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Mass segregation

About: Mass segregation is a research topic. Over the lifetime, 1024 publications have been published within this topic receiving 57729 citations.


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TL;DR: In this paper, the authors investigate the long-term dynamical evolution of GCs containing large numbers of stellar-mass black holes (BHs), and they find that significant numbers of BHs (up to ~103) are retained all the way to the present.
Abstract: Our current understanding of the stellar initial mass function and massive star evolution suggests that young globular clusters (GCs) may have formed hundreds to thousands of stellar-mass black holes (BHs), the remnants of stars with initial masses from ~20-100 M ☉. Birth kicks from supernova explosions may eject some BHs from their birth clusters, but most should be retained. Using a Monte Carlo method we investigate the long-term dynamical evolution of GCs containing large numbers of stellar BHs. We describe numerical results for 42 models, covering a broad range of realistic initial conditions, including up to 1.6 × 106 stars. In almost all models we find that significant numbers of BHs (up to ~103) are retained all the way to the present. This is in contrast to previous theoretical expectations that most BHs should be ejected dynamically within a few gigayears The main reason for this difference is that core collapse driven by BHs (through the Spitzer mass segregation instability) is easily reverted through three-body processes, and involves only a small number of the most massive BHs, while lower-mass BHs remain well-mixed with ordinary stars far from the central cusp. Thus the rapid segregation of stellar BHs does not lead to a long-term physical separation of most BHs into a dynamically decoupled inner core, as often assumed previously. Combined with the recent detections of several BH X-ray binary candidates in Galactic GCs, our results suggest that stellar BHs could still be present in large numbers in many GCs today, and that they may play a significant role in shaping the long-term dynamical evolution and the present-day dynamical structure of many clusters.

229 citations

Journal ArticleDOI
TL;DR: In this article, high-angular resolution continuum observations obtained with the IRAM Plateau de Bure interferometer at 13 and 35 mm towards the six most massive and youngest (IR-quiet) dense cores in the Cygnus X complex are presented.
Abstract: Massive dense cores (MDCs) are the high-mass equivalent of the so-called dense cores in nearby star-forming regions With typical sizes of 01 pc, they could form either a few high-mass stars, or a cluster of low-mass stars We present high-angular resolution continuum observations obtained with the IRAM Plateau de Bure interferometer at 13 and 35 mm towards the six most massive and youngest (IR-quiet) dense cores in the Cygnus X complex Located at only 17 kpc, the Cygnus X region offers the opportunity of reaching small enough scales (of the order of 1700 AU at 13 mm) to separate individual collapsing objects, and thus to observe and constrain the result of the fragmentation process The cores are sub-fragmented with a total of 23 fragments inside 5 cores Only the most compact MDC, CygX-N63, may host a single proto-stellar object with an envelope as massive as ˜60 M_ȯ The fragments in the other cores have sizes and separations similar to low-mass pre-stellar condensations and Class 0 young stellar objects in nearby protoclusters, and are most probably self-gravitating objects (M > Mvir) In addition to CygX-N63, a total of 8 objects are found to be probable precursors of OB stars with their envelope masses ranging from 84 to 30 M_ȯ inside a FWHM of 4000 AU The level of fragmentation is globally higher than in the turbulence regulated, monolithic collapse scenario, but it is also not as high as expected in a pure gravo-turbulent scenario where the distribution of mass is dominated by low-mass protostars/stars Here, the fractions of the total MDC masses in the high-mass proto-stellar fragments are found to be as high as 37, 58, and 100% in CygX-N12, CygX-N53, and CygX-N63, respectively These high fractions of mass in the proto-stellar fragments are also indicative of a high efficiency of core formation in the MDCs The increase in the core formation efficiency as a function of average density in the MDCs is proposed to be caused by the increasing importance of self-gravity leading to gravitational collapse on the scale of the MDCs At the same time, the observed MDCs tend to fragment into a few proto-stellar objects within their central regions We are therefore probably witnessing the primordial mass segregation of clusters The physical origin of the fragmentation into a few high-mass objects is not yet clear, and will be investigated in the future by studying the kinematics of the MDCs

224 citations

Journal ArticleDOI
TL;DR: In this paper, a collisional runaway scenario to form an intermediate-mass black hole (IMBH, M-BH greater than or similar to 100M(circle dot)) at the centre 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, M-BH greater than or similar to 100M(circle dot)) at the centre of a young, compact stellar cluster. The first phase is the formation of a very dense central core of massive stars (M-* similar or equal to 30-120M(circle dot)) 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 3D hydrodynamics computations. In all cases for which the core collapse happens in less than the MS lifetime of massive stars (similar or equal to 3 Myr), we obtain the growth of a single very massive star (VMS, M-* similar or equal to 400-4000M(circle dot)) through a runaway sequence of mergers. Mass loss from collisions, even for velocity dispersions as high as sigma(v) similar to 1000 km s(-1), 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 sigma(v) > 300 km s(-1), collisions accelerate (and, in extreme cases, drive) core collapse. Although the VMS grows rapidly to greater than or similar to 1000M(circle dot) 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. In the vast majority of cases, we find that the time between successive collisions becomes much shorter than the thermal time-scale of the VMS. Therefore, our assumption that all stars return quickly to the MS after a collision must eventually break down for the runaway product, and the stellar evolution of the VMS becomes very uncertain. For the same reason, the final fate of the VMS, including its possible collapse to an IMBH, remains unclear.

223 citations

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, the initial mass function (IMF) of one of the most massive Galactic star-forming regions NGC 3603 was determined using very deep, high angular resolution JHKSL' images obtained with NAOS-CONICA at the VLT at ESO.
Abstract: We study the initial mass function (IMF) of one of the most massive Galactic star-forming regions NGC 3603 to answer a fundamental question in current astrophysics: is the IMF universal, or does it vary? Using our very deep, high angular resolution JHKSL' images obtained with NAOS-CONICA at the VLT at ESO, we have successfully revealed the stellar population down to the subsolar mass range in the core of the starburst cluster. The derived IMF of NGC 3603 is reasonably fitted by a single power law with index Γ ~ − 0.74 within a mass range of 0.4-20 M☉, substantially flatter than the Salpeter-like IMF. A strong radial steepening of the IMF is observed mainly in the inner r 30'' field, indicating mass segregation in the cluster center. We estimate the total mass of NGC 3603 to be about 1.0–1.6 × 104 M☉. The derived core density is ≥6 × 104 M☉ pc−3, an order of magnitude larger than, e.g., the Orion Nebula Cluster. The estimate of the half-mass relaxation time for solar-mass stars is about 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 can be comparable to the age of the cluster. We estimate that the stars residing outside the observed field cannot steepen the IMF significantly, indicating our IMF adequately describes the whole cluster. Analyzing thoroughly the systematic uncertainties in our IMF determination, we conclude that the power-law index of the IMF of NGC 3603 is Γ = − 0.74+ 0.62−0.47. Our result thus supports the hypothesis of a potential top-heavy IMF in massive star-forming clusters and starbursts.

208 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
202336
202225
202133
202047
201943
201822