Telescope images, from which we measure half-light radii and integrated luminosities. We calculate virial masses for the SSCs, and compare the observed light-to-mass ratios to population synthesis models to constrain the initial mass function. There are apparent variations of the IMF within this single starburst galaxy. We present evidence for mass segregation despite the young ages, and discuss implications for the interpretation of the IMF.