1,407 research outputs found
Implications of climate variability for the detection of multiple equilibria and for rapid transitions in the atmosphere-vegetation system
Paleoclimatic records indicate a decline of vegetation cover in the Western Sahara at the end of the African Humid Period (about 5,500 years before present). Modelling studies have shown that this phenomenon may be interpreted as a critical transition that results from a bifurcation in the atmosphere-vegetation system. However, the stability properties of this system are closely linked to climate variability and depend on the climate model and the methods of analysis. By coupling the Planet Simulator (PlaSim), an atmosphere model of intermediate complexity, with the simple dynamic vegetation model VECODE, we assess previous methods for the detection of multiple equilibria, and demonstrate their limitations. In particular, a stability diagram can yield misleading results because of spatial interactions, and the system's steady state and its dependency on initial conditions are affected by atmospheric variability and nonlinearities. In addition, we analyse the implications of climate variability for the abruptness of a vegetation decline. We find that a vegetation collapse can happen at different locations at different times. These collapses are possible despite large and uncorrelated climate variability. Because of the nonlinear relation between vegetation dynamics and precipitation the green state is initially stabilised by the high variability. When precipitation falls below a critical threshold, the desert state is stabilised as variability is then also decreased. © 2011 The Author(s)
Proof firm downsizing and diagnosis-specific disability pensioning in Norway
<br>Background: We wanted to investigate if firm downsizing is related to an increased rate of disability pensions among the former employed, especially for those with musculoskeletal and psychiatric diagnoses, and for those having to leave the firm.</br>
<br>Methods: Statistics Norway provided a linked file with demographic information and all social security grants from the National Insurance Administration for 1992–2004 for all inhabitants in Norway. Our sample was aged 30–55 years in 1995, being alive, employed and not having a disability pension at the end of 2000. Downsizing was defined as percent change in number of employed per firm from 1995 to end 2000. Employment data were missing for 25.6% of the sample.</br>
<br>Results: Disability pension rates in the next four years were 25% higher for those experiencing a 30-59% downsizing than for those not experiencing a reduction of the workforce. 1-29% and 60-100% downsizing did not have this effect. Stayers following down-sizing had higher disability pension rates than leavers. What we have called complex musculoskeletal and psychiatric diagnoses were relatively most common.</br>
<br>Conclusion: Moderate downsizing is followed by a significant increase in disability pension rates in the following four years, often with complex musculoskeletal and psychiatric diagnoses.</br>
Analysis of the loop length distribution for the negative weight percolation problem in dimensions d=2 through 6
We consider the negative weight percolation (NWP) problem on hypercubic
lattice graphs with fully periodic boundary conditions in all relevant
dimensions from d=2 to the upper critical dimension d=6. The problem exhibits
edge weights drawn from disorder distributions that allow for weights of either
sign. We are interested in in the full ensemble of loops with negative weight,
i.e. non-trivial (system spanning) loops as well as topologically trivial
("small") loops. The NWP phenomenon refers to the disorder driven proliferation
of system spanning loops of total negative weight. While previous studies where
focused on the latter loops, we here put under scrutiny the ensemble of small
loops. Our aim is to characterize -using this extensive and exhaustive
numerical study- the loop length distribution of the small loops right at and
below the critical point of the hypercubic setups by means of two independent
critical exponents. These can further be related to the results of previous
finite-size scaling analyses carried out for the system spanning loops. For the
numerical simulations we employed a mapping of the NWP model to a combinatorial
optimization problem that can be solved exactly by using sophisticated matching
algorithms. This allowed us to study here numerically exact very large systems
with high statistics.Comment: 7 pages, 4 figures, 2 tables, paper summary available at
http://www.papercore.org/Kajantie2000. arXiv admin note: substantial text
overlap with arXiv:1003.1591, arXiv:1005.5637, arXiv:1107.174
Astrometric Positions and Proper Motions of 19 Radio Stars
We have used the Very Large Array, linked with the Pie Town Very Long
Baseline Array antenna, to determine astrometric positions of 19 radio stars in
the International Celestial Reference Frame (ICRF). The positions of these
stars were directly linked to the positions of distant quasars through phase
referencing observations. The positions of the ICRF quasars are known to 0.25
mas, thus providing an absolute reference at the angular resolution of our
radio observations. Average values for the errors in our derived positions for
all sources were 13 mas and 16 mas in R.A. and declination respectively, with
accuracies approaching 1-2 mas for some of the stars observed. Differences
between the ICRF positions of the 38 quasars, and those measured from our
observations showed no systematic offsets, with mean values of -0.3 mas in R.A.
and -1.0 mas in declination. Standard deviations of the quasar position
differences of 17 mas and 11 mas in R.A. and declination respectively, are
consistent with the mean position errors determined for the stars. Our measured
positions were combined with previous Very Large Array measurements taken from
1978-1995 to determine the proper motions of 15 of the stars in our list. With
mean errors of approximately 1.6 mas/yr, the accuracies of our proper motions
approach those derived from Hipparcos, and for a few of the stars in our
program, are better than the Hipparcos values. Comparing the positions of our
radio stars with the Hipparcos catalog, we find that at the epoch of our
observations, the two frames are aligned to within formal errors of
approximately 3 mas. This result confirms that the Hipparcos frame is inertial
at the expected level.Comment: 20 pages, 9 figures Accepted by the Astronomical Journal, 2003 March
1
The association between water kilomasers and compact radio sources in the starburst galaxy NGC2146
We report the detection of 22 GHz water vapor emission toward the starburst
galaxy NGC2146, made using the Effelsberg 100-m telescope. Interferometric
observations with the Very Large Array (VLA) show that a part of the emission
originates from two prominent sites of star formation that are associated with
compact radio continuum sources, likely ultra-compact HII regions. It is
concluded that the emission arises from the most luminous and distant water
`kilomasers' detected so far. Our data increase the number of water maser
detections in northern galaxies (Dec > -30 deg) with 100 micron IRAS point
source fluxes > 50 Jy to 18%.Comment: 4 pages including 3 figures, accepted for publication in A&A Lette
The Sizes of 1720 MHz OH Masers: VLBA and MERLIN Observations of the Supernova Remnants W44 and W28
We have used the NRAO Very Long Baseline Array (VLBA) to image OH(1720 MHz)
masers in the supernova remnants W28 and W44 at a resolution of 40 mas. We also
used MERLIN to observe the same OH(1720 MHz) masers in W44 at a resolution of
290 x 165 mas. All the masers are resolved by these VLBA and MERLIN
observations. The measured sizes range from 50 to 180 mas and yield brightness
temperature estimates from 0.3--20 x 10**8 K. We investigate whether these
measured angular sizes are intrinsic and hence originate as a result of the
physical conditions in the supernova remnant shock, or whether they are scatter
broadened sizes produced by the turbulent ionized gas along the line of sight.
While the current data on the temporal and angular broadening of pulsars,
masers and extragalactic soures toward W44 and W28 can be understood in terms
of scattering, we cannot rule out that these large sizes are intrinsic. Recent
theoretical modeling by Lockett et al. suggests that the physical parameters in
the shocked region are indicative of densities and OH abundances which lead to
estimates of sizes as large as what we measure. If the sizes and structure are
intrinsic, then the OH(1720 MHz) masrs may be more like the OH(1612 MHz) masers
in circumstellar shells than OH masers associated with HII regions. At two
locations in W28 we observe the classical S-shapes in the Stokes V profiles
caused by Zeeman splitting and use it to infer magnetic fields of order 2
milliGauss.Comment: 24 pages, 6 figures, accepted by Ap
SiS in the circumstellar envelope of IRC +10126: maser and quasi-thermal emission
We present new Effelsberg-100 m, ATCA, and VLA observations of rotational SiS
transitions in the circumstellar envelope (CSE) of IRC +10216. Thanks to the
high angular resolution achieved by the ATCA observations, we unambiguously
confirm that the molecule's J=1-0 transition exhibits maser action in this CSE,
as first suggested more than thirty years ago. The maser emission's radial
velocity peaking at a local standard of rest velocity of -39.8620.065 km/s
indicates that it arises from an almost fully accelerated shell. Monitoring
observations show time variability of the SiS (1-0) maser. The two lowest-
SiS quasi-thermal emission lines trace a much more extended emitting region
than previous high-J SiS observations. Their distributions show that the SiS
quasi-thermal emission consists of two components: one is very compact
(radius<1.5", corresponding to <3 cm), and the other extends
out to a radius >11". An incomplete shell-like structure is found in the
north-east, which is indicative of existing SiS shells. Clumpy structures are
also revealed in this CSE. The gain of the SiS (1-0) maser (optical depths of
about -5 at the blue-shifted side and, assuming inversion throughout the entire
line's velocity range, about -2 at the red-shifted side) suggests that it is
unsaturated. The SiS (1-0) maser can be explained in terms of ro-vibrational
excitation caused by infrared pumping, and we propose that infrared continuum
emission is the main pumping source.Comment: Accepted for publication in ApJ. A high-resolution version can be
found at https://gongyan2444.github.io/pdf/cw-leo-sis.pdf 3D movies of SiS
cubes can be found at https://gongyan2444.github.io/movie/sis10-3d.avi and
https://gongyan2444.github.io/movie/sis21-3d.av
Using geographic information systems for radon exposure assessment in dwellings in the Oslo region, Norway
Radon exposures were assigned to each residential address in the Oslo region
using a geographic information system (GIS) that included indoor radon
measurements. The results will be used in an epidemiologic study regarding
leukemia and brain cancer. The model is based on 6% of measured
residential buildings. High density of indoor radon measurements allowed us
to develop a buffer model where indoor radon measurements found around each
dwelling were used to assign a radon value for homes lacking radon
measurement.
<br><br>
Intraclass correlation coefficients (ICCs) were used to study the agreement
between radon values from the buffer method, from indoor
radon values of measured houses, and from a regression model constructed
with radiometric data (<i>e</i><sub>Th</sub>, <i>e</i><sub>U</sub>) and bedrock geology. We obtained good
agreement for both comparisons with ICC values between 0.54 and 0.68.
<br><br>
GIS offers a useful variety of tools to study the indoor-radon exposure
assessment. By using the buffer method it is more likely that geological
conditions are similar within the buffer and this may take more into account
the variation of radon over short distances. It is also probable that short-distance-scale correlation patterns express similarities in building styles
and living habits. Although the method has certain limitations, we regard it
as acceptable for use in epidemiological studies
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