1,721,038 research outputs found
Dynamical bar instability in rotating stars: effect of general relativity
No full textWe study the dynamical stability against bar-mode deformation of rapidly and differentially rotating stars in the first post-Newtonian approximation of general relativity. We vary the compaction of the star M/R (where M is the gravitational mass and the equatorial circumferential radius) between 0.01 and 0.05 to isolate the influence of relativistic gravitation on the instability. For compactions in this moderate range, the critical value of for the onset of the dynamical instability (where T is the rotational kinetic energy and W the gravitational binding energy) slightly decreases from ~ 0.26 to ~ 0.25 with increasing compaction for our choice of the differential rotational law. Combined with our earlier findings based on simulations in full general relativity for stars with higher compaction, we conclude that relativistic gravitation {\em enhances} the dynamical bar-mode instability, i.e. the onset of instability sets in for smaller values of in relativistic gravity than in Newtonian gravity. We also find that once a triaxial structure forms after the bar-mode perturbation saturates in dynamically unstable stars, the triaxial shape is maintained, at least for several rotational periods. To check the reliability of our numerical integrations, we verify that the general relativistic Kelvin-Helmholtz circulation is well-conserved, in addition to rest-mass energy, total mass-energy, linear and angular momentum. Conservation of circulation indicates that our code is not seriously affected by numerical viscosity. We determine the amplitude and frequency of the quasi-periodic gravitational waves emitted during the bar formation process using the quadrupole formula
One-armed spiral instability in differentially rotating stars
Full text linkWe investigate the dynamical instability of the one-armed spiral m=1 mode in differentially rotating stars by means of 3+1 hydrodynamical simulations in Newtonian gravitation. We find that both a soft equation of state and a high degree of differential rotation in the equilibrium star are necessary to excite a dynamical m=1 mode as the dominant instability at small values of the ratio of rotational kinetic to potential energy, T/|W|. We find that this spiral mode propagates outward from its point of origin near the maximum density at the center to the surface over several central orbital periods. An unstable m=1 mode triggers a secondary m=2 bar mode of smaller amplitude, and the bar mode can excite gravitational waves. As the spiral mode propagates to the surface it weakens, simultaneously damping the emitted gravitational wave signal. This behavior is in contrast to waves triggered by a dynamical m=2 bar instability, which persist for many rotation periods and decay only after a radiation-reaction damping timescale
Collapse of a rotating supermassive star to a supermassive black hole: post-Newtonian simulations
Full text linkWe study the gravitational collapse of a rotating supermassive star (SMS) by means of a (3+1) hydrodynamical simulation in a post-Newtonian (PN) approximation of general relativity. This problem is particularly challenging because of the vast dynamical range in space which must be covered in the course of col- lapse. We evolve a uniformly rotating SMS from the onset of radial instability at R_p/M=411, where R_p is the proper polar radius of the star and M is the total mass-energy, to the point at which the PN approximation breaks down. We introduce a scale factor and a "comoving" coordinate to handle the large variation in radius during the collapse and focus on the central core. Since T/W, the ratio of the rotational kinetic energy to the gravitational binding energy, is nearly proportional to 1/R_p throughout the collapse, the imploding star may ultimately exceed the critical value of T/W for dynamical instability to bar-mode formation. However, for stars rotating uniformly at the onset of collapse, we do not find any unstable growth of bars prior to the termination of our simulation. We do find that the collapse is likely to form a supermassive black hole (BH) coherently, with almost all of the matter falling into the hole, leaving very little ejected matter to form a disk. In the absence of nonaxisymmetric bar formation, the collapse of a uniformly rotating SMS does not lead to appreciable quasi-periodic gravitational wave (GW) emission by the time our integrations terminate. However, the coherent nature of the implosion suggests that rotating SMS collapse will be a promising source of GW bursts. We also expect that, following BH formation, long wavelength quasi-periodic waves will result from quasi-normal ringing. These waves may be detectable by LISA
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
Computing the Evolution of Compact Stars in 3+1 General Relativity
No full textSubmitted by Meng Tao ([email protected]) on 2012-10-11T20:49:42Z
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Numerical MHD Simulations in Dynamical Spacetimes
No full textNumerical relativity probes some of the most energetic events in the universe using computer simulations. These simulations must account for the relevant physical processes while, at the same time, evolving the spacetime according to Einstein's equations of general relativity. In particular, magnetohydrodynamics (MHD) and relativistic gravity are of crucial importance in astrophysical phenomena such as stellar collapse to black holes, binary neutron star mergers, supernovae, and gamma-ray bursts. Many of these phenomena are promising sources of GWs for detectors such as LIGO (the Laser Interferometer Gravitational Wave Observatory). The complex interplay of the magnetic field and fluid in a dynamically changing spacetime makes numerical simulations indispensable for studying such systems. We have developed a code which simultaneously solves the Einstein equations for the gravitational field, Maxwell's equations for the electromagnetic field, and the equations of relativistic magnetohydrodynamics for the fluid. We apply this code to study the evolution of magnetized differentially rotating neutron stars and magnetorotational core collapse.Made available in DSpace on 2015-09-25T20:03:02Z (GMT). No. of bitstreams: 2
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Reason: Restricted to the U of I community idenfinitely during batch ingest of legacy ETDsRestricted to the U of I community idenfinitely during batch ingest of legacy ETDsU of I Only153 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2007
Numerical MHD Simulations in Dynamical Spacetimes
No full text153 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2007.Numerical relativity probes some of the most energetic events in the universe using computer simulations. These simulations must account for the relevant physical processes while, at the same time, evolving the spacetime according to Einstein's equations of general relativity. In particular, magnetohydrodynamics (MHD) and relativistic gravity are of crucial importance in astrophysical phenomena such as stellar collapse to black holes, binary neutron star mergers, supernovae, and gamma-ray bursts. Many of these phenomena are promising sources of GWs for detectors such as LIGO (the Laser Interferometer Gravitational Wave Observatory). The complex interplay of the magnetic field and fluid in a dynamically changing spacetime makes numerical simulations indispensable for studying such systems. We have developed a code which simultaneously solves the Einstein equations for the gravitational field, Maxwell's equations for the electromagnetic field, and the equations of relativistic magnetohydrodynamics for the fluid. We apply this code to study the evolution of magnetized differentially rotating neutron stars and magnetorotational core collapse.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD
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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