130 research outputs found
The research program of the Liquid Scintillation Detector (LSD) in the Mont Blanc Laboratory
A massive (90 tons) liquid scintillation detector (LSD) has been running since October 1984 in the Mont Blanc Laboratory at a depth of 5,200 hg/sq cm of standard rock. The research program of the experiment covers a variety of topics in particle physics and astrophysics. The performance of the detector, the main fields of research are presented and the preliminary results are discussed
Origin of TeV Galactic Cosmic Rays
We consider a possibility of identification of sources of cosmic rays (CR) of
the energy above 1 TeV via observation of degree-scale extended gamma-ray
emission which traces the locations of recent sources in the Galaxy. Such
emission in the energy band above 100 GeV is produced by CR nuclei and
electrons released by the sources and spreading into the interstellar medium.
We use the data from the Fermi gamma-ray telescope to locate the degree-scale
100 GeV gamma-ray sources. We find that the number of such sources and their
overall power match to those expected when CRs injection events happen every
~100 yr in portions of ~1e50 erg. We find that most of the sources are
associated to pulsars with spin down age less than ~30 kyr and hence to the
recent supernova explosions. This supports the hypothesis of supernova origin
of Galactic CRs. We notice that the degree-scale extended emission does not
surround shell-like supernova remnants without pulsars. Based on this
observation, we argue that the presence of the pulsar is essential for the CR
acceleration process. We expect that a significant fraction of the degree-scale
sources should be detectable as extended sources with km3-scale neutrino
detectors.Comment: 14 pages, 14 figures, accepted for publication in Phys.Rev.
Mechanical activation influence on the morphological properties of La[2]O[3]-TiO[2]-B
The influence of mechanical activation of the powder mixture used to obtain the high-perfomance cathode for accelerating engineering with the SHS-method has been explored. The mechanically processed mixtures have been morphologically analyzed. The optimal modes of mechanical activation have been determined for the mixture
Velocity Shear of the Thick Disk from SPM3 Proper Motions at the South Galactic Pole
The kinematical properties of the Galactic Thick Disk are studied using
absolute proper motions from the SPM3 Catalog and 2MASS near-infrared
photometry for a sample of ~1200 red giants in the direction of the South
Galactic Pole. The photometrically-selected sample is dominated by Thick Disk
stars, as indicated by the number-density distribution that varies with
distance from the Galactic plane as a single-valued exponential over the range
1<z<4 kpc. The inferred scale height of the Thick Disk is 0.783 +/- 0.048 kpc.
The kinematics of the sample are also consistent with disk-like motion. The
U-velocity component is roughly constant, reflecting the Sun's peculiar motion,
while a considerable shear is seen in the mean rotational velocity, V. The
V-velocity profile's dependence on z is linear, with a gradient of dV/dz = -30
+/- 3 km/s/kpc. The velocity dispersions, in both U and V, show a lesser
gradient of about 9 +/- 3 km/s/kpc. We demonstrate that the derived velocity
and velocity-dispersion profiles are consistent with the assumptions of
dynamical equilibrium and reasonable models of the overall Galactic potential.Comment: 38 pages, 9 figures, accepted for publication in A
Dynamics of Gaseous Disks in a Non-axisymmetric Dark Halo
The dynamics of a galactic disk in a non-axisymmetric (triaxial) dark halo is
studied in detail using high-resolution, numerical, hydrodynamical models. A
long-lived, two-armed spiral pattern is generated for a wide range of
parameters. The spiral structure is global, and the number of turns can be two
or three, depending on the model parameters. The morphology and kinematics of
the spiral pattern are studied as functions of the halo and disk parameters.
The spiral structure rotates slowly, and its angular velocity varies
quasi-periodically. Models with differing relative halo masses, halo semi-axis
ratios, distributions of matter in the disk, Mach numbers in the gaseous
component, and angular rotational velocities of their halos are considered.Comment: 22 pages, 11 figure
Proper Motion Study of the Magellanic Clouds using SPM material
Absolute proper motions are determined for stars and galaxies to V=17.5 over
a 450 square-degree area that encloses both Magellanic Clouds. The proper
motions are based on photographic and CCD observations of the Yale/San Juan
Southern Proper Motion program, which span over a baseline of 40 years.
Multiple, local relative proper motion measures are combined in an overlap
solution using photometrically selected Galactic Disk stars to define a global
relative system that is then transformed to absolute using external galaxies
and Hipparcos stars to tie into the ICRS. The resulting catalog of 1.4 million
objects is used to derive the mean absolute proper motions of the Large
Magellanic Cloud and the Small Magellanic Cloud;
(\mu_\alpha\cos\delta,\mu_\delta)_{LMC}=(1.89,+0.39)\pm (0.27,0.27)\;\;\{mas
yr}^{-1} and (\mu_\alpha\cos\delta,\mu_\delta)_{SMC}=(0.98,-1.01)\pm
(0.30,0.29)\;\;\{mas yr}^{-1}. These mean motions are based on best-measured
samples of 3822 LMC stars and 964 SMC stars. A dominant portion (0.25 mas
yr) of the formal errors is due to the estimated uncertainty in the
inertial system of the Hipparcos Catalog stars used to anchor the bright end of
our proper motion measures. A more precise determination can be made for the
proper motion of the SMC {\it relative} to the LMC;
(\mu_{\alpha\cos\delta},\mu_\delta)_{SMC-LMC} = (-0.91,-1.49) \pm
(0.16,0.15)\;\;\{mas yr}^{-1}. This differential value is combined with
measurements of the proper motion of the LMC taken from the literature to
produce new absolute proper-motion determinations for the SMC, as well as an
estimate of the total velocity difference of the two clouds to within 54
kms.Comment: 50 pages (referee format), 13 figures. Accepted for publication in A
Gravitational stability and dynamical overheating of stellar disks of galaxies
We use the marginal stability condition for galactic disks and the stellar
velocity dispersion data published by different authors to place upper limits
on the disk local surface density at two radial scalelengths .
Extrapolating these estimates, we constrain the total mass of the disks and
compare these estimates to those based on the photometry and color of stellar
populations. The comparison reveals that the stellar disks of most of spiral
galaxies in our sample cannot be substantially overheated and are therefore
unlikely to have experienced a significant merging event in their history. The
same conclusion applies to some, but not all of the S0 galaxies we consider.
However, a substantial part of the early type galaxies do show the stellar
velocity dispersion well in excess of the gravitational stability threshold
suggesting a major merger event in the past. We find dynamically overheated
disks among both seemingly isolated galaxies and those forming pairs. The ratio
of the marginal stability disk mass estimate to the total galaxy mass within
four radial scalelengths remains within a range of 0.4---0.8. We see no
evidence for a noticeable running of this ratio with either the morphological
type or color index.Comment: 25 pages, 5 figures, accepted to Astronomy Letter
Control of star formation by supersonic turbulence
Understanding the formation of stars in galaxies is central to much of modern
astrophysics. For several decades it has been thought that stellar birth is
primarily controlled by the interplay between gravity and magnetostatic
support, modulated by ambipolar diffusion. Recently, however, both
observational and numerical work has begun to suggest that support by
supersonic turbulence rather than magnetic fields controls star formation. In
this review we outline a new theory of star formation relying on the control by
turbulence. We demonstrate that although supersonic turbulence can provide
global support, it nevertheless produces density enhancements that allow local
collapse. Inefficient, isolated star formation is a hallmark of turbulent
support, while efficient, clustered star formation occurs in its absence. The
consequences of this theory are then explored for both local star formation and
galactic scale star formation. (ABSTRACT ABBREVIATED)Comment: Invited review for "Reviews of Modern Physics", 87 pages including 28
figures, in pres
K^+ production in the reaction at incident energies from 1 to 2 AGeV
Semi-inclusive triple differential multiplicity distributions of positively
charged kaons have been measured over a wide range in rapidity and transverse
mass for central collisions of Ni with Ni nuclei. The transverse
mass () spectra have been studied as a function of rapidity at a beam
energy 1.93 AGeV. The distributions of K^+ mesons are well described by a
single Boltzmann-type function. The spectral slopes are similar to that of the
protons indicating that rescattering plays a significant role in the
propagation of the kaon. Multiplicity densities have been obtained as a
function of rapidity by extrapolating the Boltzmann-type fits to the measured
distributions over the remaining phase space. The total K^+ meson yield has
been determined at beam energies of 1.06, 1.45, and 1.93 AGeV, and is presented
in comparison to existing data. The low total yield indicates that the K^+
meson can not be explained within a hadro-chemical equilibrium scenario,
therefore indicating that the yield does remain sensitive to effects related to
its production processes such as the equation of state of nuclear matter and/or
modifications to the K^+ dispersion relation.Comment: 24 pages Latex (elsart) 7 PS figures to be submitted to Nucl. Phys
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