2,961 research outputs found
Observations of H3+ in the Diffuse Interstellar Medium
Surprisingly large column densities of H3+ have been detected using infrared
absorption spectroscopy in seven diffuse cloud sightlines (Cygnus OB2 12,
Cygnus OB2 5, HD 183143, HD 20041, WR 104, WR 118, and WR 121), demonstrating
that H3+ is ubiquitous in the diffuse interstellar medium. Using the standard
model of diffuse cloud chemistry, our H3+ column densities imply unreasonably
long path lengths (~1 kpc) and low densities (~3 cm^-3). Complimentary
millimeter-wave, infrared, and visible observations of related species suggest
that the chemical model is incorrect and that the number density of H3+ must be
increased by one to two orders of magnitude. Possible solutions include a
reduced electron fraction, an enhanced rate of H2 ionization, and/or a smaller
value of the H3+ dissociative recombination rate constant than implied by
laboratory experiments.Comment: To be published in Astrophysical Journal, March 200
Massive creation of entangled exciton states in semiconductor quantum dots
An intense laser pulse propagating in a medium of inhomogeneously broadened
quantum dots massively creates entangled exciton states. After passage of the
pulse all single-exciton states remain unpopulated (self-induced transparency)
whereas biexciton coherence (exciton entanglement) is generated through
two-photon transitions. We propose several experimental techniques for the
observation of such unexpected behavior
Enhanced cosmic-ray flux toward zeta Persei inferred from laboratory study of H3+ - e- recombination rate
The H3+ molecular ion plays a fundamental role in interstellar chemistry, as
it initiates a network of chemical reactions that produce many interstellar
molecules. In dense clouds, the H3+ abundance is understood using a simple
chemical model, from which observations of H3+ yield valuable estimates of
cloud path length, density, and temperature. On the other hand, observations of
diffuse clouds have suggested that H3+ is considerably more abundant than
expected from the chemical models. However, diffuse cloud models have been
hampered by the uncertain values of three key parameters: the rate of H3+
destruction by electrons, the electron fraction, and the cosmic-ray ionisation
rate. Here we report a direct experimental measurement of the H3+ destruction
rate under nearly interstellar conditions. We also report the observation of
H3+ in a diffuse cloud (towards zeta Persei) where the electron fraction is
already known. Taken together, these results allow us to derive the value of
the third uncertain model parameter: we find that the cosmic-ray ionisation
rate in this sightline is forty times faster than previously assumed. If such a
high cosmic-ray flux is indeed ubiquitous in diffuse clouds, the discrepancy
between chemical models and the previous observations of H3+ can be resolved.Comment: 6 pages, Nature, in pres
Universal subgap optical conductivity in quasi-one-dimensional Peierls systems
Quasi-one-dimensional Peierls systems with quantum and thermal lattice
fluctuations can be modeled by a Dirac-type equation with a Gaussian-correlated
off-diagonal disorder. A powerful new method gives the exact disorder-averaged
Green function used to compute the optical conductivity. The strong subgap tail
of the conductivity has a universal scaling form. The frequency and temperature
dependence of the calculated spectrum agrees with experiments on KCP(Br) and
trans-polyacetylene.Comment: 11 pages (+ 3 figures), LATEX (REVTEX 3.0
H3+ in Diffuse Interstellar Clouds: a Tracer for the Cosmic-Ray Ionization Rate
Using high resolution infrared spectroscopy we have surveyed twenty
sightlines for H3+ absorption. H3+ is detected in eight diffuse cloud
sightlines with column densities varying from 0.6x10^14 cm^-2 to 3.9x10^14
cm^-2. This brings to fourteen the total number of diffuse cloud sightlines
where H3+ has been detected. These detections are mostly along sightlines
concentrated in the Galactic plane, but well dispersed in Galactic longitude.
The results imply that abundant H3+ is common in the diffuse interstellar
medium. Because of the simple chemistry associated with H3+ production and
destruction, these column density measurements can be used in concert with
various other data to infer the primary cosmic-ray ionization rate, zeta_p.
Values range from 0.5x10^-16 s^-1 to 3x10^-16 s^-1 with an average of 2x10^-16
s^-1. Where H3+ is not detected the upper limits on the ionization rate are
consistent with this range. The average value of zeta_p is about an order of
magnitude larger than both the canonical rate and rates previously reported by
other groups using measurements of OH and HD. The discrepancy is most likely
due to inaccurate measurements of rate constants and the omission of effects
which were unknown when those studies were performed. We believe that the
observed column density of H3+ is the most direct tracer for the cosmic-ray
ionization rate due to its simple chemistry. Recent models of diffuse cloud
chemistry require cosmic-ray ionization rates on the order of 10^-16 s^-1 to
reproduce observed abundances of various atomic and molecular species, in rough
accord with our observational findings.Comment: Accepted to ApJ, 35 pages, 5 figures, 5 table
Iso-osmotic regulation of nitrate accumulation in lettuce (Lactuca sativa L.)
Concerns about possible health hazards arising from human consumption of lettuce and other edible vegetable crops with high concentrations of nitrate have generated demands for a greater understanding of processes involved in its uptake and accumulation in order to devise more sustainable strategies for its control. This paper evaluates a proposed iso-osmotic mechanism for the regulation of nitrate accumulation in lettuce (Lactuca sativa L.) heads. This mechanism assumes that changes in the concentrations of nitrate and all other endogenous osmotica (including anions, cations and neutral solutes) are continually adjusted in tandem to minimise differences in osmotic potential of the shoot sap during growth, with these changes occurring independently of any variations in external water potential. The hypothesis was tested using data from six new experiments, each with a single unique treatment comprising a separate combination of light intensity, N source (nitrate with or without ammonium) and nitrate concentration carried out hydroponically in a glasshouse using a butterhead lettuce variety. Repeat measurements of plant weights and estimates of all of the main soluble constituents (nitrate, potassium, calcium, magnesium, organic anions, chloride, phosphate, sulphate and soluble carbohydrates) in the shoot sap were made at intervals from about 2 weeks after transplanting until commercial maturity, and the data used to calculate changes in average osmotic potential in the shoot. Results showed that nitrate concentrations in the sap increased when average light levels were reduced by between 30 and 49 % and (to a lesser extent) when nitrate was supplied at a supra-optimal concentration, and declined with partial replacement of nitrate by ammonium in the external nutrient supply. The associated changes in the proportions of other endogenous osmotica, in combination with the adjustment of shoot water content, maintained the total solute concentrations in shoot sap approximately constant and minimised differences in osmotic potential between treatments at each sampling date. There was, however, a gradual increase in osmotic potential (ie a decline in total solute concentration) over time largely caused by increases in shoot water content associated with the physiological and morphological development of the plants. Regression analysis using normalised data (to correct for these time trends) showed that the results were consistent with a 1:1 exchange between the concentrations of nitrate and the sum of all other endogenous osmotica throughout growth, providing evidence that an iso-osmotic mechanism (incorporating both concentration and volume regulation) was involved in controlling nitrate concentrations in the shoot
Remainders of power series
Suppose ∑n=0∞anzn has radius of convergence R and σN(z)=|∑n=N∞anzn|. Suppose |z1|σN(z) for zϵT}. Two questions are asked: (a) can S be cofinite? (b) can S be infinite? This paper provides some answers to these questions. The answer to (a) is no, even if T=z2. The answer to (b) is no, for T=z2 if liman=a≠0. Examples show (b) is possible if T=z2 and for T a neighborhood of z2
3-D Photoionization Structure and Distances of Planetary Nebulae II. Menzel 1
We present the results of a spatio-kinematic study of the planetary nebula
Menzel 1 using spectro-photometric mapping and a 3-D photoionization code. We
create several 2-D emission line images from our long-slit spectra, and use
these to derive the line fluxes for 15 lines, the Halpha/Hbeta extinction map,
and the [SII] line ratio density map of the nebula. We use our photoionization
code constrained by these data to derive the three-dimensional nebular
structure and ionizing star parameters of Menzel 1 by simultaneously fitting
the integrated line intensities, the density map, and the observed morphologies
in several lines, as well as the velocity structure. Using theoretical
evolutionary tracks of intermediate and low mass stars, we derive a mass for
the central star of 0.63+-0.05 Msolar. We also derive a distance of 1050+_150
pc to Menzel 1.Comment: To be published in ApJ of 10th February 2005. 12 figure
Isochronal annealing effects on local structure, crystalline fraction, and undamaged region size of radiation damage in Ga-stabilized -Pu
The effects on the local structure due to self-irradiation damage of Ga
stabilized -Pu stored at cryogenic temperatures have been examined
using extended x-ray absorption fine structure (EXAFS) experiments. Extensive
damage, seen as a loss of local order, was evident after 72 days of storage
below 15 K. The effect was observed from both the Pu and Ga sites, although
less pronounced around Ga. Isochronal annealing was performed on this sample to
study the annealing processes that occur between cryogenic and room temperature
storage conditions, where damage is mostly reversed. Damage fractions at
various points along the annealing curve have been determined using an
amplitude-ratio method, standard EXAFS fitting, and a spherical crystallite
model, and provide information complementary to previous electrical
resistivity- and susceptibility-based isochronal annealing studies. The use of
a spherical crystallite model accounts for the changes in EXAFS spectra using
just two parameters, namely, the crystalline fraction and the particle radius.
Together, these results are discussed in terms of changes to the local
structure around Ga and Pu throughout the annealing process and highlight the
unusual role of Ga in the behavior of the lowest temperature anneals.Comment: 13 pages, 10 figure
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