937 research outputs found
A Phase-Space Approach to Collisionless Stellar Systems Using a Particle Method
A particle method for reproducing the phase space of collisionless stellar
systems is described. The key idea originates in Liouville's theorem which
states that the distribution function (DF) at time t can be derived from
tracing necessary orbits back to t=0. To make this procedure feasible, a
self-consistent field (SCF) method for solving Poisson's equation is adopted to
compute the orbits of arbitrary stars. As an example, for the violent
relaxation of a uniform-density sphere, the phase-space evolution which the
current method generates is compared to that obtained with a phase-space method
for integrating the collisionless Boltzmann equation, on the assumption of
spherical symmetry. Then, excellent agreement is found between the two methods
if an optimal basis set for the SCF technique is chosen. Since this
reproduction method requires only the functional form of initial DFs but needs
no assumptions about symmetry of the system, the success in reproducing the
phase-space evolution implies that there would be no need of directly solving
the collisionless Boltzmann equation in order to access phase space even for
systems without any special symmetries. The effects of basis sets used in SCF
simulations on the reproduced phase space are also discussed.Comment: 16 pages w/4 embedded PS figures. Uses aaspp4.sty (AASLaTeX v4.0). To
be published in ApJ, Oct. 1, 1997. This preprint is also available at
http://www.sue.shiga-u.ac.jp/WWW/prof/hozumi/papers.htm
Maximum entropy models for antibody diversity
Recognition of pathogens relies on families of proteins showing great
diversity. Here we construct maximum entropy models of the sequence repertoire,
building on recent experiments that provide a nearly exhaustive sampling of the
IgM sequences in zebrafish. These models are based solely on pairwise
correlations between residue positions, but correctly capture the higher order
statistical properties of the repertoire. Exploiting the interpretation of
these models as statistical physics problems, we make several predictions for
the collective properties of the sequence ensemble: the distribution of
sequences obeys Zipf's law, the repertoire decomposes into several clusters,
and there is a massive restriction of diversity due to the correlations. These
predictions are completely inconsistent with models in which amino acid
substitutions are made independently at each site, and are in good agreement
with the data. Our results suggest that antibody diversity is not limited by
the sequences encoded in the genome, and may reflect rapid adaptation to
antigenic challenges. This approach should be applicable to the study of the
global properties of other protein families
Evolution and instabilities of disks harboring super massive black holes
The bar formation is still an open problem in modern astrophysics. In this
paper we present numerical simulation performed with the aim of analyzing the
growth of the bar instability inside stellar-gaseous disks, where the star
formation is triggered, and a central black hole is present. The aim of this
paper is to point out the impact of such a central massive black hole on the
growth of the bar. We use N-body-SPH simulations of the same isolated
disk-to-halo mass systems harboring black holes with different initial masses
and different energy feedback on the surrounding gas. We compare the results of
these simulations with the one of the same disk without black hole in its
center. We make the same comparison (disk with and without black hole) for a
stellar disk in a fully cosmological scenario. A stellar bar, lasting 10 Gyrs,
is present in all our simulations. The central black hole mass has in general a
mild effect on the ellipticity of the bar but it is never able to destroy it.
The black holes grow in different way according their initial mass and their
feedback efficiency, the final values of the velocity dispersions and of the
black hole masses are near to the phenomenological constraints.Comment: 10 pages, 8 figures, accepted for pubblication in "Astrophysics and
Space Science
The Radial Orbit Instability in Collisionless N-Body Simulations
Using a suite of self-gravitating, collisionless N-body models, we
systematically explore a parameter space relevant to the onset and behavior of
the radial orbit instability (ROI), whose strength is measured by the systemic
axis ratios of the models. We show that a combination of two initial
conditions, namely the velocity anisotropy and the virial ratio, determines
whether a system will undergo ROI and exactly how triaxial the system will
become. A third initial condition, the radial shape of the density profile,
plays a smaller, but noticeable role. Regarding the dynamical development of
the ROI, the instability a) begins after systems collapse to their most compact
configuration and b) evolves fastest when a majority of the particles have
radially anisotropic orbits while there is a lack of centrally-concentrated
isotropic orbits. We argue that this is further evidence that self-reinforcing
torques are the key to the onset of the ROI. Our findings support the idea that
a separate orbit instability plays a role in halting the ROI.Comment: accepted for publication in ApJ. 9 figures in emulateapj styl
The Self-Regulated Growth of Supermassive Black Holes
We present a series of simulations of the self--regulated growth of
supermassive black holes (SMBHs) in galaxies via three different fueling
mechanisms: major mergers, minor mergers, and disk instabilities. The SMBHs in
all three scenarios follow the same black hole fundamental plane (BHFP) and
correlation with bulge binding energy seen in simulations of major mergers, and
observed locally. Furthermore, provided that the total gas supply is
significantly larger than the mass of the SMBH, its limiting mass is not
influenced by the amount of gas available or the efficiency of black hole
growth. This supports the assertion that SMBHs accrete until they reach a
critical mass at which feedback is sufficient to unbind the gas locally,
terminating the inflow and stalling further growth. At the same time, while
minor and major mergers follow the same projected correlations (e.g., the
and Magorrian relations), SMBHs grown via disk instabilities do
not, owing to structural differences between the host bulges. This finding is
supported by recent observations of SMBHs in pseudobulges and bulges in barred
systems, as compared to those hosted by classical bulges. Taken together, this
provides support for the BHFP and binding energy correlations as being more
"fundamental" than other proposed correlations in that they reflect the
physical mechanism driving the co-evolution of SMBHs and spheroids.Comment: 15 pages, 16 figures, accepted for publication in Ap
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A Protocol for Space Charge Measurements in Full-size HVDC Extruded Cables
This position paper, prepared by the IEEE DEIS HVDC Cable Systems Technical Committee, illustrates a protocol recommended for the measurement of space charges in full-size HVDC extruded cables during load cycle qualification tests (either prequalification load cycles or type test load cycles). The protocol accounts for the experimental practices of space charge measurements in the thick insulation of coaxial cables in terms of poling time, depolarization time, heating and cooling of specimens, as well as for the experience gained very recently from such kind of measurements performed in the framework of qualification tests relevant to ongoing HVDC cable system projects. The goal of the protocol is not checking the compliance with any maximum acceptable limit of either space charge or electric field. Rather, this protocol aims at assessing the variation of the electric field profile in the cable insulation wall during poling time at the beginning and at the end of load cycle qualification tests for full-size HVDC extruded cables. Indeed, in the design stage the electric field distributions are determined by the cable geometry and by temperature gradient in the insulation. Thus, the design is based on macroscopic parameters conductivity and permittivity and how they depend upon temperature. Any disturbance of the electric field due to space charge accumulation will only be revealed during space charge measurements either in as-manufactured state or in the aged state after load cycle qualification tests
Factors Affecting Outcomes of Stereoacuity in Partially Refractive Accommodative Esotropia with Motor Success Treated by Preoperative Prism Correction and Surgery
Teiji Yagasaki,1,2 Yoshimi Yokoyama,2 Ayaka Yagasaki,1,2 Kenta Hozumi,2 Sho Ichikawa2,3 1Yagasaki Eye Clinic, Ichinomiya, Aichi, Japan; 2Department of Ophthalmology, Japan Community Health Care Organization Chukyo Hospital, Nagoya, Aichi, Japan; 3Chukyo Eye Clinic, Nagoya, Aichi, JapanCorrespondence: Teiji Yagasaki, Email [email protected]: Favorable stereoacuity does not develop in all patients with partially refractive accommodative esotropia (PRAET) successfully aligned, and there have been few previous reports on the factors influencing stereoacuity outcomes in patients with PRAET treated with prismatic correction (PPC) and/or surgery. This study aimed to analyze factors affecting stereoacuity outcomes in patients of PRAET treated with PPC and surgery.Study Design: Retrospective study.Methods: Sixty-six patients with alignment within 10 prism diopters at final visit with PPC and surgery were included. According to the final stereoacuity, patients were grouped into the fine group (≤ 60 arcsec (“)), the coarse group (60 “ 6 months, ≤ 2 years; late: > 2 years) was carried out with the Kruskal–Wallis test.Results: There were no differences in ages at initial PPC, at surgery, at final visit, durations of misalignment, of PPC, or after surgery; however, significant differences in ages at onset and initial visit were found. Age at onset in the absent group was significantly earlier than those of the fine and the coarse groups (p 2 years, 18 patients (72%) showed fine or coarse stereoacuity (p 2 years.Keywords: partially refractive accommodative esotropia, prism correction, stereoacuity, critical period, surger
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