2,262 research outputs found
AN ECONOMIC MODEL FOR IRRIGATION WELL MANAGEMENT IN A DECLINING AQUIFER
A computerized model is developed that uses the aquifer characteristics and irrigation requirements to estimate hydraulic properties of a well pumping from an unconfined aquifer with a steadily declining water table. The model simulates electrically-powered well operation under the most economical conditions. High versatility of model inputs allows examination of many facets of well management. One example is the effect of electricity price on the ratio of energy cost of total cost of supplying water at different average total heads. At current electricity prices, energy accounted for 65 to 70 percent of total water costs. In a second example, as average annual efficiency decreased, average annual cost per acre-foot increased and average annual pumping volume decreased, leaving average annual total energy costs almost constant.Resource /Energy Economics and Policy,
VLA Observations of the "Eye of the Tornado"- the High Velocity \HII Region G357.63-0.06
The unusual supernova remnant candidate G357.7-0.1 and the compact source
G357.63-0.06 have been observed with the Very Large Array at 1.4 and 8.3 GHz.
The H92 line (8.3 GHz) was detected from the compact source with a
surprising velocity of about -210 km/s indicating that this source is an \HII
region, is most likely located at the Galactic center, and is unrelated to the
SNR. The \HI absorption line (1.4 GHz) data toward these sources supports this
picture and suggests that G357.7-0.1 lies farther away than the Galactic
center.Comment: Latex, 14 pages including 4 figures. Accepted to A
Radio Recombination Lines from Starbursts: NGC 3256, NGC 4945 and the Circinus Galaxy
A renewed attempt to detect radio recombination lines from external galaxies
has resulted in the measurement of lines from several bright starburst
galaxies. The lines are produced by hydrogen ionized by young, high-mass stars
and are diagnostic of the conditions and gas dynamics in the starburst regions
without problems of dust obscuration. We present here detections of the lines
H91alpha and H92alpha near 8.6 GHz from the starburst nuclei in NGC 3256, NGC
4945, and the Circinus galaxy using the ATCA and VLA. Modelling the line
emitting region as a collection of H II regions, we derive the required number
of H II regions, their temperature, density, and distribution.Comment: 6 pages, to appear in "Proc 331. Heraeus Seminar: The Evolution of
Starbursts", Bad Honnef, Germany, Aug 16 - 20, 2004, Eds: S. Huettemeister,
S. Aalto, D.J. Bomans, and E. Manthe
Spatial Variations in Galactic H I Structure on AU-Scales Toward 3C 147 Observed with the Very Long Baseline Array
This paper reports dual-epoch, Very Long Baseline Array observations of H I
absorption toward 3C 147. One of these epochs (2005) represents new
observations while one (1998) represents the reprocessing of previous
observations to obtain higher signal-to-noise results. Significant H I opacity
and column density variations, both spatially and temporally, are observed with
typical variations at the level of \Delta\tau ~ 0.20 and in some cases as large
as \Delta\tau ~ 0.70, corresponding to column density fluctuations of order 5 x
10^{19} cm^{-2} for an assumed 50 K spin temperature. The typical angular scale
is 15 mas; while the distance to the absorbing gas is highly uncertain, the
equivalent linear scale is likely to be about 10 AU. Approximately 10% of the
face of the source is covered by these opacity variations, probably implying a
volume filling factor for the small-scale absorbing gas of no more than about
1%. Comparing our results with earlier results toward 3C 138 (Brogan et al.),
we find numerous similarities, and we conclude that small-scale absorbing gas
is a ubiquitous phenomenon, albeit with a low probability of intercept on any
given line of sight. Further, we compare the volumes sampled by the line of
sight through the Galaxy between our two epochs and conclude that, on the basis
of the motion of the Sun alone, these two volumes are likely to be
substantially different. In order to place more significant constraints on the
various models for the origin of these small-scale structures, more frequent
sampling is required in any future observations.Comment: 16 pages with 10 figures in 24 files; AASTeX format; accepted by A
PKS B1400-33: an unusual radio relic in a poor cluster
We present new arcminute resolution radio images of the low surface
brightness radio source PKS B1400-33 that is located in the poor cluster Abell
S753. The observations consist of 330 MHz VLA, 843 MHz MOST and 1398 and 2378
MHz ATCA data. These new images, with higher surface brightness sensitivity
than previous observations, reveal that the large scale structure consists of
extended filamentary emission bounded by edge-brightened rims. The source is
offset on one side of symmetrically distributed X-ray emission that is centered
on the dominant cluster galaxy NGC 5419. PKS B1400-33 is a rare example of a
relic in a poor cluster with radio properties unlike those of most relics and
halos observed in cluster environments.
The diffuse source appears to have had an unusual origin and we discuss
possible mechanisms. We examine whether the source could be re-energized relic
radio plasma or a buoyant synchrotron bubble that is a relic of activity in NGC
5419. The more exciting prospect is that the source is relic plasma preserved
in the cluster gaseous environment following the chance injection of a radio
lobe into the ICM as a result of activity in a galaxy at the periphery of the
cluster.Comment: 26 pages, 8 figures, accepted for publication in the Astronomical
Journa
Detection of HI 21 cm-line absorption in the Warm Neutral Medium and in the Outer Arm of the Galaxy
Using the Westerbork Synthesis Radio Telescope, we have detected HI 21
cm-line absorption in the Warm Neutral Medium of the Galaxy toward the
extragalactic source 3C147. This absorption, at an LSR velocity of -29+/-4 km/s
with a full width at half maximum of 53+/-6 km/s, is associated with the
Perseus Arm of the Galaxy. The observed optical depth is (1.9+/-0.2)*10**(-3).
The estimated spin temperature of the gas is 3600+/-360 K. The volume density
is 0.4 per cc assuming pressure equilibrium. Toward two other sources, 3C273
and 3C295, no wide HI 21 cm-line absorption was detected. The highest of the
3sigma lower limits on the spin temperature of the Warm Neutral Medium is 2600
K. In addition, we have also detected HI 21 cm-line absorption from high
velocity clouds in the Outer Arm toward 3C147 and 3C380 at LSR velocities of
-117.3, -124.5 and -113.7 km/s respectively. We find two distinct temperature
components in the high velocity clouds with spin temperatures of greater than
1000 K and less than 200 K, respectively.Comment: 21 pages inclusive of 7 figures and 2 table
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