4,847 research outputs found
Iron oxide nanoparticles fabricated by electric explosion of wire: Focus on magnetic nanofluids
Nanoparticles of iron oxides (MNPs) were prepared using the electric explosion of wire technique (EEW). The main focus was on the fabrication of de-aggregated spherical nanoparticles with a narrow size distribution. According to XRD the major crystalline phase was magnetite with an average diameter of MNPs, depending on the fraction. Further separation of air-dry EEW nanoparticles was performed in aqueous suspensions. In order to provide the stability of magnetite suspension in water, we found the optimum concentration of the electrostatic stabilizer (sodium citrate and optimum pH level) based on zeta-potential measurements. The stable suspensions still contained a substantial fraction of aggregates which were disintegrated by the excessive ultrasound treatment. The separation of the large particles out of the suspension was performed by centrifuging. The structural features, magnetic properties and microwave absorption of MNPs and their aqueous solutions confirm that we were able to obtain an ensemble in which the magnetic contributions come from the spherical MNPs. The particle size distribution in fractionated samples was narrow and they showed a similar behaviour to that expected of the superparamagnetic ensemble. Maximum obtained concentration was as high as 5 % of magnetic material (by weight). Designed assembly of de-aggregated nanoparticles is an example of on-purpose developed magnetic nanofluid. Copyright © 2012 Author(s)
Bar-driven Transport of Molecular Gas to Galactic Centers and Its Consequences
We study the characteristics of molecular gas in the central regions of
spiral galaxies on the basis of our CO(J=1-0) imaging survey of 20 nearby
spiral galaxies using the NRO and OVRO millimeter arrays. Condensations of
molecular gas at galactic centers with sizescales < 1 kpc and CO-derived masses
M_gas(R<500pc) = 10^8 - 10^9 M_sun are found to be prevalent in the gas-rich
L^* galaxies. Moreover, the degree of gas concentration to the central kpc is
found to be higher in barred systems than in unbarred galaxies. This is the
first statistical evidence for the higher central concentration of molecular
gas in barred galaxies, and it strongly supports the theory of bar-driven gas
transport. It is most likely that more than half of molecular gas within the
central kpc of a barred galaxy was transported there from outside by the bar.
The supply of gas has exceeded the consumption of gas by star formation in the
central kpc, resulting in the excess gas in the centers of barred systems. The
mean rate of gas inflow is statistically estimated to be larger than 0.1 - 1
M_sun/yr.
The correlation between gas properties in the central kpc and the type of
nuclear spectrum (HII, LINER, or Seyfert) is investigated. A correlation is
found in which galaxies with larger gas-to-dynamical mass ratios tend to have
HII nuclear spectra, while galaxies with smaller ratios show spectra indicating
AGN.
Also, the theoretical prediction of bar-dissolution by condensation of gas to
galactic centers is observationally tested. It is suggested that the timescale
for bar dissolution is larger than 10^8 - 10^10 yr, or a bar in a L^* galaxy is
not destroyed by a condensation of 10^8 - 10^9 M_sun gas in the central kpc.Comment: AASTeX, 20 pages, 8 eps figs, ApJ in press (10 Nov. 1999 issue
Star Formation in Isolated Disk Galaxies. I. Models and Characteristics of Nonlinear Gravitational Collapse
We model gravitational collapse leading to star formation in a wide range of
isolated disk galaxies using a three-dimensional, smoothed particle
hydrodynamics code. The model galaxies include a dark matter halo and a disk of
stars and isothermal gas. Absorbing sink particles are used to directly measure
the mass of gravitationally collapsing gas. They reach masses characteristic of
stellar clusters. In this paper, we describe our galaxy models and numerical
methods, followed by an investigation of the gravitational instability in these
galaxies. Gravitational collapse forms star clusters with correlated positions
and ages, as observed, for example, in the Large Magellanic Cloud.
Gravitational instability alone acting in unperturbed galaxies appears
sufficient to produce flocculent spiral arms, though not more organized
patterns. Unstable galaxies show collapse in thin layers in the galactic plane;
associated dust will form thin dust lanes in those galaxies, in agreement with
observations. (abridged)Comment: 49 pages, 22 figures, to appear in ApJ (July, 2005), version with
high quality color images can be fond in
http://research.amnh.org/~yuexing/astro-ph/0501022.pd
Radial Profiles of Star Formation in the Far Outer Regions of Galaxy Disks
Star formation in galaxies is triggered by a combination of processes,
including gravitational instabilities, spiral wave shocks, stellar compression,
and turbulence compression. Some of these persist in the far outer regions
where the column density is far below the threshold for instabilities, making
the outer disk cutoff somewhat gradual. We show that in a galaxy with a single
exponential gas profile the star formation rate can have a double exponential
with a shallow one in the inner part and a steep one in the outer part. Such
double exponentials have been observed recently in the broad-band intensity
profiles of spiral and dwarf Irregular galaxies. The break radius in our model
occurs slightly outside the threshold for instabilities provided the Mach
number for compressive motions remains of order unity to large radii. The ratio
of the break radius to the inner exponential scale length increases for higher
surface brightness disks because the unstable part extends further out. This is
also in agreement with observations. Galaxies with extended outer gas disks
that fall more slowly than a single exponential, such as 1/R, can have their
star formation rate scale approximately as a single exponential with radius,
even out to 10 disk scale lengths. Halpha profiles should drop much faster than
the star formation rate as a result of the rapidly decreasing ambient density.Comment: To appear in ApJ. Available from
ftp.lowell.edu/pub/dah/papers/sfouterdisks
The Eastern Arm of M83 Revisited: High-Resolution Mapping of 12CO 1-0 Emission
We have used the Owens Valley Millimeter Array to map 12CO (J=1-0) along a
3.5 kpc segment of M83's eastern spiral arm at resolutions of 6.5"x3.5", 10",
and 16". The CO emission in most of this segment lies along the sharp dust lane
demarking the inner edge of the spiral arm, but beyond a certain point along
the arm the emission shifts downstream from the dust lane to become better
aligned with the young stars seen in blue and H-beta images. This morphology
resembles that of the western arm of M100. Three possibilities, none of which
is wholly satisfactory, are considered to explain the deviation of the CO arm
from the dust lane: heating of the CO by UV radiation from young stars, heating
by low-energy cosmic rays, and a molecular medium consisting of two (diffuse
and dense) components which react differently to the density wave. Regardless,
the question of what CO emission traces along this spiral arm is a complicated
one. Strong tangential streaming is observed where the arm crosses the
kinematic major axis of the galaxy, implying that the shear becomes locally
prograde in the arms. Inferred from the streaming is a very high gas surface
density of about 230 solar masses/pc**2 and an arm-interarm contrast greater
than 2.3 in the part of the arm near the major axis. Using two different
criteria, we find that the gas at this location is well above the threshold for
gravitational instability -- much more clearly so than in either M51 or M100.Comment: Accepted for publication in ApJ. 25 pages, 5 figures. Manuscript in
LaTeX, figures in pdf. Fig 3 in colo
Derived coisotropic structures II: stacks and quantization
We extend results about -shifted coisotropic structures from part I of
this work to the setting of derived Artin stacks. We show that an intersection
of coisotropic morphisms carries a Poisson structure of shift one less. We also
compare non-degenerate shifted coisotropic structures and shifted Lagrangian
structures and show that there is a natural equivalence between the two spaces
in agreement with the classical result. Finally, we define quantizations of
-shifted coisotropic structures and show that they exist for .Comment: 45 pages. Contains the second half of arXiv:1608.01482v1 with new
material adde
Surface modified Ni nanoparticles produced by the electrical explosion of wire
Spherical nickel nanoparticles were prepared by the electrical explosion of wire. The as prepared nanoparticles were modified immediately after fabrication at room temperature in order to provide tunable surface properties with focus on the development of composites filled with nanoparticles. Following liquid modificators were used: hexane, toluene and the solution of polystyrene in toluene. In one case the surface modification by carbon was made in gas phase as a result of hydrocarbon injection. The average size of the nanoparticles was about 50 nm and unit cell parameters were close to 0.351 nm. Detailed characterization was done by X-ray diffraction, transmission electron microscopy, and magnetization measurements. Sphericity was also checked using microwave resonant absorption. © (2015) Trans Tech Publications, Switzerland.Electromagnetics of Russian Academy of Sciences;et al;Institute for Theoretical and Applied;Japan Society for the Promotion of Science;Lomonosov Moscow State University;Russian Foundation for Basic Researc
Target mass number dependence of subthreshold antiproton production in proton-, deuteron- and alpha-particle-induced reactions
Data from KEK on subthreshold \bar{\mrm{p}} as well as on and
\mrm{K}^\pm production in proton-, deuteron- and -induced reactions
at energies between 2.0 and 12.0 A GeV for C, Cu and Pb targets are described
within a unified approach. We use a model which considers a nuclear reaction as
an incoherent sum over collisions of varying numbers of projectile and target
nucleons. It samples complete events and thus allows for the simultaneous
consideration of all final particles including the decay products of the
nuclear residues. The enormous enhancement of the \bar{\mrm{p}} cross
section, as well as the moderate increase of meson production in deuteron and
induced compared to proton-induced reactions, is well reproduced for
all target nuclei. In our approach, the observed enhancement near the
production threshold is mainly due to the contributions from the interactions
of few-nucleon clusters by simultaneously considering fragmentation processes
of the nuclear residues. The ability of the model to reproduce the target mass
dependence may be considered as a further proof of the validity of the cluster
concept.Comment: 9 pages, 4 figure
Polyacrylamide ferrogels with Ni nanowires
Nickel magnetic nanowires (NWs) have attracted significant attention due to their unique properties, which are useful for basic studies and technological applications, for example in biomedicine. Their structure and magnetic properties were systematically studied in the recent years. In this work, Ni NWs with high aspect ratios (length/diameter ~250) were fabricated by electrodeposition into commercial anodic aluminum oxide templates. The templates were then etched and the NWs were suspended in water, where their hydrodynamic size was evaluated by dynamic light scattering. The magnetic response of these NWs as a function of an external magnetic field indicates a dominant shape anisotropy with propagation of the vortex domain wall as the main magnetization reversal process. The suspension of Ni NWs was used in the synthesis of two types of polyacrylamide ferrogels (FGs) by free radical polymerization, with weight fractions of Ni NWs in FGs of 0.036% and 0.169%. The FGs were reasonably homogeneous. The magnetic response of these FGs (hysteresis loops) indicated that the NWs are randomly oriented inside the FG, and their magnetic response remains stable after embedding. © 2019 by the authors.Russian Science Foundation, RSF: MAT2017-83631-C3-R, 18-19-00090Funding: This work was supported by the Russian Science Foundation grant 18-19-00090. Part of this work has been performed under the financial support of the Spanish Government under project MAT2017-83631-C3-R
Subthreshold antiproton production in proton-carbon reactions
Data from KEK on subthreshold antiproton as well as on pi(+-) and K(+-)
production in proton-nucleus reactions are described at projectile energies
between 3.5 and 12.0 GeV. We use a model which considers a hadron-nucleus
reaction as an incoherent sum over collisions of the projectile with a varying
number of target nucleons. It samples complete events and allows thus for the
simultaneous consideration of all particle species measured. The overall
reproduction of the data is quite satisfactory. It is shown that the
contributions from the interaction of the projectile with groups of several
target nucleons are decisive for the description of subthreshold production.
Since the collective features of subthreshold production become especially
significant far below the threshold, the results are extrapolated down to COSY
energies. It is concluded that an antiproton measurement at ANKE-COSY should be
feasible, if the high background of other particles can be efficiently
suppressed.Comment: 15 pages, 5 figures, gzipped tar file, submitted to J. Phys. G v2:
Modification of text due to demands of referee
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