9,622 research outputs found
Electron and ion densities in interstellar clouds
A quantitative theory of ionization in diffuse clouds is developed which includes H(+) charge exchange with O. Dissociative charge exchange of He(+) with H2 plays an important role in the densities of H(+) and He(+). The abundance of HD is also discussed
Model calculations for diffuse molecular clouds
A steady state isobaric cloud model is developed. The pressure, thermal, electrical, and chemical balance equations are solved simultaneously with a simple one dimensional approximation to the equation of radiative transfer appropriate to diffuse clouds. Cooling is mainly by CII fine structure transitions, and a variety of heating mechanisms are considered. Particular attention is given to the abundance variation of H2. Inhomogeneous density distributions are obtained because of the attenuation of the interstellar UV field and the conversion from atomic to molecular hyrodgen. The effects of changing the model parameters are described and the applicability of the model to OAO-3 observations is discussed. Good qualitative agreement with the fractional H2 abundance determinations has been obtained. The observed kinetic temperatures near 80 K can also be achieved by grain photoelectron heating. The problem of the electron density is solved taking special account of the various hydrogen ions as well as heavier ones
Silicon chemistry in interstellar clouds
Interstellar SiO was discovered shortly after CO but it has been detected mainly in high density and high temperature regions associated with outflow sources. A new model of interstellar silicon chemistry that explains the lack of SiO detections in cold clouds is presented which contains an exponential temperature dependence for the SiO abundance. A key aspect of the model is the sensitivity of SiO production by neutral silicon reactions to density and temperature, which arises from the dependence of the rate coefficients on the population of the excited fine structure levels of the silicon atom. This effect was originally pointed out in the context of neutral reactions of carbon and oxygen by Graff, who noted that the leading term in neutral atom-molecule interactions involves the quadrupole moment of the atom. Similar to the case of carbon, the requirement that Si has a quadrupole moment requires population of the J = 1 level, which lies 111K above the J = 0 ground state and has a critical density n(cr) equal to or greater than 10(6)/cu cm. The SiO abundance then has a temperature dependence proportional to exp(-111/T) and a quadratic density dependence for n less than n(cr). As part of the explanation of the lack of SiO detections at low temperatures and densities, this model also emphasizes the small efficiencies of the production routes and the correspondingly long times needed to reach equilibrium. Measurements of the abundance of SiO, in conjunction with theory, can provide information on the physical properties of interstellar clouds such as the abundances of oxygen bearing molecules and the depletion of interstellar silicon
Morphology of Polyanhydride Microsphere Delivery Systems
Scanning electron microscopy (SEM) was used to elucidate the mechanism of polymer degradation and drug release in polyanhydride microspheres. Three different fabrication methods — solvent removal, solvent evaporation, and hot melt microencapsulation — were used to prepare polyanhydride microspheres containing a variety of drugs. The morphology of these microspheres releasing drug in vitro and in vivo was studied by SEM and compared with degradation and release data measured by conventional methods. Microspheres prepared by the three techniques were shown to have distinctive morphological characteristics induced by the nature of the fabrication method. In addition, SEM analysis could be used to explain the drug release profiles and polymer degradation behavior seen in vitro as well as the in vivo effects of insulin-loaded microspheres on diabetic rats. This study has shown SEM to be an important and powerful tool for analyzing the effects of microsphere fabrication method on drug release
Metastability in Two Dimensions and the Effective Potential
We study analytically and numerically the decay of a metastable phase in
(2+1)-dimensional classical scalar field theory coupled to a heat bath, which
is equivalent to two-dimensional Euclidean quantum field theory at zero
temperature. By a numerical simulation we obtain the nucleation barrier as a
function of the parameters of the potential, and compare it to the theoretical
prediction from the bounce (critical bubble) calculation. We find the
nucleation barrier to be accurately predicted by theory using the bounce
configuration obtained from the tree-level (``classical'') effective action.
Within the range of parameters probed, we found that using the bounce derived
from the one-loop effective action requires an unnaturally large prefactor to
match the lattice results. Deviations from the tree-level prediction are seen
in the regime where loop corrections would be expected to become important.Comment: 13pp, LaTex with Postscript figs, CLNS 93/1202, DART-HEP-93/0
Thermal Timescale Mass Transfer and the Evolution of White Dwarf Binaries
The evolution of binaries consisting of evolved main sequence stars (1 <
M_d/Msun < 3.5) with white dwarf companions (0.7 < M_wd/Msun < 1.2) is
investigated through the thermal mass transfer phase. Taking into account the
stabilizing effect of a strong, optically thick wind from the accreting white
dwarf surface, we have explored the formation of several evolutionary groups of
systems for progenitors with initial orbital periods of 1 and 2 days. The
numerical results show that CO white dwarfs can accrete sufficient mass to
evolve to a Type Ia supernova and ONeMg white dwarfs can be built up to undergo
accretion induced collapse for donors more massive than about 2 Msun. For
donors less massive than ~2 Msun the system can evolve to form a He and CO or
ONeMg white dwarf pair. In addition, sufficient helium can be accumulated (~0.1
Msun) in systems characterized by 1.6 < M_d/Msun < 1.9 and 0.8 < M_wd/Msun < 1
such that sub Chandrasekhar mass models for Type Ia supernovae, involving off
center helium ignition, are possible for progenitor systems evolving via the
Case A mass transfer phase. For systems characterized by mass ratios > 3 the
system likely merges as a result of the occurrence of a delayed dynamical mass
transfer instability. A semi-analytical model is developed to delineate these
phases which can be easily incorporated in population synthesis studies of
these systems.Comment: 9 pages, 6 figures, Latex, emulateapj style, ApJ accepte
Rate dependent shear bands in a shear transformation zone model of amorphous solids
We use Shear Transformation Zone (STZ) theory to develop a deformation map
for amorphous solids as a function of the imposed shear rate and initial
material preparation. The STZ formulation incorporates recent simulation
results [Haxton and Liu, PRL 99 195701 (2007)] showing that the steady state
effective temperature is rate dependent. The resulting model predicts a wide
range of deformation behavior as a function of the initial conditions,
including homogeneous deformation, broad shear bands, extremely thin shear
bands, and the onset of material failure. In particular, the STZ model predicts
homogeneous deformation for shorter quench times and lower strain rates, and
inhomogeneous deformation for longer quench times and higher strain rates. The
location of the transition between homogeneous and inhomogeneous flow on the
deformation map is determined in part by the steady state effective
temperature, which is likely material dependent. This model also suggests that
material failure occurs due to a runaway feedback between shear heating and the
local disorder, and provides an explanation for the thickness of shear bands
near the onset of material failure. We find that this model, which resolves
dynamics within a sheared material interface, predicts that the stress weakens
with strain much more rapidly than a similar model which uses a single state
variable to specify internal dynamics on the interface.Comment: 10 pages, 13 figures, corrected typos, added section on rate
strengthening vs. rate weakening material
Dynamics and Thermodynamics of the Glass Transition
The principal theme of this paper is that anomalously slow, super-Arrhenius
relaxations in glassy materials may be activated processes involving chains of
molecular displacements. As pointed out in a preceding paper with A. Lemaitre,
the entropy of critically long excitation chains can enable them to grow
without bound, thus activating stable thermal fluctuations in the local density
or molecular coordination of the material. I argue here that the intrinsic
molecular-scale disorder in a glass plays an essential role in determining the
activation rate for such chains, and show that a simple disorder-related
correction to the earlier theory recovers the Vogel-Fulcher law in three
dimensions. A key feature of this theory is that the spatial extent of
critically long excitation chains diverges at the Vogel-Fulcher temperature. I
speculate that this diverging length scale implies that, as the temperature
decreases, increasingly large regions of the system become frozen and do not
contribute to the configurational entropy, and thus ergodicity is partially
broken in the super-Arrhenius region above the Kauzmann temperature . This
partially broken ergodicity seems to explain the vanishing entropy at and
other observed relations between dynamics and thermodynamics at the glass
transition.Comment: 20 pages, no figures, some further revision
Flux penetration in slab shaped Type-I superconductors
We study the problem of flux penetration into type--I superconductors with
high demagnetization factor (slab geometry).Assuming that the interface between
the normal and superconducting regions is sharp, that flux diffuses rapidly in
the normal regions, and that thermal effects are negligible, we analyze the
process by which flux invades the sample as the applied field is increased
slowly from zero.We find that flux does not penetrate gradually.Rather there is
an instability in the process and the flux penetrates from the boundary in a
series of bursts, accompanied by the formation of isolated droplets of the
normal phase, leading to a multiply connected flux domain structure similar to
that seen in experiments.Comment: 4 pages, 2 figures, Fig 2.(b) available upon request from the
authors, email - [email protected]
Hypnotics' association with mortality or cancer: a matched cohort study
Objectives: An estimated 6%e10 % of US adults took a hypnotic drug for poor sleep in 2010. This study extends previous reports associating hypnotics with excess mortality. Setting: A large integrated health system in the USA. Design: Longitudinal electronic medical records were extracted for a one-to-two matched cohort survival analysis. Subjects: Subjects (mean age 54 years) were 10 529 patients who received hypnotic prescriptions and 23 676 matched controls with no hypnotic prescriptions, followed for an average of 2.5 year
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