3,892 research outputs found
The H-test probability distribution revisited: Improved sensitivity
Aims: To provide a significantly improved probability distribution for the
H-test for periodicity in X-ray and -ray arrival times, which is
already extensively used by the -ray pulsar community. Also, to obtain
an analytical probability distribution for stacked test statistics in the case
of a search for pulsed emission from an ensemble of pulsars where the
significance per pulsar is relatively low, making individual detections
insignificant on their own. This information is timely given the recent rapid
discovery of new pulsars with the Fermi-LAT t -ray telescope. Methods:
Approximately realisations of the H-statistic () for random
(white) noise is calculated from a random number generator for which the
repitition cycle is . From these numbers the probability
distribution is calculated. Results: The distribution of is is
found to be exponential with parameter so that the cumulative
probability distribution . If we stack independent
values for , the sum of such values would follow the Erlang-K
distribution with parameter for which the cumulative probability
distribution is also a simple analytical expression. Conclusion: Searches for
weak pulsars with unknown pulse profile shapes in the Fermi-LAT, Agile or other
X-ray data bases should benefit from the {\it H-test} since it is known to be
powerful against a broad range of pulse profiles, which introduces only a
single statistical trial if only the {\it H-test} is used. The new probability
distribution presented here favours the detection of weaker pulsars in terms of
an improved sensitivity relative to the previously known distribution.Comment: 4 pages, two figures, to appear in Astronomy and Astrophysics,
Letter
Predicting the Viability of Fish Populations in a Modified Riverine Environment
Riverine fishes evolved to life in a highly variable, flow-driven environment. During the two past centuries, large rivers have been substantially altered by human activities. This has resulted in declines of fish populations that depend on the large river environment.
The research described here uses models to evaluate the effects of human activities on the viability of fish populations in rivers. I focused on five modifications of the river environment associated with impoundment: (1) seasonal allocation of river flow; (2) diversion of river flow; (3) fragmentation of the river habitat by dams; (4) conversion of free-flowing river to reservoir habitat; and (5) alteration of migration patterns.
To understand the role of flow regulation on chinook salmon (Oncorhynchus tshawytscha) recruitment, I developed an individual-based model to predict recruitment as a function of seasonal flow patterns in the Tuolumne River, California. I used simulated annealing to find flow patterns that maximize chinook recruitment under wet and dry hydrologic conditions. As water availability increased, I found that the optimal flow pattern shifted from allocating low flows uniformly across seasons to a pattern with high spring flows. When I considered a new objective: maximizing the variance of spawning times among recruits, the optimal flow regime called for a winter pulse in flow just before the peak spawning date for the minority (late-fall) run.
To evaluate the recovery options for chinook salmon in the Tuolumne River, Ideveloped an age-based model to conduct a population viability analysis (PVA). I developed a flow-dependent spawner-recruitment relationship from the recruitment model. Its shape depended on the flow regime, suggesting that such relationships are not fixed properties of species, but depend on environmental conditions. The PVA model suggested that recovery, in the absence of straying, would be enhanced most by significantly reducing ocean harvest, followed by reduced diversion of water from the river.
For white sturgeon (Acipenser transmontanus) populations in the Snake River, Idaho a main concern is habitat fragmentation by dams resulting in smaller, isolated populations. Simulation experiments to evaluate the effects of fragmentation suggested that population viability was higher when dams were spaced widely enough apart to retain free-flowing habitat. A simulation experiment to evaluate the effects of altered migration patterns associated with impoundment showed that both the likelihood of persistence and the genetic diversity among white sturgeon populations were enhanced by balanced upstream and downstream migration rates.
Models that simulate the responses of fish populations to modified river habitat do not consider the potential for an evolutionary response. I designed a PVA model simulating the genetic basis of age at maturity for individual fish. Simulated individual variation in this trait lead to increased population viability only when the variation was heritable and subjected to an altered selective regime. The results support the idea that predicting population viability depends on estimating the potential for evolution in fitness-related traits for populations exposed to anthropogenic changes in the environment that impose strong, directional selective forces
A Cosmic Ray Positron Anisotropy due to Two Middle-Aged, Nearby Pulsars?
Geminga and B0656+14 are the closest pulsars with characteristic ages in the
ran ge of 100 kyr to 1 Myr. They both have spindown powers of the order 3e34
erg/s at present. The winds of these pulsars had most probably powered pulsar
wind nebulae (PWNe) that broke up less than about 100 kyr after the birth of
the pulsars. Assuming that leptonic particles accelerated by the pulsars were
confined in th e PWNe and were released into the interstellar medium (ISM) on
breakup of the PW Ne, we show that, depending on the pulsar parameters, both
pulsars make a non-ne gligible contribution to the local cosmic ray (CR)
positron spectrum, and they m ay be the main contributors above several GeV.
The relatively small angular dist ance between Geminga and B0656+14 thus
implies an anisotropy in the local CR po sitron flux at these energies. We
calculate the contribution of these pulsars to the locally observed CR electr
on and positron spectra depending on the pulsar birth period and the magnitude
o f the local CR diffusion coefficient. We further give an estimate of the
expecte d anisotropy in the local CR positron flux. Our calculations show that
within the framework of our model, the local CR posit ron spectrum imposes
constraints on pulsar parameters for Geminga and B0656+14, notably the pulsar
period at birth, and also the local interstellar diffusion co efficient for CR
leptons.Comment: accepted for publication in ApJ
Shadowing Effects on the Nuclear Suppression Factor, R_dAu, in d+Au Interactions
We explore how nuclear modifications to the nucleon parton distributions
affect production of high transverse momentum hadrons in deuteron-nucleus
collisions. We calculate the charged hadron spectra to leading order using
standard fragmentation functions and shadowing parameterizations. We obtain the
d+Au to pp ratio both in minimum bias collisions and as a function of
centrality. The minimum bias results agree reasonably well with the BRAHMS data
while the calculated centrality dependence underestimates the data and is a
stronger function of p_T than the data indicate.Comment: 18 pages, 3 figures, final version, Phys. Rev. C in pres
Condensation and Evaporation of Mutually Repelling Particles :Steady states and limit cycles
We study condensation and evaporation of particles which repel each other,
using a simple set of rules on a square lattice. Different results are obtained
for a mobile and an immobile surface layer.A two point limit cycle is observed
for high temperature and low pressure in both cases. Here the coverage
oscillates between a high and a low value without ever reaching a steady state.
The results for the immobile case depend in addition on the initial coverage.Comment: 8 pages, 3 figure
Spectral Energy Distributions of T Tauri and Herbig Ae Disks: Grain Mineralogy, Parameter Dependences, and Comparison with ISO LWS Observations
We improve upon the radiative, hydrostatic equilibrium models of passive
circumstellar disks constructed by Chiang & Goldreich (1997). New features
include (1) account for a range of particle sizes, (2) employment of
laboratory-based optical constants of representative grain materials, and (3)
numerical solution of the equations of radiative and hydrostatic equilibrium
within the original 2-layer (disk surface + disk interior) approximation. We
explore how the spectral energy distribution (SED) of a face-on disk depends on
grain size distributions, disk geometries and surface densities, and stellar
photospheric temperatures. Observed SEDs of 3 Herbig Ae and 2 T Tauri stars,
including spectra from the Long Wavelength Spectrometer (LWS) aboard the
Infrared Space Observatory (ISO), are fitted with our models. Silicate emission
bands from optically thin, superheated disk surface layers appear in nearly all
systems. Water ice emission bands appear in LWS spectra of 2 of the coolest
stars. Infrared excesses in several sources are consistent with vertical
settling of photospheric grains. While this work furnishes further evidence
that passive reprocessing of starlight by flared disks adequately explains the
origin of infrared-to-millimeter wavelength excesses of young stars, we
emphasize how the SED alone does not provide sufficient information to
constrain particle sizes and disk masses uniquely.Comment: Accepted to ApJ, 35 pages inc. 14 figures, AAS preprin
Reflexology versus reflexology and colour therapy combined for treating chronic sinusitis
Published ArticleAccording to Wills reflexology entails the division of the body into ten zones, concentrating on the pressure points on the feet, with each foot representing five zones (2006: Online). Colour therapy uses the vibrational frequencies of colour to restore the client's health. Colour zone therapy on the other hand, is a combination of reflexology and colour therapy. Zone refers to the working of key points on the feet, where colour refers to the assessment of the condition and treating it with the correct colour's frequency (Gimbel, 1993: 2-3). Reflexology uses a physical stimulus and colour therapy, emotional stimulus, thus colour zone therapy addresses both.
In this study the researcher's objective was to investigate treatments with reflexology compared with treatments of colour zone therapy, by treating chronic sinusitis to explore the influence of colour on the outcome of reflexology treatments. Ten chronic sinusitis-suffering clients were treated with reflexology in a white cubicle. Another ten clients were treated with colour zone therapy (thus colour therapy as well as reflexology). They had indigo coloured paper in their hands. Results were obtained as case studies, with clients reporting how they felt before and after each treatment. Each client received five treatments.
The results indicated that the reflexology clients did experience an improvement after the fourth treatment. By the fifth treatment three of the ten clients had discharged some mucus. However, clients treated with colour zone therapy showed results after the second treatment. At the fourth treatment, already seven of the ten colour zone therapy clients had discharged a large amount of mucus.
This concludes that treating a condition with colour, in combination with another treatment provides better and quicker results
s-Process Nucleosynthesis in Advanced Burning Phases of Massive Stars
We present a detailed study of s-process nucleosynthesis in massive stars of
solar-like initial composition and masses 15, 20,25, and 30 Msun. We update our
previous results of s-process nucleosynthesis during the core He-burning of
these stars and then focus on an analysis of the s-process under the physical
conditions encountered during the shell-carbon burning. We show that the recent
compilation of the Ne22(alpha,n)Mg25 rate leads to a remarkable reduction of
the efficiency of the s-process during core He-burning. In particular, this
rate leads to the lowest overproduction factor of Kr80 found to date during
core He-burning in massive stars. The s-process yields resulting from shell
carbon burning turn out to be very sensitive to the structural evolution of the
carbon shell. This structure is influenced by the mass fraction of C12 attained
at the end of core helium burning, which in turn is mainly determined by the
C12(alpha,gamma)O16 reaction. The still present uncertainty in the rate for
this reaction implies that the s-process in massive stars is also subject to
this uncertainty. We identify some isotopes like Zn70 and Rb87 as the
signatures of the s-process during shell carbon burning in massive stars. In
determining the relative contribution of our s-only stellar yields to the solar
abundances, we find it is important to take into account the neutron exposure
of shell carbon burning. When we analyze our yields with a Salpeter Initial
Mass Function, we find that massive stars contribute at least 40% to s-only
nuclei with mass A 90, massive stars
contribute on average ~7%, except for Gd152, Os187, and Hg198 which are ~14%,
\~13%, and ~11%, respectively.Comment: 52 pages, 16 figures, accepted for publication in Ap
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