345 research outputs found
Environmental Changes Produced by Cold-Water Outlets from Three Arkansas Reservoirs
Water qualities of two natural streams (Buffalo and Kings Rivers), one new coId-tailwater (Beaver), and two old coId-tailwaters (Norfork and Bull Shoals) in northwestern Arkansas were studied from July 1965 through October 1968. The essential difference between the old cold-tailwaters and natural streams is a change in water quality which allows the development of a new productive ecological environment. Features which typify the old tailwaters are as follows: (1) relatively homioithermal temperatures; (2) stream beds scoured by strong hydoelectric power generation currents; (3) abundant phytoplankton and benthic macroinvertebrates; and (4) absence of warm water game fishes. Environmental factors characterizing natural streams are as follows: (1) high summer temperatures; (2) seasonal and individual current fluctuations at the various stations; (3) a greater variety of benthic macroinvertebrates and ichthyofauna; (4) abundant zooplankters; and (5) a tendency toward an equal distribution of the phyla Chrysophyta, Cyanophyta, and Chlorophyta. By October 1968, the new Beaver coId-tailwater had lost all of its warm-water characteristics but had not developed the biotic features of the old tailwaters
Limnological, Ichthyological, and Parasitological Investigations on Arkansas Reservoris in Relation to Water Quality
Lake Fort Smith, a 525 acre (212 ha) reservoir, was impounded in 1936 as a water supply for the city of Fort Smith. The reservoir is located on Clear Creek (Frog Bayou), a tributary of the Arkansas River, in the Boston Mountains 28 miles (45 km) northeast of the city of Fort Smith in Crawford County, Arkansas. A map and morphometric characteristics of Lake Fort Smith are given in Fig. 1 and Table I (Hoffman, 1951; Nelson, 1952). In 1956 Lake Shepherd Springs, a 750 acre (304 ha) impoundment, was created one mile upstream of Lake Fort Smith (Rorie, 1961). Both lakes have a shale substrate and are subject to periods of high turbidity. The 2 two lakes have a water shed of 65 square mile area (168 km ) of mountainous oak-hickory forest. Lake Shepherd Springs has not acted as a settling basin for sediments; thus, the upper portion of Lake Fort Smith has numerous shallow areas with a mud bottom supporting various submergent and emergent aquatic plants. The lower portion of the lake has a rocky, shale substrate with only limited emergent vegetation
The many streams of the Magellanic Stream
We present results from neutral hydrogen (HI) observations of the tip of the
Magellanic Stream (MS), obtained with the Arecibo telescope as a part of the
on-going survey by the Consortium for Galactic studies with the Arecibo L-band
Feed Array. We find four large-scale, coherent HI streams, extending
continously over a length of 20 degrees, each stream possessing different
morphology and velocity gradients. The newly discovered streams provide strong
support for the tidal model of the MS formation by Connors et al. (2006), which
suggested a spatial and kinematic bifurcation of the MS. The observed
morphology and kinematics suggest that three of these streams could be
interpreted as a 3-way splitting of the main MS filament, while the fourth
stream appears much younger and may have originated from the Magellanic Bridge.
We find an extensive population of HI clouds at the tip of the MS. Two thirds
of clouds have an angular size in the range 3.5'--10'. We interpret this as
being due to thermal instability, which would affect a warm tail of gas
trailing through the Galactic halo over a characteristic timescale of a few
Myrs to a few hundred Myrs. We show that thermal fragments can survive in the
hot halo for a long time, especially if surrounded by a <10^6 K halo gas. If
the observed clumpy structure is mainly due to thermal instability, then the
tip of the MS is at a distance of ~70 kpc. A significant fraction of HI clouds
at the tip of the MS show multi-phase velocity profiles, indicating the
co-existence of cooler and warmer gas.Comment: Accepted by Ap
SN2012ab: A Peculiar Type IIn Supernova with Aspherical Circumstellar Material
We present photometry, spectra, and spectropolarimetry of supernova (SN)
2012ab, mostly obtained over the course of days after discovery. SN
2012ab was a Type IIn (SN IIn) event discovered near the nucleus of spiral
galaxy 2MASXJ12224762+0536247. While its light curve resembles that of SN
1998S, its spectral evolution does not. We see indications of CSM interaction
in the strong intermediate-width emission features, the high luminosity (peak
at absolute magnitude ), and the lack of broad absorption features in
the spectrum. The H emission undergoes a peculiar transition. At early
times it shows a broad blue emission wing out to km
and a truncated red wing. Then at late times (
100days) it shows a truncated blue wing and a very broad red emission wing
out to roughly km . This late-time broad red wing
probably arises in the reverse shock. Spectra also show an asymmetric
intermediate-width H component with stronger emission on the red side
at late times. The evolution of the asymmetric profiles requires a density
structure in the distant CSM that is highly aspherical. Our spectropolarimetric
data also suggest asphericity with a strong continuum polarization of % and depolarization in the H line, indicating asphericity in the
CSM at a level comparable to that in other SNe IIn. We estimate a mass-loss
rate of for km extending back at least 75yr prior to the
SN. The strong departure from axisymmetry in the CSM of SN 2012ab may suggest
that the progenitor was an eccentric binary system undergoing eruptive mass
loss.Comment: 18 pages, 12 figure
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