6,696 research outputs found
Functional Methods and Effective Potentials for Nonlinear Composites
A formulation of variational principles in terms of functional integrals is
proposed for any type of local plastic potentials. The minimization problem is
reduced to the computation of a path integral. This integral can be used as a
starting point for different approximations. As a first application, it is
shown how to compute to second-order the weak-disorder perturbative expansion
of the effective potentials in random composite. The three-dimensional results
of Suquet and Ponte-Casta\~neda (1993) for the plastic dissipation potential
with uniform applied tractions are retrieved and extended to any space
dimension, taking correlations into account. In addition, the viscoplastic
potential is also computed for uniform strain rates.Comment: 20 pages, accepted for publication in JMP
The Effect of the AGN Feedback on the Interstellar Medium of Early-Type Galaxies: 2D Hydrodynamical Simulations of the Low-Rotation Case
We present 2D hydrodynamical simulations for the evolution of early-type
galaxies containing central massive black holes (MBHs), starting at age 2 Gyr.
The code contains accurate and physically consistent radiative and mechanical
AGN wind feedback, with parsec-scale central resolution. Mass input comes from
stellar evolution; energy input includes Type Ia and II supernova and stellar
heating; star-formation is included. Realistic, axisymmetric dynamical models
for the galaxies are built solving the Jeans' equations. The lowest mass models
(Mstar = 8 10^{10}Msun) develop global outflows sustained by SNIa's heating,
ending with a significantly lower amount of hot gas and new stars. In more
massive models, nuclear outbursts last to the present epoch, with large and
frequent fluctuations in nuclear emission and from the gas (Lx). Each burst
last ~ 10^{7.5} yr, during which (for r < 2-3 kpc) cold, inflowing, and hot,
outflowing gas phases coexist. The Lx-T relation for the gas matches that of
local galaxies. AGN activity causes positive feedback for star formation.
Roughly half of the total mass loss is recycled into new stars (DeltaMstar),
just ~ 3% of it is accreted on the MBH, the remainder being ejected from the
galaxy. The ratio between the mass of gas expelled to that in to new stars, the
load factor, is ~0.6. Rounder galaxies shapes lead to larger final MBH masses,
DeltaMstar, and Lx. Almost all the time is spent at very low nuclear
luminosities, yet one quarter of the total energy is emitted at an Eddington
ratio > 0.1. The duty-cycle of AGN activity approximates 4% (Abridged).Comment: 26 pages, 15 figure, submitted to ApJ. Comments welcom
About the Reaction to âStyles of Thought on the Continental Drift Debateâ
The article appearing previously in this journal entitled âStyles of Thought on the Continental Drift Debateâ (Pellegrini 2019) prompted a response from Weber and Ć eĆĄelja (2020) which they termed as âa defence of rationalist accountsâ. They argue that their self-designated âsophisticated rationalismâ explains the closure of the continental-drift debate without being affected by my critiques to rationalist approaches. While ignoring the empirical evidence that shows the complexity of the debate and the necessity to include broader social elements in the analysis (such as scientists denying continental drift even after the plate tectonics theory, others supporting it without being familiarized with the literature), they proclaim to be unconvinced about the analysis of the styles of thought. In order to clarify differences in the approach to the continental-drift historical controversy, I respond here to the criticism my paper drew while discussing the place of rationalism when explaining the acceptance of a theory. I will argue that their distinction between âcrudeâ and âsophisticatedâ rationalism does not solve the problem of social aspects being left aside by rationalists in view of the acceptance of a theory. I will also argue that in order to understand what leads people to embrace a belief (namely scientists in accepting a theory), the analysis of mere cognitive or epistemic arguments is not enough and it leads to a reductionist explanation as to social behaviour.Fil: Pellegrini, Pablo Ariel. Universidad Nacional de Quilmes. Departamento de Ciencias Sociales. Instituto de Estudios Sociales de la Ciencia y la TecnologĂa; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; Argentin
Cooling flows and quasars: II. Detailed models of feedback modulated accretion flows
Most elliptical galaxies contain central black holes (BHs), and most also
contain significant amounts of hot gas capable of accreting on to the central
BH due to cooling times short compared to the Hubble time. Why therefore do we
not see AGNs at the center of most elliptical galaxies rather than in only (at
most) a few per cent of them? We propose here the simple idea that feedback
from accretion events heats the ambient gas retarding subsequent infall. In
this context, we present a class of 1D hydrodynamical evolutionary sequences
for the gas flows in elliptical galaxies with a massive central BH. The
resulting evolution is characterized by strong oscillations, in which very fast
and energetic bursts of the BH are followed by longer periods in which the
X-ray galaxy emission comes from the coronal gas. We also allow for departures
from spherical symmetry by examining scenarios in which the central engine is
either an ADAF or a more conventional accretion disk that is optically thick
except for a polar region. In all cases the duty cycle (fraction of the time
that the system will be seen as an AGN) is quite small and in the range 10^{-4}
- 10^{-3}. Thus, for any reasonable value of the efficiency, the presence of a
massive BH at the center of a galaxy seems to be incompatible with the presence
of a long-lived cooling flow.Comment: 43 pages, 10 figures. Main additions concern observed Compton
temperatures and few extra numerical models. Conclusions unchanged. 1 new
table and 3 new figures. Accepted for publication on ApJ (main journal
Non Markovian Quantum Repeated Interactions and Measurements
A non-Markovian model of quantum repeated interactions between a small
quantum system and an infinite chain of quantum systems is presented. By
adapting and applying usual pro jection operator techniques in this context,
discrete versions of the integro-differential and time-convolutioness Master
equations for the reduced system are derived. Next, an intuitive and rigorous
description of the indirect quantum measurement principle is developed and a
discrete non Markovian stochastic Master equation for the open system is
obtained. Finally, the question of unravelling in a particular model of
non-Markovian quantum interactions is discussed.Comment: 22 page
Properties of Very Luminous Galaxies
Recent analysis of the SSRS2 data based on cell-counts and two-point
correlation function has shown that very luminous galaxies are much more
strongly clustered than fainter galaxies. In fact, the amplitude of the
correlation function of very luminous galaxies () asymptotically
approaches that of clusters. In this paper we investigate the
properties of the most luminous galaxies, with blue absolute magnitude . We find that: 1) the population mix is comparable to that in other ranges
of absolute magnitudes; 2) only a small fraction are located in bona fide
clusters; 3) the bright galaxy-cluster cross-correlation function is
significantly higher on large scales than that measured for fainter galaxies;
4) the correlation length of galaxies brighter than \MB ,
expressed as a function of the mean interparticle distance, appears to follow
the universal dimensionless correlation function found for clusters and radio
galaxies; 5) a large fraction of the bright galaxies are in interacting pairs,
others show evidence for tidal distortions, while some appear to be surrounded
by faint satellite galaxies. We conclude that very luminous optical galaxies
differ from the normal population of galaxies both in the clustering and other
respects. We speculate that this population is highly biased tracers of mass,
being associated to dark halos with masses more comparable to clusters than
typical loose groups.Comment: 29 pages (6 figures) + 2 tables; paper with all figures and images
available at http://boas5.bo.astro.it/~cappi/papers.html; The Astronomical
Journal, in pres
X-ray properties expected from AGN feedback in elliptical galaxies
The ISM evolution of elliptical galaxies experiencing feedback from accretion
onto a central black hole was studied recently with high-resolution 1D
hydrodynamical simulations including radiative heating and pressure effects, a
RIAF-like radiative efficiency, mechanical input from AGN winds, and
accretion-driven starbursts. Here we focus on the observational properties of
the models in the X-ray band (nuclear luminosity; hot ISM luminosity and
temperature; temperature and brightness profiles during quiescence and during
outbursts). The nuclear bursts last for ~10^7 yr, with a duty-cycle of a few X
(10^-3-10^-2); the present epoch bolometric nuclear emission is very
sub-Eddington. The ISM thermal luminosity \lx oscillates in phase with the
nuclear one; this helps reproduce statistically the observed large \lx
variation. In quiescence the temperature profile has a negative gradient;
thanks to past outbursts, the brightness profile lacks the steep shape typical
of inflowing models. Outbursts produce disturbances in these profiles. Most
significantly, a hot bubble from shocked hot gas is inflated at the galaxy
center; the bubble would be conical in shape, and show radio emission. The ISM
resumes a smooth appearance on a time-scale of ~200 Myr; the duty-cycle of
perturbances in the ISM is of the order of 5-10%. From the present analysis,
additional input physics is important in the ISM-black hole coevolution, to
fully account for the properties of real galaxies, as a confining external
medium and a jet. The jet will reduce further the mass available for accretion
(and then the Eddington ratio ), and may help, together with an external
pressure, to produce flat or positive temperature gradient profiles (observed
in high density environments). Alternatively, can be reduced if the switch
from high to low radiative efficiency takes place at a larger than
routinely assumed.Comment: 40 pages, submitted to Ap
Energy Consumption Minimization Problem In A Railway Network
EWGT 2016 - 19th EURO Working Group on Transportation Meeting, Istanbul, TURQUIE, 05-/09/2016 - 07/09/2016; When train operations are perturbed, a new working timetable needs to be computed in real-time. In the literature, several algorithms have been proposed for optimizing this computation. This optimization usually does not consider energy consumption. However, minimizing energy consumption is a central issue both from the environmental and economic perspective. In this paper, we address the real-time problem of minimizing the energy consumption. The energy consumption depends on driving regimes used by the train drivers. Hence, we focus on the decision of the appropriate driving regimes throughout each train's travel along a given infrastructure. A model and solution approach for this problem are provided. We show a proof of concept on the applicability of this solution approach on a simple test case
The Hot Interstellar Medium in Normal Elliptical Galaxies III: The Thermal Structure of the Gas
This is the third paper in a series analyzing X-ray emission from the hot
interstellar medium in a sample of 54 normal elliptical galaxies observed by
Chandra, focusing on 36 galaxies with sufficient signal to compute radial
temperature profiles. We distinguish four qualitatively different types of
profile: positive gradient (outwardly rising), negative gradients (falling),
quasi-isothermal (flat) and hybrid (falling at small radii, then rising). We
measure the mean logarithmic temperature gradients in two radial regions: from
0--2 -band effective radii (excluding the central point source), and
from 2--. We find the outer gradient to be uncorrelated with intrinsic
host galaxy properties, but strongly influenced by the environment: galaxies in
low-density environments tend to show negative outer gradients, while those in
high-density environments show positive outer gradients, suggesting influence
of circumgalactic hot gas. The inner temperature gradient is unaffected by the
environment but strongly correlated with intrinsic host galaxy characteristics:
negative inner gradients are more common for smaller, optically faint, low
radio-luminosity galaxies, whereas positive gradients are found in bright
galaxies with stronger radio sources. There is no evidence for bimodality in
the distribution of inner or outer gradients. We propose three scenarios to
explain the inner temperature gradients: (1) Weak AGN heat the ISM locally,
higher-luminosity AGN heat the system globally through jets inflating cavities
at larger radii; (2) The onset of negative inner gradients indicates a
declining importance of AGN heating relative to other sources, such as
compressional heating or supernovae; (3) The variety of temperature profiles
are snapshots of different stages of a time-dependent flow.Comment: 18 pages, emulateapj, 55 figures (36 online-only figures included in
astro-ph version), submitted to Ap
- âŠ