197 research outputs found
Green underwater wireless communications using hybrid optical-acoustic technologies
Underwater wireless communication is a rapidly growing field, especially with the recent emergence of technologies such as autonomous underwater vehicles (AUVs) and remotely operated vehicles (ROVs). To support the high-bandwidth applications using these technologies, underwater optics has attracted significant attention, alongside its complementary technology – underwater acoustics. In this paper, we propose a hybrid opto-acoustic underwater wireless communication model that reduces network power consumption and supports high-data rate underwater applications by selecting appropriate communication links in response to varying traffic loads and dynamic weather conditions. Underwater optics offers high data rates and consumes less power. However, due to the severe absorption of light in the medium, the communication range is short in underwater optics. Conversely, acoustics suffers from low data rate and high power consumption, but provides longer communication ranges. Since most underwater equipment relies on battery power, energy-efficient communication is critical for reliable underwater communications. In this work, we derive analytical models for both underwater acoustics and optics, and calculate the required transmit power for reliable communications in various underwater communication environments. We then formulate an optimization problem that minimizes the network power consumption for carrying data from underwater nodes to surface sinks under varying traffic loads and weather conditions. The proposed optimization model can be solved offline periodically, hence the additional computational complexity to find the optimum solution for larger networks is not a limiting factor for practical applications. Our results indicate that the proposed technique yields up to 35% power savings compared to existing opto-acoustic solutions
A Direct Imaging Survey of Spitzer detected debris disks: Occurrence of giant planets in dusty systems
We describe a joint high contrast imaging survey for planets at Keck and VLT
of the last large sample of debris disks identified by the Spitzer Space
Telescope. No new substellar companions were discovered in our survey of 30
Spitzer-selected targets. We combine our observations with data from four
published surveys to place constraints on the frequency of planets around 130
debris disk single stars, the largest sample to date. For a control sample, we
assembled contrast curves from several published surveys targeting 277 stars
which do not show infrared excesses. We assumed a double power law distribution
in mass and semi-major axis of the form f(m,a) = , where
we adopted power law values and logarithmically flat values for the mass and
semi-major axis of planets. We find that the frequency of giant planets with
masses 5-20 and separations 10-1000 AU around stars with debris
disks is 6.27% (68% confidence interval 3.68 - 9.76%), compared to 0.73% (68%
confidence interval 0.20 - 1.80%) for the control sample of stars without
disks. These distributions differ at the 88% confidence level, tentatively
suggesting distinctness of these samples.Comment: Accepted to A
Monopoles and Knots in Skyrme Theory
We show that the Skyrme theory actually is a theory of monopoles which allows
a new type of solitons, the topological knots made of monopole-anti-monopole
pair,which is different from the well-known skyrmions. Furthermore, we derive a
generalized Skyrme action from the Yang-Mills action of QCD, which we propose
to be an effective action of QCD in the infra-red limit. We discuss the
physical implications of our results.Comment: 4 pages. Phys. Rev. Lett. in pres
Jet quenching in shock waves
We study the propagation of an ultrarelativistic light quark jet inside a
shock wave using the holographic principle. The maximum stopping distance and
its dependency on the energy of the jet is obtained
The Spitzer c2d Survey of Nearby Dense Cores: III: Low Mass Star Formation in a Small Group, L1251B
We present a comprehensive study of a low-mass star-forming region,L1251B, at
wavelengths from the near-infrared to the millimeter. L1251B, where only one
protostar, IRAS 22376+7455, was known previously, is confirmed to be a small
group of protostars based on observations with the Spitzer Space Telescope. The
most luminous source of L1251B is located 5" north of the IRAS position. A
near-infrared bipolar nebula, which is not associated with the brightest object
and is located at the southeast corner of L1251B, has been detected in the IRAC
bands. OVRO and SMA interferometric observations indicate that the brightest
source and the bipolar nebula source in the IRAC bands are deeply embedded disk
sources.Submillimeter continuum observations with single-dish telescopes and
the SMA interferometric observations suggest two possible prestellar objects
with very high column densities. Outside of the small group, many young stellar
object candidates have been detected over a larger region of 12' x 12'.
Extended emission to the east of L1251B has been detected at 850 micron; this
"east core" may be a site for future star formation since no point source has
been detected with IRAC or MIPS. This region is therefore a possible example of
low-mass cluster formation, where a small group of pre- and protostellar
objects (L1251B) is currently forming, alongside a large starless core (the
east core).Comment: 35 pages, 15 figures, accepted for publication in ApJ, for the full
resolution paper, visit
"http://peggysue.as.utexas.edu/SIRTF/PAPERS/pap27.pub.pdf
The Chiral Model of Sakai-Sugimoto at Finite Baryon Density
In the context of holographic QCD we analyze Sakai-Sugimoto's chiral model at
finite baryon density and zero temperature. The baryon number density is
introduced through compact D4 wrapping S^4 at the tip of D8-\bar{D8}. Each
baryon acts as a chiral point-like source distributed uniformly over R^3, and
leads a non-vanishing U(1)_V potential on the brane. For fixed baryon charge
density n_B we analyze the bulk energy density and pressure using the canonical
formalism. The baryonic matter with point like sources is always in the
spontaneously broken phase of chiral symmetry, whatever the density. The
point-like nature of the sources and large N_c cause the matter to be repulsive
as all baryon interactions are omega mediated. Through the induced DBI action
on D8-\bar{D8}, we study the effects of the fixed baryon charge density n_B on
the pion and vector meson masses and couplings. Issues related to vector
dominance in matter in the context of holographic QCD are also discussed.Comment: V3: 39 pages, 16 figures, minor corrections, version to appear in
JHEP. V2: references added, typos correcte
Causes of Infant Mortality in Babol, Northern Iran
Background and Objective: Infant mortality is an important health indicator. In order to reduce infant mortality, its causes should be considered in each region so that steps can be taken to improve the quality of health care and treatment. Therefore, this study was conducted in order to determine the causes of infant mortality in Babol, northern Iran.
Methods: This cross-sectional study was conducted on all infants who died in public and private hospitals of Babol. The cause of infant death was extracted and analyzed based on the International Classification of Diseases 10th Revision (ICD-10) and also based on laboratory, clinical and paraclinical evidence recorded in the files.
Findings: Out of a total of 16,005 live births in Babol, 123 infants (7.6%) died, of which 53 (43.1%) died in the first 24 hours of life. The mean gestational age of the deceased cases was 31.4±5.5 weeks, the mean birth weight was 1705.4±1045.3 grams, and their mean life span was 5.4±6.6 days. The most common causes of infant mortality were prematurity (30%), congenital anomalies (25.3%), sepsis (17.1%) and respiratory distress syndrome (13%).
Conclusion: The results of the study showed that the most common causes of infant mortality are prematurity and congenital anomalies
Topology of the gauge-invariant gauge field in two-color QCD
We investigate solutions to a nonlinear integral equation which has a central
role in implementing the non-Abelian Gauss's Law and in constructing
gauge-invariant quark and gluon fields. Here we concern ourselves with
solutions to this same equation that are not operator-valued, but are functions
of spatial variables and carry spatial and SU(2) indices. We obtain an
expression for the gauge-invariant gauge field in two-color QCD, define an
index that we will refer to as the ``winding number'' that characterizes it,
and show that this winding number is invariant to a small gauge transformation
of the gauge field on which our construction of the gauge-invariant gauge field
is based. We discuss the role of this gauge field in determining the winding
number of the gauge-invariant gauge field. We also show that when the winding
number of the gauge field is an integer , the gauge-invariant
gauge field manifests winding numbers that are not integers, and are
half-integers only when .Comment: 26 pages including 6 encapsulated postscript figures. Numerical
errors have been correcte
Baryonic Response of Dense Holographic QCD
The response function of a homogeneous and dense hadronic system to a
time-dependent (baryon) vector potential is discussed for holographic dense QCD
(D4/D8 embedding) both in the confined and deconfined phases. Confined
holographic QCD is an uncompressible and static baryonic insulator at large N_c
and large \lambda, with a gapped vector spectrum and a massless pion.
Deconfined holographic QCD is a diffusive conductor with restored chiral
symmetry and a gapped transverse baryonic current. Similarly, dense D3/D7 is
diffusive for any non-zero temperature at large N_c and large \lambda. At zero
temperature dense D3/D7 exhibits a baryonic longitudinal visco-elastic mode
with a first sound speed \lambda/\sqrt{3} and a small width due to a shear
viscosity to baryon ratio \eta/n_B=\hbar/4. This mode is turned diffusive by
arbitrarily small temperatures, a hallmark of holography.Comment: V2: 47 pages, 7 figures, references added, typos correcte
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