3,223 research outputs found
The optical spectrum of a large isolated polycyclic aromatic hydrocarbon: hexa-peri-hexabenzocoronene, C42H18
The first optical spectrum of an isolated polycyclic aromatic hydrocarbon
large enough to survive the photophysical conditions of the interstellar medium
is reported. Vibronic bands of the first electronic transition of the all
benzenoid polycyclic aromatic hydrocarbon hexa-peri-hexabenzocoronene were
observed in the 4080-4530 Angstrom range by resonant 2-color 2-photon
ionization spectroscopy. The strongest feature at 4264 Angstrom is estimated to
have an oscillator strength of f=1.4x10^-3, placing an upper limit on the
interstellar abundance of this polycyclic aromatic hydrocarbon at 4x10^12
cm^-2, accounting for a maximum of ~0.02% of interstellar carbon. This study
opens up the possibility to rigorously test neutral polycyclic aromatic
hydrocarbons as carriers of the diffuse interstellar bands in the near future.Comment: 9 pages, 1 figure. Fixed a typo on the frequency of the 'b' ban
Angular Correlations in Top Quark Pair Production and Decay at Hadron Colliders
We show how to observe sizable angular correlations between the decay
products of the top quark and those of the anti-top quark in top quark pair
production and decay at hadron colliders. These correlations result from the
large asymmetry in the rate for producing like-spin versus unlike-spin top
quark pairs provided the appropriate spin axes are used. The effects of new
physics at production or decay on these correlations are briefly discussed.Comment: 34 pages, revtex, including 12 uuencoded postscript figure
Soybean Cyst Nematode Reduces Soybean Yield Without Causing Obvious Aboveground Symptoms
Field experiments were conducted at locations in northern and southern Illinois, central Iowa, and central Missouri from 1997 to 1999 to investigate the effects of Heterodera glycines on soybean growth, development, and yield. A wide range of infestation levels was present at all locations. Two locally adapted cultivars, one resistant to H. glycines, were grown at each location. Cultivars were planted in alternating four-row strips with 76 cm between rows. For each cultivar, 20 1-m-long single-row plots were sampled every 2 weeks starting 4 weeks after planting. Infection by H. glycines reduced plant height and leaf and stem weight on the resistant cultivars in the first 12 weeks after planting, and delayed pod and seed development 12 to 14 weeks after planting. Biomass accumulation was not reduced on the susceptible cultivars until 10 weeks after planting; reduction in pod and seed development occurred throughout the reproductive stages. Susceptible cultivars produced significantly lower yields than resistant cultivars, but the yield reductions were not accompanied by visually detectable symptoms
Changes in temperature and precipitation extremes in the IPCC ensemble of global coupled model simulations
Temperature and precipitation extremes and their potential future changes are evaluated in an ensemble of global coupled climate models participating in the Intergovernmental Panel on Climate Change (IPCC) diagnostic exercise for the Fourth Assessment Report (AR4). Climate extremes are expressed in terms of 20-yr return values of annual extremes of near-surface temperature and 24-h precipitation amounts. The simulated changes in extremes are documented for years 2046–65 and 2081–2100 relative to 1981–2000 in experiments with the Special Report on Emissions Scenarios (SRES) B1, A1B, and A2 emission scenarios. Overall, the climate models simulate present-day warm extremes reasonably well on the global scale, as compared to estimates from reanalyses. The model discrepancies in simulating cold extremes are generally larger than those for warm extremes, especially in sea ice–covered areas. Simulated present-day precipita-tion extremes are plausible in the extratropics, but uncertainties in extreme precipitation in the Tropics are very large, both in the models and the available observationally based datasets. Changes in warm extremes generally follow changes in the mean summertime temperature. Cold ex-tremes warm faster than warm extremes by about 30%–40%, globally averaged. The excessive warming of cold extremes is generally confined to regions where snow and sea ice retreat with global warming. With th
Topological Defects in Gravitational Theories with Non Linear Lagrangians
The gravitational field of monopoles, cosmic strings and domain walls is
studied in the quadratic gravitational theory with , and is compared with the result in Einstein's theory. The metric
aquires modifications which correspond to a short range `Newtonian' potential
for gauge cosmic strings, gauge monopoles and domain walls and to a long range
one for global monopoles and global cosmic strings. In this theory the
corrections turn out to be attractive for all the defects. We explain, however,
that the sign of these corrections in general depends on the particular higher
order derivative theory and topological defect under consideration. The
possible relevance of our results to the study of the evolution of topological
defects in the early universe is pointed out.Comment: LaTeX (uses revrex macros), 13 page
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