3,646 research outputs found
On the orientation and magnitude of the black hole spin in galactic nuclei
Massive black holes in galactic nuclei vary their mass M and spin vector J
due to accretion. In this study we relax, for the first time, the assumption
that accretion can be either chaotic, i.e. when the accretion episodes are
randomly and isotropically oriented, or coherent, i.e. when they occur all in a
preferred plane. Instead, we consider different degrees of anisotropy in the
fueling, never confining to accretion events on a fixed direction. We follow
the black hole growth evolving contemporarily mass, spin modulus a and spin
direction. We discover the occurrence of two regimes. An early phase (M <~ 10
million solar masses) in which rapid alignment of the black hole spin direction
to the disk angular momentum in each single episode leads to erratic changes in
the black hole spin orientation and at the same time to large spins (a ~ 0.8).
A second phase starts when the black hole mass increases above >~ 10 million
solar masses and the accretion disks carry less mass and angular momentum
relatively to the hole. In the absence of a preferential direction the black
holes tend to spin-down in this phase. However, when a modest degree of
anisotropy in the fueling process (still far from being coherent) is present,
the black hole spin can increase up to a ~ 1 for very massive black holes (M >~
100 million solar masses), and its direction is stable over the many accretion
cycles. We discuss the implications that our results have in the realm of the
observations of black hole spin and jet orientations.Comment: 14 pages, 7 figures, accepted for publication in Ap
A path to radio-loudness through gas-poor galaxy mergers and the role of retrograde accretion
In this proceeding we explore a pathway to radio-loudness under the
hypothesis that retrograde accretion onto giant spinning black holes leads to
the launch of powerful jets, as seen in radio loud QSOs and recently in
LAT/Fermi and BAT/Swift Blazars. Counter-rotation of the accretion disc
relative to the BH spin is here associated to gas-poor galaxy mergers
progenitors of giant (missing-light) ellipticals. The occurrence of retrograde
accretion enters as unifying element that may account for the
radio-loudness/galaxy morphology dichotomy observed in AGN.Comment: To appear in the proceedings of the conference "Accretion and
Ejection in AGN: A global view, June 22-26 2009 - Como, Italy
Neutrino processes in partially degenerate neutron matter
We investigate neutrino processes for conditions reached in simulations of
core-collapse supernovae. Where neutrino-matter interactions play an important
role, matter is partially degenerate, and we extend earlier work that addressed
the degenerate regime. We derive expressions for the spin structure factor in
neutron matter, which is a key quantity required for evaluating rates of
neutrino processes. We show that, for essentially all conditions encountered in
the post-bounce phase of core-collapse supernovae, it is a very good
approximation to calculate the spin relaxation rates in the nondegenerate
limit. We calculate spin relaxation rates based on chiral effective field
theory interactions and find that they are typically a factor of two smaller
than those obtained using the standard one-pion-exchange interaction alone.Comment: 41 pages, 9 figures, NORDITA-2011-116; added comparison figures and
fit function for use in simulations, to appear in Astrophys.
Pushing 1D CCSNe to explosions: model and SN 1987A
We report on a method, PUSH, for triggering core-collapse supernova
explosions of massive stars in spherical symmetry. We explore basic explosion
properties and calibrate PUSH such that the observables of SN1987A are
reproduced. Our simulations are based on the general relativistic hydrodynamics
code AGILE combined with the detailed neutrino transport scheme IDSA for
electron neutrinos and ALS for the muon and tau neutrinos. To trigger
explosions in the otherwise non-exploding simulations, we rely on the
neutrino-driven mechanism. The PUSH method locally increases the energy
deposition in the gain region through energy deposition by the heavy neutrino
flavors. Our setup allows us to model the explosion for several seconds after
core bounce. We explore the progenitor range 18-21M. Our studies
reveal a distinction between high compactness (HC) and low compactness (LC)
progenitor models, where LC models tend to explore earlier, with a lower
explosion energy, and with a lower remnant mass. HC models are needed to obtain
explosion energies around 1 Bethe, as observed for SN1987A. However, all the
models with sufficiently high explosion energy overproduce Ni. We
conclude that fallback is needed to reproduce the observed nucleosynthesis
yields. The nucleosynthesis yields of Ni depend sensitively on the
electron fraction and on the location of the mass cut with respect to the
initial shell structure of the progenitor star. We identify a progenitor and a
suitable set of PUSH parameters that fit the explosion properties of SN1987A
when assuming 0.1M of fallback. We predict a neutron star with a
gravitational mass of 1.50M. We find correlations between explosion
properties and the compactness of the progenitor model in the explored
progenitors. However, a more complete analysis will require the exploration of
a larger set of progenitors with PUSH.Comment: revised version as accepted by ApJ (results unchanged, text modified
for clarification, a few references added); 26 pages, 20 figure
Mass and spin co-evolution during the alignment of a black hole in a warped accretion disc
In this paper, we explore the gravitomagnetic interaction of a black hole (BH) with a misaligned accretion disc to study BH spin precession and alignment jointly with BH mass MBH and spin parameter a evolution, under the assumption that the disc is continually fed, in its outer region, by matter with angular momentum fixed on a given direction . We develop an iterative scheme based on the adiabatic approximation to study the BH-disc co-evolution: in this approach, the accretion disc transits through a sequence of quasi-steady warped states (Bardeen-Petterson effect) and interacts with the BH until the spin JBH aligns with . For a BH aligning with a corotating disc, the fractional increase in mass is typically less than a few per cent, while the spin modulus can increase up to a few tens of per cent. The alignment time-scale is of ∼105-106 yr for a maximally rotating BH accreting at the Eddington rate. BH-disc alignment from an initially counter-rotating disc tends to be more efficient compared to the specular corotating case due to the asymmetry seeded in the Kerr metric: counter-rotating matter carries a larger and opposite angular momentum when crossing the innermost stable orbit, so that the spin modulus decreases faster and so the relative inclination angl
An Advanced Leakage Scheme for Neutrino Treatment in Astrophysical Simulations
We present an Advanced Spectral Leakage (ASL) scheme to model neutrinos in the context of core-collapse supernovae (CCSNe) and compact binary mergers. Based on previous gray leakage schemes, the ASL scheme computes the neutrino cooling rates by interpolating local production and diffusion rates (relevant in optically thin and thick regimes, respectively) separately for discretized values of the neutrino energy. Neutrino trapped components are also modeled, based on equilibrium and timescale arguments. The better accuracy achieved by the spectral treatment allows a more reliable computation of neutrino heating rates in optically thin conditions. The scheme has been calibrated and tested against Boltzmann transport in the context of Newtonian spherically symmetric models of CCSNe. ASL shows a very good qualitative and a partial quantitative agreement for key quantities from collapse to a few hundreds of milliseconds after core bounce. We have proved the adaptability and flexibility of our ASL scheme, coupling it to an axisymmetric Eulerian and to a three-dimensional smoothed particle hydrodynamics code to simulate core collapse. Therefore, the neutrino treatment presented here is ideal for large parameter-space explorations, parametric studies, high-resolution tests, code developments, and long-term modeling of asymmetric configurations, where more detailed neutrino treatments are not available or are currently computationally too expensive
Neutron star binary orbits in their host potential: Effect on early R-process enrichment
Coalescing neutron star binary (NSB) systems are primary candidates for r-process enrichment of galaxies. The recent detection of r-process elements in ultrafaint dwarf (UFD) galaxies and the abundances measured in classical dwarfs challenges the NSB merger scenario both in terms of coalescence time-scales and merger locations. In this paper, we focus on the dynamics of NSBs in the gravitational potentials of different types of host galaxies and on its impact on the subsequent galactic enrichment. We find that, for a ∼t−1 delay time distribution, even when receiving a low kick (∼10 km s−1) from the second supernova explosion, in shallow dwarf galaxy potentials NSBs tend to merge with a large off-set from the host galaxy. This results in a significant geometrical dilution of the amount of produced r-process elements that fall back and pollute the host galaxy gas reservoir. The combination of dilution and small number statistics produces a large scatter in the expected r-process enrichment within a single UFD or classical dwarf galaxy. Comparison between our results and observed europium abundances reveals a tension that even a systematic choice of optimistic parameters in our models cannot release. Such a discrepancy could point to the need of additional r-process production sites that suffer less severe dilution or to a population of extremely fast merging binaries
An explicit dynamics GPU structural solver for thin shell finite elements
With the availability of user oriented software tools, dedicated architectures, such as the parallel computing
platform and programming model CUDA (Compute Unified Device Architecture) released by NVIDIA,
one of the main producers of graphics cards, and of improved, highly performing GPU (Graphics
Processing Unit) boards, GPGPU (General Purpose programming on GPU) is attracting increasing interest
in the engineering community, for the development of analysis tools suitable to be used in validation/
verification and virtual reality applications. For their inherent explicit and decoupled structure, explicit
dynamics finite element formulations appear to be particularly attractive for implementations on hybrid
CPU/GPU or pure GPU architectures. The issue of an optimized, double-precision finite element GPU
implementation of an explicit dynamics finite element solver for elastic shell problems in small strains
and large displacements and rotations, using unstructured meshes, is here addressed. The conceptual
difference between a GPU implementation directly adapted from a standard CPU approach and a new
optimized formulation, specifically conceived for GPUs, is discussed and comparatively assessed. It is
shown that a speedup factor of about 5 can be achieved by an optimized algorithm reformulation and
careful memory management. A speedup of more than 40 is achieved with respect of state-of-the art
commercial codes running on CPU, obtaining real-time simulations in some cases, on commodity hardware.
When a last generation GPU board is used, it is shown that a problem with more than 16 millions
degrees of freedom can be solved in just few hours of computing time, opening the way to virtualization
approaches for real large scale engineering problems
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