105,844 research outputs found
Properties of Quasi-Relaxed Stellar Systems in an External Tidal Field
No full textIn a separate paper (Bertin & Varri) we have described a general method for the construction of self-consistent non-spherical models of quasi-relaxed stellar systems, and, in particular, of those that extend the spherical King models to the case in which an external tidal field is taken into account explicitly. Here we describe the resulting two parameter family of physically justified triaxial models, characterized by a concentration parameter (as for the spherical case) and by a second parameter, the tidal strength, defined as the square of the ratio of the orbital revolution frequency to the central dynamical frequency of the stellar system. The most significant departures from spherical symmetry occur when the truncation radius is of the order of the tidal radius, which, for a given concentration, sets a maximum value to the allowed tidal strength. We illustrate several properties that characterize the intrinsic and the projected structure of the models. Under the guidance of this analytical framework, by means of N-body simulations we also show that, as a result of the presence of a tidal field, an initially spherical configuration tends to evolve into configurations similar to those predicted by our analytical equilibrium models
A dynamical study of Galactic globular clusters under different relaxation conditions
No full textAims. We perform a systematic combined photometric and kinematic analysis of a sample of globular clusters under different relaxation conditions, based on their core relaxation time (as listed in available catalogs), by means of two well-known families of spherical stellar dynamical models. Systems characterized by shorter relaxation time scales are expected to be better described by isotropic King models, while less relaxed systems might be interpreted by means of non-truncated, radially-biased anisotropic f models, originally designed to represent stellar systems produced by a violent relaxation formation process and applied here for the first time to the study of globular clusters. Methods. The comparison between dynamical models and observations is performed by fitting simultaneously surface brightness and velocity dispersion profiles. For each globular cluster, the best-fit model in each family is identified, along with a full error analysis on the relevant parameters. Detailed structural properties and mass-to-light ratios are also explicitly derived. Results. We find that King models usually offer a good representation of the observed photometric profiles, but often lead to less satisfactory fits to the kinematic profiles, independently of the relaxation condition of the systems. For some less relaxed clusters, f models provide a good description of both observed profiles. Some derived structural characteristics, such as the total mass or the half-mass radius, turn out to be significantly model-dependent. The analysis confirms that, to answer some important dynamical questions that bear on the formation and evolution of globular clusters, it would be highly desirable to acquire larger numbers of accurate kinematic data-points, well distributed over the cluster field
LA RIVOLUZIONE INCOMPIUTA DI KEYNES
No full textIL LAVORO E' UNA RECENSIONE DEL VOLUME DI L. PASINETTI "KEYNES E I KEYNESIANI DI CAMBRIDGE. UNA RIVOLUZIONE IN ECONOMIA DA PORTARE A COMPIMENTO", LATERZA, 2010 (ed. originale CAMBRIDGE UNIVERSITY PRESS, 2007). IL VOLUME ESAMINA IN MODO APPROPRIATO LE RAGIONI DELLA CRISI DELLA TEORIA ECONOMICA KEYNESIANA NEL CONFRONTO CON LE TEORIE ECONOMICHE NEO-LIBERISTE E FA NOTARE COME L'INSTABILITA' DELLE ECONOMIE DI MERCATO RIPORTA CONTINUAMENTE ALLA ATTENZIONE DELLA POLITICA ECONOMICA LA TERZA VIA KEYNESIANA PER CORREGGERE DISOCCUPAZIONE E DISUGUAGLIANZA NELLA DISTRIBUZIONE DEL REDDITO. LA GENERALIZZAZIONE DELLA TEORIA KEYNESIANA PROPOSTA NEL VOLUME VA NELLA DIREZIONE DELLA DINAMICA ECONOMICA STRUTTURALE E CIOE' DI UNA ESTENSIONE DELLA MACROECONOMIA DELLA CRESCITA REGOLARE A STRUTTURA COSTANTE
Self-consistent models of quasi-relaxed rotating stellar systems
No full textAims: Two new families of self-consistent axisymmetric truncated equilibrium models for the description of quasi-relaxed rotating stellar systems are presented. The first extends the well-known spherical King models to the case of solid-body rotation. The second is characterized by differential rotation, designed to be rigid in the central regions and to vanish in the outer parts, where the imposed energy truncation becomes effective. Methods: The models are constructed by solving the relevant nonlinear Poisson equation for the self-consistent mean-field potential. For rigidly rotating configurations, the solutions are obtained by an asymptotic expansion based on the rotation strength parameter, following a procedure developed earlier by us for the case of tidally generated triaxial models. The differentially rotating models are constructed by means of a spectral iterative approach, with a numerical scheme based on a Legendre series expansion of the density and the potential. Results: The two classes of models exhibit complementary properties. The rigidly rotating configurations are flattened toward the equatorial plane, with deviations from spherical symmetry that increase with the distance from the center. For models of the second family, the deviations from spherical symmetry are strongest in the central region, whereas the outer parts tend to be quasi-spherical. The relevant parameter spaces are thoroughly explored and the corresponding intrinsic and projected structural properties are described. Special attention is given to the effect of different options for the truncation of the distribution function in phase space. Conclusions: Models in the moderate rotation regime are best suited to applications to globular clusters. For general interest in stellar dynamics, at high values of the rotation strength the differentially rotating models tend to exhibit a toroidal core embedded in an otherwise quasi-spherical configuration. Physically simple analytical models of the kind presented here provide insights into dynamical mechanisms and may be a useful basis for more realistic investigations carried out with the help of N-body simulations
The flattening of globular clusters : internal rotation or velocity anisotropy?
No full textInternal rotation is considered to play a major role in determining the structure and dynamics of some globular clusters. We present a dynamical analysis of the photometry and three-dimensional kinematics of 47 Tuc and ω
Cen, by means of a new family of self-consistent axisymmetric rotating models. The combined use of line-of-sight velocities and
proper motions allows us to obtain a global description of the internal dynamical structure of the objects together with an estimate of their dynamical distances. The well-relaxed cluster 47 Tuc is very well interpreted by our dynamical models; in particular, internal rotation is found to explain the observed morphology. For the partially relaxed cluster ω Cen, the selected model provides a good representation of its complex three-dimensional kinematics, in general qualitative agreement with the observed anisotropy profile, which is characterized by tangential anisotropy in the outer parts; discrepancies are found between the observed
and the expected ellipticity profile and are ascribed to the presence of a high degree of radial anisotropy in the intermediate region and to its interplay with rotation
First Phase Space Portrait of a Hierarchical Stellar Structure in the Milky Way
No full textWe present the first detailed observational picture of a possible ongoing massive cluster hierarchical assembly in the Galactic disk as revealed by the analysis of the stellar full phase space (3D positions and kinematics and spectro-photometric properties) of an extended area (6 diameter) surrounding the well-known h and χ Persei double stellar cluster in the Perseus Arm. Gaia-EDR3 shows that the area is populated by seven comoving clusters, three of which were previously unknown, and by an extended and quite massive (M ∼ 105 M o ̇) halo. All stars and clusters define a complex structure with evidence of possible mutual interactions in the form of intra-cluster overdensities and/or bridges. They share the same chemical abundances (half-solar metallicity) and age (t ∼ 20 Myr) within a small confidence interval and the stellar density distribution of the surrounding diffuse stellar halo resembles that of a cluster-like stellar system. The combination of these pieces of evidence suggests that stars distributed within a few degrees from h and χ Persei are part of a common, substructured stellar complex that we named LISCA I. Comparison with results obtained through direct N-body simulations suggest that LISCA I may be at an intermediate stage of an ongoing cluster assembly that can eventually evolve in a relatively massive (a few times 105 M o ̇) stellar system. We argue that such a cluster formation mechanism may be quite efficient in the Milky Way and disk-like galaxies and, as a consequence, it has a relevant impact on our understanding of cluster formation efficiency as a function of the environment and redshift
Dynamical stability and long-term evolution of rotating stellar systems
No full textWe present the first results of an extensive survey of N-body simulations designed to investigate the dynamical stability and the long-term evolution of two new families of self-consistent stellar dynamical models, characterized by the presence of internal rotation.
The first family extends the well-known King models to the case of axisymmetric systems flattened by solid-body rotation while the second family is characterized by differential rotation. The equilibrium configurations thus obtained can be described in terms of two dimensionless parameters, which measure the concentration and the amount of rotation, respectively.
Slowly rotating configurations are found to be dynamically stable and we followed their long-term evolution, in order to evaluate the interplay between collisional relaxation and angular momentum transport. We also studied the stability of rapidly rotating models, which are characterized by the presence of a toroidal core embedded in an otherwise quasi-spherical configuration. In both cases, a description in terms of the radial and global properties, such as the ratio between the ordered kinetic energy and the gravitational energy of the system, is provided.
Because the role of angular momentum in the process of cluster formation is only partly understood, we also undertook a preliminary investigation of the violent relaxation of simple systems initially characterized by approximate solid-body rotation. The properties of the final equilibrium configurations thus obtained are compared with those of the above-described family of differentially rotating models
Models of partially relaxed and quasi-relaxed stellar systems
No full textWe present solutions of the Vlasov-Poisson equations in the presence of external fields that break the natural symmetry associated with the one-component problem, with application to the dynamics of globular clusters and of spiral galaxies
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe 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
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