6,190 research outputs found
Arkansas Cotton Variety Test 2003
The primary aim of the Arkansas Cotton Variety Test is to provide unbiased data regarding the agronomic performance of cotton varieties and advanced breeding lines in the major cotton-growing areas of Arkansas. This information helps seed dealers establish marketing strategies and assists producers in choosing varieties to plant. In this way, the annual test facilitates the inclusion of new, improved genetic material in Arkansas cotton production
Effects of Collisions with Rocky Planets on the Properties of Hot Jupiters
Observed Hot Jupiters exhibit a wide range of physical properties. For a
given mass, many planets have inflated radii, while others are surprisingly
compact and may harbor large central cores. Motivated by the observational
sample, this paper considers possible effects from collisions of smaller rocky
planets with gas giant planets. In this scenario, the Jovian planets migrate
first and enter into (approximately) 4 day orbits, whereas rocky planets (mass
= 0.1-20 that of Earth) migrate later and then encounter the gaseous giants.
Previous work indicates that the collision rates are high for such systems.
This paper calculates the trajectories of incoming rocky planets as they orbit
within the gaseous planets and are subjected to gravitational, frictional, and
tidal forces. These collisions always increase the metallicity of the Jovian
planets. If the incoming rocky bodies survive tidal destruction and reach the
central regions, they provide a means of producing large planetary cores. Both
the added metallicity and larger cores act to decrease the radii of the gas
giants at fixed mass. The energy released during these collisions provides the
Jovian planet with an additional heat source; here we determine the radial
layers where kinetic energy of the colliding body is dissipated, including the
energy remaining upon impact with the existing core. This process could have
long-term effects if the colliding body deposits significant energy deep in the
interior, in regions of high opacity. Both Hot Jupiters and newly formed gas
giants have inflated radii, large enough to allow incoming rocky planets to
survive tidal disruption, enhance the central core mass, and deposit
significant energy (in contrast, denser giant planets with the mass and radius
of Jupiter are expected to tidally destroy incoming rocky bodies).Comment: 35 pages, 11 figures, accepted to PAS
Arkansas Cotton Variety Test 2004
The primary aim of the Arkansas Cotton Variety Test is to provide unbiased data regarding the agronomic performance of cotton varieties and advanced breeding lines in the major cotton-growing areas of Arkansas. This information helps seed dealers establish marketing strategies and assists producers in choosing varieties to plant
A Theoretical Model for the Relation for Supermassive Black Holes in Galaxies
We construct a model for the formation of black holes within galactic bulges.
The initial state is a slowly rotating isothermal sphere, characterized by
effective transport speed \aeff and rotation rate . The black hole
mass is determined when the centrifugal radius of the collapse flow exceeds the
capture radius of the central black hole. This model reproduces the observed
correlation between black hole masses and galactic velocity dispersions, \mbh
\approx 10^8 M_\odot (\sigma/200 \kms)^4, where \sigma = \sqrt{2} \aeff.
This model also predicts the ratio \mrat of black hole mass to host mass:
\mrat 0.004 (\sigma/200 \kms).Comment: 9 pages, 2 figures, submitted to Astrophysical Journal Letter
C, N, O Abundances in the Most Metal-Poor Damped Lyman alpha Systems
This study focuses on some of the most metal-poor damped Lyman alpha
absorbers known in the spectra of high redshift QSOs, using new and archival
observations obtained with UV-sensitive echelle spectrographs on the Keck and
VLT telescopes. The weakness and simple velocity structure of the absorption
lines in these systems allows us to measure the abundances of several elements,
and in particular those of C, N, and O, a group that is difficult to study in
DLAs of more typical metallicities. We find that when the oxygen abundance is
less than about 1/100 of solar, the C/O ratio in high redshift DLAs and
sub-DLAs matches that of halo stars of similar metallicity and shows higher
values than expected from galactic chemical evolution models based on
conventional stellar yields. Furthermore, there are indications that at these
low metallicities the N/O ratio may also be above simple expectations and may
exhibit a minimum value, as proposed by Centurion and her collaborators in
2003. Both results can be interpreted as evidence for enhanced production of C
and N by massive stars in the first few episodes of star formation, in our
Galaxy and in the distant proto-galaxies seen as QSO absorbers. The higher
stellar yields implied may have an origin in stellar rotation which promotes
mixing in the stars' interiors, as considered in some recent model
calculations. We briefly discuss the relevance of these results to current
ideas on the origin of metals in the intergalactic medium and the universality
of the stellar initial mass function.Comment: 17 pages, 9 Figures, Accepted for publication in Monthly Notices of
the Royal Astronomical Societ
The Nature of the UV/X-Ray Absorber in PG 2302+029
We present Chandra X-ray observations of the radio-quiet QSO PG 2302+029.
This quasar has a rare system of ultra-high velocity (-56,000 km/s) UV
absorption lines that form in an outflow from the active nucleus (Jannuzi et
al. 2003). The Chandra data indicate that soft X-ray absorption is also
present. We perform a joint UV and X-ray analysis, using photoionization
calculations, to detemine the nature of the absorbing gas. The UV and X-ray
datasets were not obtained simultaneously. Nonetheless, our analysis suggests
that the X-ray absorption occurs at high velocities in the same general region
as the UV absorber. There are not enough constraints to rule out multi-zone
models. In fact, the distinct broad and narrow UV line profiles clearly
indicate that multiple zones are present. Our preferred estimates of the
ionization and total column density in the X-ray absorber (log U=1.6,
N_H=10^22.4 cm^-2) over predict the O VI 1032, 1038 absorption unless the X-ray
absorber is also outflowing at ~56,000 km/s, but they over predict the Ne VIII
770, 780 absorption at all velocities. If we assume that the X-ray absorbing
gas is outflowing at the same velocity of the UV-absorbing wind and that the
wind is radiatively accelerated, then the outflow must be launched at a radius
of < 10^15 cm from the central continuum source. The smallness of this radius
casts doubts on the assumption of radiative acceleration.Comment: Accepted for Publication in Ap
On the forward cone quantization of the Dirac field in "longitudinal boost-invariant" coordinates with cylindrical symmetry
We obtain a complete set of free-field solutions of the Dirac equation in a
(longitudinal) boost-invariant geometry with azimuthal symmetry and use these
solutions to perform the canonical quantization of a free Dirac field of mass
. This coordinate system which uses the 1+1 dimensional fluid rapidity and the fluid proper time is
relevant for understanding particle production of quarks and antiquarks
following an ultrarelativistic collision of heavy ions, as it incorporates the
(approximate) longitudinal "boost invariance" of the distribution of outgoing
particles. We compare two approaches to solving the Dirac equation in
curvilinear coordinates, one directly using Vierbeins, and one using a
"diagonal" Vierbein representation
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