4,622 research outputs found
Velocity map imaging of the dynamics of reactions of Cl atoms with neopentane and tetramethyl silane
Detecting a rotation in the epsilon Eridani debris disc
The evidence for a rotation of the epsilon Eridani debris disc is examined.
Data at 850 micron wavelength were previously obtained using the Submillimetre
Common User Bolometer Array (SCUBA) over periods in 1997-1998 and 2000-2002. By
chi-square fitting after shift and rotation operations, images from these two
epochs were compared to recover proper motion and orbital motion of the disc.
The same procedures were then performed on simulated images to estimate the
accuracy of the results.
Minima in the chi-square plots indicate a motion of the disc of approximately
0.6'' per year in the direction of the star's proper motion. This
underestimates the true value of 1'' per year, implying that some of the
structure in the disc region is not associated with epsilon Eridani,
originating instead from background galaxies. From the chi-square fitting for
orbital motion, a counterclockwise rotation rate of ~2.75 degrees per year is
deduced. Comparisons with simulated data in which the disc is not rotating show
that noise and background galaxies result in approximately Gaussian
fluctuations with a standard deviation +/-1.5 degrees per year. Thus
counterclockwise rotation of disc features is supported at approximately a
2-sigma level, after a 4-year time difference. This rate is faster than the
Keplerian rate of 0.65 degrees per year for features at ~65 AU from the star,
suggesting their motion is tracking a planet inside the dust ring.
Future observations with SCUBA-2 can rule out no rotation of the epsilon
Eridani dust clumps with ~4-sigma confidence. Assuming a rate of about 2.75
degrees per year, the rotation of the features after a 10-year period could be
shown to be >1 degree per year at the 3-sigma level.Comment: 8 pages, 6 figure
Velocity map imaging of the dynamics of the CH3 + HCl -> CH4 + Cl reaction using a dual molecular beam method
International audienceThe reactions CH3 + HCl → CH4 + Cl(<sup>2</sup>P<sub>3/2</sub>) and CD<sub>3</sub> + HCl → CD<sub>3</sub>H + Cl(<sup>2</sup>P<sub>3/2</sub>) have been studied by photo-initiation (by CH<sub>3</sub>I or CD<sub>3</sub>I photolysis at 266 nm) in a dual molecular beam apparatus. Product Cl(<sup>2</sup>P</sub>3/2</sub>) atoms were detected using resonance enhanced multi-photon ionisation and velocity map imaging, revealing product translational energy and angular scattering distributions in the centre-of-mass frame. Image analysis is complicated by the bimodal speed distribution of CH<sub>3</sub> (and CD<sub>3</sub>) radicals formed in coincidence with I(<sup>2</sup>P<sub>3/2</sub>) and I(<sup>2</sup>P<sub>1/2</sub>) atoms from CH<sub>3</sub>I (CD<sub>3</sub>I) photodissociation, giving overlapping Newton diagrams with displaced centre of mass velocities. The relative reactivities to form Cl atoms are greater for the slower CH<sub>3</sub> speed group than the faster group by factors of ~1.5 for the reaction of CH<sub>3</sub> and ~2.5 for the reaction of CD<sub>3</sub>, consistent with the greater propensity of the faster methyl radicals to undergo electronically adiabatic reactions to form Cl(<sup>2</sup>P<sub>1/2</sub>). The average fraction of the available energy becoming product translational energy is = 0.48 ± 0.05 and 0.50 ± 0.03 for reaction of the faster and slower sets of CH<sub>3</sub> radicals, respectively. The Cl atoms are deduced to be preferentially forward scattered with respect to the HCl reagents, but the angular distributions from the dual beam imaging experiments require correction for under-detection of forward scattered Cl products
Equiangular lines in Euclidean spaces
We obtain several new results contributing to the theory of real equiangular
line systems. Among other things, we present a new general lower bound on the
maximum number of equiangular lines in d dimensional Euclidean space; we
describe the two-graphs on 12 vertices; and we investigate Seidel matrices with
exactly three distinct eigenvalues. As a result, we improve on two
long-standing upper bounds regarding the maximum number of equiangular lines in
dimensions d=14, and d=16. Additionally, we prove the nonexistence of certain
regular graphs with four eigenvalues, and correct some tables from the
literature.Comment: 24 pages, to appear in JCTA. Corrected an entry in Table
Predicting the frequencies of diverse exo-planetary systems
Extrasolar planetary systems range from hot Jupiters out to icy comet belts
more distant than Pluto. We explain this diversity in a model where the mass of
solids in the primordial circumstellar disk dictates the outcome. The star
retains measures of the initial heavy-element (metal) abundance that can be
used to map solid masses onto outcomes, and the frequencies of all classes are
correctly predicted. The differing dependences on metallicity for forming
massive planets and low-mass cometary bodies are also explained. By
extrapolation, around two-thirds of stars have enough solids to form Earth-like
planets, and a high rate is supported by the first detections of low-mass
exo-planets.Comment: 5 pages, 2 figures; accepted by MNRA
ALMA and Herschel Observations of the Prototype Dusty and Polluted White Dwarf G29-38
ALMA Cycle 0 and Herschel PACS observations are reported for the prototype,
nearest, and brightest example of a dusty and polluted white dwarf, G29-38.
These long wavelength programs attempted to detect an outlying, parent
population of bodies at 1-100 AU, from which originates the disrupted
planetesimal debris that is observed within 0.01 AU and which exhibits L_IR/L =
0.039. No associated emission sources were detected in any of the data down to
L_IR/L ~ 1e-4, generally ruling out cold dust masses greater than 1e24 - 1e25 g
for reasonable grain sizes and properties in orbital regions corresponding to
evolved versions of both asteroid and Kuiper belt analogs. Overall, these null
detections are consistent with models of long-term collisional evolution in
planetesimal disks, and the source regions for the disrupted parent bodies at
stars like G29-38 may only be salient in exceptional circumstances, such as a
recent instability. A larger sample of polluted white dwarfs, targeted with the
full ALMA array, has the potential to unambiguously identify the parent
source(s) of their planetary debris.Comment: 8 pages, 5 figures and 1 table. Accepted to MNRA
Bulletin No. 305 - The Composition of Sumer Range Plants in Utah
In Utah, a vast industry of livestock grazing, which is the backbone of the state\u27s agriculture, has arisen during the past 75 years. Range land furnishes between 6 and 7 million animal unit months of forage to some 2 1/2 million sheep and 275 thousand range cattle. Income from meat, wool, and range livestock sales in Utah is about 11,700,000 is calculated to be obtained from range lands exclusive of cultivated pastures.
These range lands can be used economically in no other way than by grazing livestock and, because of heavy winter snows and protracted dry periods, most of these lands are distinctly seasonal in character. Livestock, then, must be supported for Iong periods upon farm lands or upon other range lands. The animals must be driven or shipped over distances sometimes well in excess of 100 miles from one range to another. The specific seasonal nature of these lands makes important the study of seasonal variations in forage value. That animals can make the most efficient use of the range lands, it is important to understand the forage value, the balance of various chemical constituents, and the importance of deficiencies in the diet of animals existing wholly upon these native plants
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