1,720,978 research outputs found
Gravitational waves from collapsing globular cluster systems
Full text linkThe evolution of globular cluster systems in some galaxies can be cause of merging of globulars in the very central regions. This high stellar density favours the growth of a central nucleus via swallowing of surrounding stars. The infall of stars into a nuclear black hole is here shown to be, under certain conditions, not only source of electromagnetic radiation but also a significant source of gravitational waves
Dynamical friction in multi-component evolving Globular Clusters
Full text linkWe use the Chandrasekhar formalism and direct N-body simulations to study the effect of dynamical friction
on a test object only slightly more massive than the field stars, orbiting a spherically symmetric background of
particles with a mass spectrum. The main goal is to verify whether the dynamical friction time (tDF) develops a nonmonotonic radial dependence that could explain the bimodality of the blue straggler radial distributions observed
in globular clusters. In these systems, in fact, relaxation effects lead to a mass and velocity radial segregation of the
different mass components, so that mass-spectrum effects on tDF are expected to be dependent on radius. We find
that in spite of the presence of different masses, tDF is always a monotonic function of radius, at all evolutionary
times and independently of the initial concentration of the simulated cluster. This is because the radial dependence
of tDF is largely dominated by the total mass density profile of the background stars (which is monotonically
decreasing with radius). Hence, a progressive temporal erosion of the blue straggler star (BSS) population at larger
and larger distances from the cluster center remains the simplest and the most likely explanation of the shape of
the observed BSS radial distributions, as suggested in previous works. We also confirm the theoretical expectation
that approximating a multi-mass globular cluster as made of (averaged) equal-mass stars can lead to significant
overestimations of tDF within the half-mass radius
The Surface Density Profile of NGC 6388: A Good Candidate for Harboring an Intermediate-Mass Black Hole
No full textWe have used a combination of HST high-resolution and ground-based wide-field observations of the Galactic globular cluster NGC 6388 to derive its center of gravity, projected density profile, and central surface brightness profile. While the overall projected profiles are well fit by a King model with an intermediate concentration (c=1.8) and a sizable core radius (rc = 7.2"), a significant power-law (with slope ?=-0.2) deviation from flat-core behavior has been detected within the inner 1". These properties suggest the presence of a central intermediate-mass black hole. The observed profiles are well reproduced by a multimass isotropic, spherical model, including a black hole with a mass of ~5.7 × 103 Msolar
Merging of Globular Clusters in Inner Galactic Regions. I. Do They Survive the Tidal Interaction?
No full textThe main topic of this paper is the investigation of the modes of interaction of globular clusters (GCs) moving in the inner part of a galaxy. This is tackled by means of high-resolution N-body simulations, whose first results are presented in this article. Our simulations dealt with primordial, very massive (of the order of 107 Msolar) GCs that were able to decay, because of dynamical friction, into the inner regions of triaxial galaxies on a timescale much shorter than their internal relaxation time. To check the disruptive roles of both tidal forces and GC-GC collisions, their effects were maximized by considering clusters on quasi-radial orbits and choosing the initial conditions so as to give head-on collisions at each passage through the center. The available CPU resources allowed us to simulate clusters with different structural parameters and to follow them on quasi-radial orbits during eight passages across the center. The main findings are as follows: (1) clusters with an initial high-enough King concentration parameter (c>=1.2) preserve up to 50% of their initial mass, (2) the inner density distribution of the surviving clusters keeps a King model profile, (3) GC-GC collisions have a negligible effect compared with that caused by the passage through the galactic center, (4) the orbital energy dissipation due to the tidal interaction is of the same order as that caused by dynamical friction, (5) complex substructures like ``ripples'' and ``clumps'' are formed, as observed around real clusters. These findings support the validity of the hypothesis of merging of GCs in the galactic central region, with modes that deserve further careful investigation
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Evidence of tidal distortions and mass-loss from the old open cluster NGC 6791
Full text linkWe present the first evidence of clear signatures of tidal distortions in the density distribution of the fascinating open cluster NGC 6791. We used deep and wide-field data obtained with the Canada-France-Hawaii Telescope covering a 2° × 2° area around the cluster. The 2D density map obtained with the optimal matched filter technique shows a clear elongation and an irregular distribution starting from ̃300 arcsec from the cluster centre. At larger distances, two tails extending in opposite directions beyond the tidal radius are also visible. These features are aligned to both the absolute proper motion and to the Galactic Centre directions. Moreover, other overdensities appear to be stretched in a direction perpendicular to the Galactic plane. Accordingly to the behaviour observed in the density map, we find that both the surface brightness and the star count density profiles reveal a departure from a King model starting from ̃600 arcsec from the centre. These observational evidence suggest that NGC 6791 is currently experiencing mass-loss likely due to gravitational shocking and interactions with the tidal field. We use this evidence to argue that NGC 6791 should have lost a significant fraction of its original mass. A larger initial mass would in fact explain why the cluster survived so long. Using available recipes based on analytic studies and N-body simulations, we derived the expected mass-loss due to stellar evolution and tidal interactions and estimated the initial cluster mass to be Mini = (1.5-4) × 105 M⊙
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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