43,679 research outputs found
Fluctuations and Relationships of Selected Physiochemical Parameters in Dardanelle Reservoir, Arkansas, 1975-1982
Annual and seasonal fluctuations and relationships are described for discharge, turbidity, chloride, total hardness, conductivity and suspended solids over an eight-year period in Dardanelle Reservoir. The parameters fluctuated rather widely primarily in response to seasonal patterns of rainfall. Chloride and conductivity were related and generally fluctuated together as did turbidity and suspended solids. Hardness appeared to vary independently of the others prior to 1979 then varied more closely with chloride after March 1979. Inherent differences between the Illinois Bayou arm and the main Arkansas River sections complicated the precise identification of any overall impact of power plant operation. No significant long term changes were seen, but chloride declined gradually whereas hardness and conductivity increased slightly. Suspended solids exhibited a significant rise in 1982
Phytoplankton Community Structure in Dardanelle Reservoir, Arkansas, 1975-1982
Phytoplankton data were collected with standard equipment and procedures over an eight-year period (1975-1982) in Dardanelle Reservoir, Arkansas. Community abundance and diversity at the genus level are described. Sixty-five genera representing 35 families and five divisions were identified. Total phytoplankton abundance and diversity were quite uniform among the stations but fluctuated considerably with time. These fluctuations did not correspond clearly with season. Dominant taxa were seasonal, though, with diatoms being usually dominant in January, April and October, and blue-greens dominant in July. The phytoplankton community structure has not been significantly altered by the operation of ANO Unit I
A comparison of matrix methods for calculating eigenvalues in acoustically lined ducts
Three approximate methods - finite differences, weighted residuals, and finite elements - were used to solve the eigenvalue problem which arises in finding the acoustic modes and propagation constants in an absorptively lined two-dimensional duct without airflow. The matrix equations derived for each of these methods were solved for the eigenvalues corresponding to various values of wall impedance. Two matrix orders, 20 x 20 and 40 x 40, were used. The cases considered included values of wall admittance for which exact eigenvalues were known and for which several nearly equal roots were present. Ten of the lower order eigenvalues obtained from the three approximate methods were compared with solutions calculated from the exact characteristic equation in order to make an assessment of the relative accuracy and reliability of the three methods. The best results were given by the finite element method using a cubic polynomial. Excellent accuracy was consistently obtained, even for nearly equal eigenvalues, by using a 20 x 20 order matrix
Annona muricata (graviola): toxic or therapeutic
This paper examines annona muricata (graviola): toxic or therapeutic
Temporal properties of short and long gamma-ray bursts
A temporal analysis was performed on a sample of 100 bright short GRBs with
T90 < 2s from the BATSE Current Catalog along with a similar analysis on 319
long bright GRBs with T90 > 2s from the same catalog. The short GRBs were
denoised using a median filter and the long GRBs were denoised using a wavelet
method. Both samples were subjected to an automated pulse selection algorithm
to objectively determine the effects of neighbouring pulses. The rise times,
fall times, FWHM, pulse amplitudes and areas were measured and their frequency
distributions are presented. The time intervals between pulses were also
measured. The frequency distributions of the pulse properties were found to be
similar and consistent with lognormal distributions for both the short and long
GRBs. The time intervals between the pulses and the pulse amplitudes of
neighbouring pulses were found to be correlated with each other. The same
emission mechanism can account for the two sub-classes of GRBs.Comment: 3 pages, 8 figures; Proceedings of "Gamma-Ray Burst and Afterglow
Astronomy 2001", Woods Hol
Registration of Heat Capacity Mapping Mission day and night images
Neither iterative registration, using drainage intersection maps for control, nor cross correlation techniques were satisfactory in registering day and night HCMM imagery. A procedure was developed which registers the image pairs by selecting control points and mapping the night thermal image to the daytime thermal and reflectance images using an affine transformation on a 1300 by 1100 pixel image. The resulting image registration is accurate to better than two pixels (RMS) and does not exhibit the significant misregistration that was noted in the temperature-difference and thermal-inertia products supplied by NASA. The affine transformation was determined using simple matrix arithmetic, a step that can be performed rapidly on a minicomputer
Letter from D. L. Watson to Barnum (B. R.) Colson
Letter from D. L. Watson to Barnum (B. R.) Colson. The one-page typewritten letter is dated 20 September 1912
A far-infrared study of N/O abundance ratio in galactic H 2 regions
Far-infrared lines of N++ and O++ in several galactic H II regions were measured in an effort to probe the abundance ratio N/O. New measurements are presented for W32 (630.8-0.0), Orion A, and G75.84+0.4. The combination of (N III) 57.3 millimicrons and (O III) 88.4 and 51.8 millimicrons yields measurements of N++/O++ that are largely insensitive to electron temperature, density uncertainties, and to clumping of the ionized gas, due to the similarity of the critical densities for these transitions. In the observed nebulae, N++/O++ should be indicative of N/O, a ratio that is of special importance in nucleosynthesis theory. Measurements are compared with previous measurements of M17 and W51. For nebulae in the solar circle, N++/O++ is greater than the N/O values derived from optical studies of N+/O+ in low ionization zones of the same nebulae. We find that N++/O++ in W43 is significantly higher than for the other H II regions in the sample. Since W43 is located at R = 5 kpc, which is the smallest galactocentric distance in our sample, our data appear consistent with the presence of a negative abundance gradient d(N/O)dR
High Redshift Standard Candles: Predicted Cosmological Constraints
We investigate whether future measurements of high redshift standard candles
(HzSCs) will be a powerful probe of dark energy, when compared to other types
of planned dark energy measurements. Active galactic nuclei and gamma ray
bursts have both been proposed as potential HzSC candidates. Due to their high
luminosity, they can be used to probe unexplored regions in the expansion
history of the universe. Information from these regions can help constrain the
properties of dark energy, and in particular, whether it varies over time.
We consider both linear and piecewise parameterizations of the dark energy
equation of state, , and assess the optimal redshift distribution a
high-redshift standard-candle survey could take to constrain these models.
The more general the form of the dark energy equation of state being
tested, the more useful high-redshift standard candles become. For a linear
parameterization of , HzSCs give only small improvements over planned
supernova and baryon acoustic oscillation measurements; a wide redshift range
with many low redshift points is optimal to constrain this linear model.
However to constrain a general, and thus potentially more informative, form of
, having many HzSCs can significantly improve limits on the nature of
dark energy.Comment: Accepted MNRAS, 27 Pages, 15 figures, matches published versio
Near-Infrared, Adaptive Optics Observations of the T Tauri Multiple-Star System
With high-angular-resolution, near-infrared observations of the young stellar
object T Tauri at the end of 2002, we show that, contrary to previous reports,
none of the three infrared components of T Tau coincide with the compact radio
source that has apparently been ejected recently from the system (Loinard,
Rodriguez, and Rodriguez 2003). The compact radio source and one of the three
infrared objects, T Tau Sb, have distinct paths that depart from orbital or
uniform motion between 1997 and 2000, perhaps indicating that their interaction
led to the ejection of the radio source. The path that T Tau Sb took between
1997 and 2003 may indicate that this star is still bound to the presumably more
massive southern component, T Tau Sa. The radio source is absent from our
near-infrared images and must therefore be fainter than K = 10.2 (if located
within 100 mas of T Tau Sb, as the radio data would imply), still consistent
with an identity as a low-mass star or substellar object.Comment: 11 pages, 3 figures, submitted to ApJ
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