1,433 research outputs found
Effect of type of otolith and preparation technique on age estimation of larval and juvenile spot (Leiostomus xanthurus)
Otoliths of larval and juvenile fish provide a record of age, size, growth, and development (Campana and Neilson,
1985; Thorrold and Hare, 2002). However, determining the time of first increment formation in otoliths (Campana, 2001) and assessing the accuracy (deviation from real age)
and precision (repeatability of increment counts from the same otolith) of increment counts are prerequisites for using otoliths to study the life history of fish (Campana and Moksness, 1991). For most fish species, first increment deposition occurs either at hatching, a day after hatching, or after first feeding and yolksac absorption (Jones, 1986; Thorrold and Hare, 2002). Increment deposition before
hatching also occurs (Barkmann and Beck, 1976; Radtke and Dean, 1982). If first increment deposition does not occur at hatching, the standard procedure is to add a predetermined number to increment counts to estimate fish age (Campana and Neilson, 1985)
The Celestial Reference Frame at 24 and 43 GHz. II. Imaging
We have measured the sub-milli-arcsecond structure of 274 extragalactic
sources at 24 and 43 GHz in order to assess their astrometric suitability for
use in a high frequency celestial reference frame (CRF). Ten sessions of
observations with the Very Long Baseline Array have been conducted over the
course of 5 years, with a total of 1339 images produced for the 274
sources. There are several quantities that can be used to characterize the
impact of intrinsic source structure on astrometric observations including the
source flux density, the flux density variability, the source structure index,
the source compactness, and the compactness variability. A detailed analysis of
these imaging quantities shows that (1) our selection of compact sources from
8.4 GHz catalogs yielded sources with flux densities, averaged over the
sessions in which each source was observed, of about 1 Jy at both 24 and 43
GHz, (2) on average the source flux densities at 24 GHz varied by 20%-25%
relative to their mean values, with variations in the session-to-session flux
density scale being less than 10%, (3) sources were found to be more compact
with less intrinsic structure at higher frequencies, and (4) variations of the
core radio emission relative to the total flux density of the source are less
than 8% on average at 24 GHz. We conclude that the reduction in the effects due
to source structure gained by observing at higher frequencies will result in an
improved CRF and a pool of high-quality fiducial reference points for use in
spacecraft navigation over the next decade.Comment: 63 pages, 18 figures, 6 tables, accepted by the Astronomical Journa
The approach to criticality in sandpiles
A popular theory of self-organized criticality relates the critical behavior
of driven dissipative systems to that of systems with conservation. In
particular, this theory predicts that the stationary density of the abelian
sandpile model should be equal to the threshold density of the corresponding
fixed-energy sandpile. This "density conjecture" has been proved for the
underlying graph Z. We show (by simulation or by proof) that the density
conjecture is false when the underlying graph is any of Z^2, the complete graph
K_n, the Cayley tree, the ladder graph, the bracelet graph, or the flower
graph. Driven dissipative sandpiles continue to evolve even after a constant
fraction of the sand has been lost at the sink. These results cast doubt on the
validity of using fixed-energy sandpiles to explore the critical behavior of
the abelian sandpile model at stationarity.Comment: 30 pages, 8 figures, long version of arXiv:0912.320
The Gaussian Plasma Lens in Astrophysics. Refraction
We consider the geometrical optics for refraction of a distant radio source
by an interstellar plasma lens, with application to a lens with a Gaussian
electron column density profile. The refractive properties of the lens are
specified completely by a dimensionless parameter, alpha, which is a function
of the wavelength of observation, the lens' electron column density, the
lens-observer distance, and the transverse diameter of the lens. Relative
motion of the observer and lens produces modulations in the source's light
curve. Plasma lenses are diverging so the light curve displays a minimum, when
the lens is on-axis, surrounded by enhancements above the unlensed flux
density. Lensing can also produce caustics, multiple imaging, and angular
position wander of the background source. If caustics are formed, the
separation of the outer caustics can constrain alpha, while the separation of
the inner caustics can constrain the size of the lens. We apply our analysis to
0954+654, a source for which we can identify caustics in its light curve, and
1741-038, for which polarization observations were obtained during and after
the scattering event. We find general agreement between modelled and observed
light curves at 2.25 GHz, but poor agreement at 8.1 GHz. The discrepancies may
result from a combination of lens substructure or anisotropic shape, a lens
that only grazes the source, or unresolved source substructure. Our analysis
places the following constraints on the lenses: Toward 0954+654 (1741-038) the
lens was 0.38 AU (0.065 AU) in diameter, with a peak column density of 0.24 pc
cm^{-3} (1E-4 pc cm^{-3}) and an electron density of 1E5 cm^{-3} (300 cm^{-3}).
The angular wander caused by the lens was 250 mas (0.4 mas) at 2.25 GHz. For
1741-038, we place an upper limit of 100 mG on the lens' magnetic field.Comment: 26 pages, LaTeX2e using AASTeX macro aaspp4, 11 PostScript figures;
to be published in Ap
Efeito do sistema plantio direto sobre a produtividade da mandioca (Manihot esculenta Crantz) em Sangão - SC.
A mandioca constitui parte fundamental da alimentação de mais de 500 milhões de pessoas no mundo, sendo também matéria-prima para uma série de produtos, sejam eles minimamente processados até quimicamente modificados (TAKAHASHI; CONÇALO, 2001). Para o cultivo da mandioca é recomendável que o solo tenha profundidade de 30 a 40 cm; ausência de camadas impermeáveis e material rochoso e; seja solto, poroso e friável para possibilitar o fácil crescimento das raízes (MONTALDO, 1979; SOUZA & SOUZA, 2000). Para propiciar estas condições, realiza-se o preparo do solo que, de acordo com HOWELER et al (1993), deve proporcionar um bom rompimento do solo para possibilitar melhor drenagem e aeração, reduzir a podridão de raízes e aumentar a produtividade, facilitar a colheita, reduzir os danos as raízes durante a colheita, proporcionando redução da deterioração durante o armazenamento. Entretanto, a associação de sistemas de preparo que resultam em pequena quantidade de palha remanescente sobre o solo com as características fitotécnicas da mandioca, a qual oferece reduzida proteção à superfície do solo no período inicial de desenvolvimento (HOWELER et al, 1993), agrava os problemas com erosão, compactação, oxidação da matéria orgânica, entre outros, resultando no empobrecimento crescente dos solos (SOUZA, SOUZA & GOMES, 2006). Nas regiões em que as áreas apresentam declividades mais acentuadas como é comum se observar no município de Sangão e regiões próximas, as perdas de solo por erosão são preocupantes. Em outras regiões, preocupados com estes aspectos, vários estudos foram realizados avaliando-se a produtividade de mandioca em diferentes sistemas de preparo, onde foi possível constatar que na maioria (OHIRI e EZUMAH, 1990; CADAVID et al, 1998; MATE, 2002; JONGRUAYSUP et al., 2003; FEY et al, 2007 e; OUSUBO et al, 2008) se obtiveram resultados de produtividade da mandioca equivalentes e até superiores em sistema plantio direto, enquanto OLIVEIRA et al (2001) e PEQUENO et al (2007), encontraram resultados equivalentes de produtividade em apenas um ano e, na média de todos os anos, o preparo mínimo e sistema plantio direto foram estatisticamente inferiores, apontado-se como possível causa destes resultados a maior densidade e menor macroporosidade encontrada nesses sistemas. Visando amenizar este problema que pode afetar o desenvolvimento, a produtividade e a colheita da mandioca, FEY (2009) desenvolveu um sulcador alado para a implantação da cultura da mandioca em sistema plantio direto, cujo objetivo é romper uma maior área de solo no sulco de plantio deixando-o solto para facilitar o desenvolvimento das raízes e melhorar a infiltração de água. Nos estudos realizados pelo autor não foram encontradas diferenças de produtividade na cultura da mandioca implantada em sistema de preparo convencional do solo e sistema plantio direto na região oeste do Paraná. Nesse contexto, o trabalho objetivou avaliar a produtividade da mandioca em sistema de preparo convencional e sistema plantio direto
Angular Broadening of Intraday Variable AGN. II. Interstellar and Intergalactic Scattering
We analyze a sample of 58 multi-wavelength, Very Long Baseline Array
observations of active galactic nuclei (AGN) to determine their scattering
properties. Approximately 75% of the sample consists of AGN that exhibit
centimeter-wavelength intraday variability (interstellar scintillation) while
the other 25% do not show intraday variability. We find that interstellar
scattering is measurable for most of these AGN, and the typical broadening
diameter is 2 mas at 1 GHz. We find that the scintillating AGN are typically at
lower Galactic latitudes than the non-scintillating AGN, consistent with the
scenario that intraday variability is a propagation effect from the Galactic
interstellar medium. The magnitude of the inferred interstellar broadening
measured toward the scintillating AGN, when scaled to higher frequencies, is
comparable to the diameters inferred from analyses of the light curves for the
more well-known intraday variable sources. However, we find no difference in
the amount of scattering measured toward the scintillating versus
non-scintillating AGN. A consistent picture is one in which the scintillation
results from localized regions ("clumps") distributed throughout the Galactic
disk, but which individually make little contribution to the angular
broadening. Of the 58 AGN observed, 37 (64%) have measured redshifts. At best,
a marginal trend is found for scintillating (non-scintillating) AGN to have
smaller (larger) angular diameters at higher redshifts. We also use our
observations to try to constrain the possibility of intergalactic scattering.
While broadly consistent with the scenario of a highly turbulent intergalactic
medium, our observations do not place significant constraints on its
properties.Comment: 13 pages, 4 figures; AASTeX format; ApJ in pres
Symbolic Manipulators Affect Mathematical Mindsets
Symbolic calculators like Mathematica are becoming more commonplace among
upper level physics students. The presence of such a powerful calculator can
couple strongly to the type of mathematical reasoning students employ. It does
not merely offer a convenient way to perform the computations students would
have otherwise wanted to do by hand. This paper presents examples from the work
of upper level physics majors where Mathematica plays an active role in
focusing and sustaining their thought around calculation. These students still
engage in powerful mathematical reasoning while they calculate but struggle
because of the narrowed breadth of their thinking. Their reasoning is drawn
into local attractors where they look to calculation schemes to resolve
questions instead of, for example, mapping the mathematics to the physical
system at hand. We model the influence of Mathematica as an integral part of
the constant feedback that occurs in how students frame, and hence focus, their
work
VSOP and Ground-based VLBI Imaging of the TeV Blazar Markarian 421 at Multiple Epochs
We present thirty VLBI images of the TeV blazar Markarian 421 (1101+384) at
fifteen epochs spanning the time range from 1994 to 1997, and at six different
frequencies from 2.3 to 43 GHz. The imaged observations include a
high-resolution 5 GHz VLBI Space Observatory Programme (VSOP) observation with
the HALCA satellite on 1997 November 14; full-track VLBA observations from 1994
April, 1996 November, and 1997 May at frequencies between 5 and 43 GHz; six
epochs of VLBA snapshot observations at frequencies between 2 and 15 GHz from
Radio Reference Frame studies; and five geodetic VLBI observations at 2 and 8
GHz from the archive of the Washington VLBI Correlator Facility located at the
U.S. Naval Observatory. The dense time coverage of the images allows us to
unambiguously track components in the parsec-scale jet over the observed time
range. We measure the speeds of three inner jet components located between 0.5
and 5 mas from the core (0.3 to 3 pc projected linear distance) to be 0.19 +/-
0.27, 0.30 +/- 0.07, and -0.07 +/- 0.07 c (H_{0}=65 km s^{-1} Mpc^{-1}). If the
sole 43 GHz image is excluded, all measured speeds are consistent with no
motion. These speeds differ from tentative superluminal speeds measured by
Zhang & B\aa\aa th from three epochs of data from the early 1980's. Possible
interpretations of these subluminal speeds in terms of the high Doppler factor
demanded by the TeV variability of this source are discussed.Comment: 18 pages, including 7 figures, emulateapj.sty, accepted by The
Astrophysical Journal; modified text describing Radio Reference Frame
observation
Absolute kinematics of radio source components in the complete S5 polar cap sample. III. First wide-field high-precision astrometry at 15.4 GHz
We report on the first wide-field, high-precision astrometric analysis of the
13 extragalactic radio sources of the complete S5 polar cap sample at 15.4 GHz.
We describe new algorithms developed to enable the use of differenced phase
delays in wide-field astrometric observations and discuss the impact of using
differenced phase delays on the precision of the wide-field astrometric
analysis. From this global fit, we obtained estimates of the relative source
positions with precisions ranging from 14 to 200 as at 15.4 GHz, depending
on the angular separation of the sources (from 1.6 to 20.8
degrees). These precisions are 10 times higher than the achievable
precisions using the phase-reference mapping technique.Comment: 9 pages, 7 figure
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