30,257 research outputs found
Vanishing of Gravitational Particle Production in the Formation of Cosmic Strings
We consider the gravitationally induced particle production from the quantum
vacuum which is defined by a free, massless and minimally coupled scalar field
during the formation of a gauge cosmic string. Previous discussions of this
topic estimate the power output per unit length along the string to be of the
order of ergs/sec/cm in the s-channel. We find that this production
may be completely suppressed. A similar result is also expected to hold for the
number of produced photons.Comment: 10 pages, Plain LaTex. Minor improvements. To appear in PR
Quiescence: a mechanism for escaping the effects of drug on cell populations
We point out that a simple and generic strategy to lower the risk for
extinction consists in the developing a dormant stage in which the organism is
unable to multiply but may die. The dormant organism is protected against the
poisonous environment. The result is to increase the survival probability of
the entire population by introducing a type of zero reproductive fitness. This
is possible, because the reservoir of dormant individuals act as a buffer that
can cushion fatal fluctuations in the number of births and deaths which without
the dormant population would have driven the entire population to extinction.Comment: 18 pages and 9 figure
Longitudinal dependence of middle and low latitude zonal plasma drifts measured by DE-2
We used ion drift observations from the DE-2 satellite to study for the first time the longitudinal variations of middle and low latitude <i>F</i> region zonal plasma drifts during quiet and disturbed conditions. The quiet-time middle latitude drifts are predominantly westward; the low latitude drifts are westward during the day and eastward at night. The daytime quiet-time drifts do not change much with longitude; the nighttime drifts have strong season dependent longitudinal variations. In the dusk-premidnight period, the equinoctial middle latitude westward drifts are smallest in the European sector and the low latitude eastward drifts are largest in the American-Pacific sector. The longitudinal variations of the late night-early morning drifts during June and December solstice are anti-correlated. During geomagnetically active times, there are large westward perturbation drifts in the late afternoon-early night sector at upper middle latitudes, and in the midnight sector at low latitudes. The largest westward disturbed drifts during equinox occur in European sector, and the smallest in the Pacific region. These results suggest that during equinox SAPS events occur most often at European longitudes. The low latitude perturbation drifts do not show significant longitudina
Low-density series expansions for directed percolation IV. Temporal disorder
We introduce a model for temporally disordered directed percolation in which
the probability of spreading from a vertex , where is the time and
is the spatial coordinate, is independent of but depends on . Using
a very efficient algorithm we calculate low-density series for bond percolation
on the directed square lattice. Analysis of the series yields estimates for the
critical point and various critical exponents which are consistent with a
continuous change of the critical parameters as the strength of the disorder is
increased.Comment: 11 pages, 3 figure
Transverse Asymmetry A_T′ from the Quasielastic ^3He(e,e′) Process and the Neutron Magnetic Form Factor
We have measured the transverse asymmetry A_T′ in ^3He(e,e′) quasielastic scattering in Hall A at Jefferson Laboratory with high precision for Q^2 values from 0.1 to 0.6 (GeV/c)^2. The neutron magnetic form factor GMn was extracted based on Faddeev calculations for Q^2 = 0.1 and 0.2 (GeV/c)^2 with an experimental uncertainty of less than 2%
Precision Measurement of the Spin-Dependent Asymmetry in the Threshold Region of ^3He(e, e')
We present the first precision measurement of the spin-dependent asymmetry in the threshold region of ^3He(e,e′) at Q^2 values of 0.1 and 0.2(GeV/c)^2. The agreement between the data and nonrelativistic Faddeev calculations which include both final-state interactions and meson-exchange current effects is very good at Q^2 = 0.1(GeV/c)^2, while a small discrepancy at Q^2 = 0.2(GeV/c)^2 is observed
An axisymmetric time-domain spectral-element method for full-wave simulations: Application to ocean acoustics
The numerical simulation of acoustic waves in complex 3D media is a key topic
in many branches of science, from exploration geophysics to non-destructive
testing and medical imaging. With the drastic increase in computing
capabilities this field has dramatically grown in the last twenty years.
However many 3D computations, especially at high frequency and/or long range,
are still far beyond current reach and force researchers to resort to
approximations, for example by working in 2D (plane strain) or by using a
paraxial approximation. This article presents and validates a numerical
technique based on an axisymmetric formulation of a spectral finite-element
method in the time domain for heterogeneous fluid-solid media. Taking advantage
of axisymmetry enables the study of relevant 3D configurations at a very
moderate computational cost. The axisymmetric spectral-element formulation is
first introduced, and validation tests are then performed. A typical
application of interest in ocean acoustics showing upslope propagation above a
dipping viscoelastic ocean bottom is then presented. The method correctly
models backscattered waves and explains the transmission losses discrepancies
pointed out in Jensen et al. (2007). Finally, a realistic application to a
double seamount problem is considered.Comment: Added a reference, and fixed a typo (cylindrical versus spherical
Comparison of photosensitivity in germanium doped silica fibers using 244 nm and 266 nm continuous wave lasers
The Globular Cluster System of the Coma cD Galaxy NGC 4874 from Hubble Space Telescope ACS and WFC3/IR Imaging
We present new HST optical and near-infrared (NIR) photometry of the rich
globular cluster (GC) system of NGC 4874, the cD galaxy in the core of the Coma
cluster (Abell 1656). NGC 4874 was observed with the HST Advanced Camera for
Surveys in the F475W (g) and F814W (I) passbands and the Wide Field Camera 3 IR
Channel in F160W (H). The GCs in this field exhibit a bimodal optical color
distribution with more than half of the GCs falling on the red side at g-I > 1.
Bimodality is also present, though less conspicuously, in the optical-NIR I-H
color. Consistent with past work, we find evidence for nonlinearity in the g-I
versus I-H color-color relation. Our results thus underscore the need for
understanding the detailed form of the color-metallicity relations in
interpreting observational data on GC bimodality. We also find a very strong
color-magnitude trend, or "blue tilt," for the blue component of the optical
color distribution of the NGC 4874 GC system. A similarly strong trend is
present for the overall mean I-H color as a function of magnitude; for M_814 <
-10 mag, these trends imply a steep mass-metallicity scaling with , but the scaling is not a simple power law and becomes
much weaker at lower masses. As in other similar systems, the spatial
distribution of the blue GCs is more extended than that of the red GCs, partly
because of blue GCs associated with surrounding cluster galaxies. In addition,
the center of the GC system is displaced by 4+/-1 kpc towards the southwest
from the luminosity center of NGC 4874, in the direction of NGC 4872. Finally,
we remark on a dwarf elliptical galaxy with a noticeably asymmetrical GC
distribution. Interestingly, this dwarf has a velocity of nearly -3000 km/s
with respect to NGC 4874; we suggest it is on its first infall into the cluster
core and is undergoing stripping of its GC system by the cluster potential.Comment: 24 pages, 20 figures, accepted for publication in Ap
- …