5,780 research outputs found
Vegetation-soil relations in a highly sodic landscape, Yelarbon, southern Queensland
Soil and vegetation data were collected from a sodic-scald near Yelarbon in southern Queensland. The surface of the landscape includes relatively light textured pedestals of the A-horizon with slightly alkaline pH and slopes leading down to scalded basement representing the surface of the strongly alkaline B-horizon. The strongest gradient within the floristic patterns was associated with wetland vegetation in drainage lines, but a secondary and orthogonal gradient was related to soil pH, which was probably a function of lower alkalinity on the more stable and weathered A-horizons. There were few significant differences between soil or vegetation characteristics from plot data comparing parts of the landscape with differing historical grazing regimes. Sites included stock routes heavily grazed between the 1920s and 1970s, and subsequently almost ungrazed; and grazed paddocks that have had moderate use throughout this period. There is clear evidence that the area is naturally active in terms of erosion and deposition during flooding regardless of grazing
Operator pencil passing through a given operator
Let be a linear differential operator acting on the space of
densities of a given weight \lo on a manifold . One can consider a pencil
of operators \hPi(\Delta)=\{\Delta_\l\} passing through the operator
such that any \Delta_\l is a linear differential operator acting on densities
of weight \l. This pencil can be identified with a linear differential
operator \hD acting on the algebra of densities of all weights. The existence
of an invariant scalar product in the algebra of densities implies a natural
decomposition of operators, i.e. pencils of self-adjoint and anti-self-adjoint
operators. We study lifting maps that are on one hand equivariant with respect
to divergenceless vector fields, and, on the other hand, with values in
self-adjoint or anti-self-adjoint operators. In particular we analyze the
relation between these two concepts, and apply it to the study of
\diff(M)-equivariant liftings. Finally we briefly consider the case of
liftings equivariant with respect to the algebra of projective transformations
and describe all regular self-adjoint and anti-self-adjoint liftings.Comment: 32 pages, LaTeX fil
Parsec-scale HI absorption structure in a low-redshift galaxy seen against a Compact Symmetric Object
We present global VLBI observations of the 21-cm transition of atomic
hydrogen seen in absorption against the radio source J0855+5751. The foreground
absorber (SDSS~J085519.05+575140.7) is a dwarf galaxy at = 0.026. As the
background source is heavily resolved by VLBI, the data allow us to map the
properties of the foreground HI gas with a spatial resolution of 2pc. The
absorbing gas corresponds to a single coherent structure with an extent
35pc, but we also detect significant and coherent variations, including a
change in the HI optical depth by a factor of five across a distance of
6pc. The large size of the structure provides support for the Heiles &
Troland model of the ISM, as well as its applicability to external galaxies.
The large variations in HI optical depth also suggest that caution should be
applied when interpreting measurements from radio-detected DLAs. In
addition, the distorted appearance of the background radio source is indicative
of a strong jet-cloud interaction in its host galaxy. We have measured its
redshift ( = 0.54186) using optical spectroscopy on the William Herschel
Telescope and this confirms that J0855+5751 is a FRII radio source with a
physical extent of 1kpc and supports the previous identification of this
source as a Compact Symmetric Object. These sources often show absorption
associated with the host galaxy and we suggest that both HI and OH should be
searched for in J0855+5751.Comment: 14 pages and 10 figures. Accepted for publication in MNRA
Functional centrality in graphs
In this paper we introduce the functional centrality as a generalization of
the subgraph centrality. We propose a general method for characterizing nodes
in the graph according to the number of closed walks starting and ending at the
node. Closed walks are appropriately weighted according to the topological
features that we need to measure
Deep Radio Imaging of Globular Clusters and the Cluster Pulsar Population
We have obtained deep multifrequency radio observations of seven globular
clusters using the Very Large Array and the Australia Telescope Compact Array.
Five of these, NGC 6440, NGC 6539, NGC 6544, NGC 6624 and Terzan 5 had
previously been detected in a shallower survey for steep spectrum radio sources
in globular clusters (Fruchter and Goss 1990). The sixth, the rich globular
cluster, Liller 1, had heretofore been undetected in the radio, and the
seventh, 47 Tucanae, was not included in our original survey. High resolution 6
and 20 cm images of three of the clusters, NGC 6440, NGC 6539, NGC 6624 reveal
only point sources coincident with pulsars which have been discovered
subsequent to our first imaging survey. 21 and 18 cm images reveal several
point sources within a few core-radii of the center of 47 Tuc. Two of these are
identified pulsars, and a third, which is both variable and has a steep
spectrum, is also most likely a pulsar previously identified by a pulsed
survey. However, the 6, 20 and 90 cm images of NGC 6544, Liller 1 and Terzan 5
display strong steep-spectrum emission which cannot be associated with known
pulsars. The image of the rich cluster Terzan 5 displays numerous point sources
within , or 4 core radii of the cluster center. The density of these
objects rises rapidly toward the core, where an elongated region of emission is
found. The brightest individual sources, as well as the extended emission,
possess the steep spectra expected of pulsars. Furthermore, the flux
distribution of the sources agrees well with the standard pulsar luminosity
function. The total luminosity and number of objects observed suggest that
Terzan 5 contains more pulsars than any other Galactic globular cluster.Comment: 33 pages, 6 Postscript figures; Accepted for publication in the
Astrophysical Journal; abstract abridged. PDF version also available at
http://nemesis.stsci.edu/~fruchter/fg99/fg99.pd
A catalogue of micro-Jy radio sources in northern legacy fields
We present catalogues of faint 1.4-GHz radio sources from extremely-deep Very
Large Array pointings in the Lockman Hole, the Hubble Deep Field North (HDF-N)
and ELAIS N2. Our analysis of the HDF-N data has produced maps that are
significantly deeper than those previously published and we have used these to
search for counterparts to submm sources. For each of the fields we have
derived normalised differential source counts and in the case of the HDF-N find
no evidence for the previously-reported under-density of sources; our counts
are entirely consistent with those found for the majority of other fields. The
catalogues are available as an online supplement to this paper and the maps are
also available for download.Comment: 10 pages, 6 figures. Accepted for publication in MNRAS. Full
resolution versions of Fig. 3 are available from
ftp://ftp.roe.ac.uk/pub/adb/vla_deep_fields/ . Catalogues and maps can be
obtained from the same ur
Pulsar Radio Emission Altitude from Curvature Radiation
We assume that the relativistic sources moving along the dipolar magnetic
field lines emit curvature radiation. The beamed emission occurs in the
direction of tangents to the field lines, and to receive it, the sight line
must align with the tangent within the beaming angle 1/gamma, where gamma is
the particle Lorentz factor. By solving the viewing geometry in an inclined and
rotating dipole magnetic field, we show that, at any given pulse phase,
observer tends to receive radiation only from the specific heights allowed by
the geometry. We find outer conal components are emitted at higher altitudes
compared to inner components including the core. At any pulse phase, low
frequency emission comes from higher altitudes than high frequency emission. We
have modeled the emission heights of pulse components of PSR B0329+54, and
estimated field line curvature radii and particle Lorentz factors in the
emission regions.Comment: 14 pages, 3 figures. Accepted for Astrophysical Journal, 200
Some Exact Results on the Potts Model Partition Function in a Magnetic Field
We consider the Potts model in a magnetic field on an arbitrary graph .
Using a formula of F. Y. Wu for the partition function of this model as a
sum over spanning subgraphs of , we prove some properties of concerning
factorization, monotonicity, and zeros. A generalization of the Tutte
polynomial is presented that corresponds to this partition function. In this
context we formulate and discuss two weighted graph-coloring problems. We also
give a general structural result for for cyclic strip graphs.Comment: 5 pages, late
Modeling the Images of Relativistic Jets Lensed by Galaxies with Different Mass Surface Density Distributions
The images of relativistic jets from extragalactic sources produced by
gravitational lensing by galaxies with different mass surface density
distributions are modeled. In particular, the following models of the
gravitational lens mass distribution are considered: a singular isothermal
ellipsoid, an isothermal ellipsoid with a core, two- and three-component models
with a galactic disk, halo, and bulge. The modeled images are compared both
between themselves and with available observations. Different sets of
parameters are shown to exist for the gravitationally lensed system B0218+357
in multicomponent models. These sets allow the observed geometry of the system
and the intensity ratio of the compact core images to be obtained, but they
lead to a significant variety in the Hubble constant determined from the
modeling results.Comment: 26 pages, 9 figures, will be published in the Astronomy Letters,
2011, v.37, N4, pp. 233-24
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