2,700,385 research outputs found
Determining the cosmic ray ionization rate in dynamically evolving clouds
The ionization fraction is an important factor in determining the chemical
and physical evolution of star forming regions. In the dense, dark starless
cores of such objects, the ionization rate is dominated by cosmic rays; it is
therefore possible to use simple analytic estimators, based on the relative
abundances of different molecular tracers, to determine the cosmic ray
ionization rate.
This paper uses a simple model to investigate the accuracy of two well-known
estimators in dynamically evolving molecular clouds. It is found that, although
the analytical formulae based on the abundances of H3+,H2,CO,O,H2O and HCO+
give a reasonably accurate measure of the cosmic ray ionization rate in static,
quiescent clouds, significant discrepancies occur in rapidly evolving
(collapsing) clouds. As recent evidence suggests that molecular clouds may
consist of complex, dynamically evolving sub-structure, we conclude that simple
abundance ratios do not provide reliable estimates of the cosmic ray ionization
rate in dynamically active regions.Comment: Accepted by A&A. 17 pages, 4 figure
Thermonuclear explosions of rapidly rotating white dwarfs - I. Deflagrations
Context: Turbulent deflagrations of Chandrasekhar mass White Dwarfs are
commonly used to model Type Ia Supernova explosions. In this context, rapid
rotation of the progenitor star is plausible but has so far been neglected.
Aims: The aim of this work is to explore the influence of rapid rotation on the
deflagration scenario. Methods: We use three dimensional hydrodynamical
simulations to model turbulent deflagrations ignited within a variety of
rapidly rotating CO WDs obeying rotation laws suggested by accretion studies.
Results: We find that rotation has a significant impact on the explosion. The
flame develops a strong anisotropy with a preferred direction towards the
stellar poles, leaving great amounts of unburnt matter along the equatorial
plane. Conclusions: The large amount of unburnt matter is contrary to observed
spectral features of SNe Ia. Thus, rapid rotation of the progenitor star and
the deflagration scenario are incompatible in order to explain SNe Ia.Comment: 13 pages, 10 figures, accepted for publication by A&
Formation of a Metallic Contact: Jump to Contact Revisited
The transition from tunneling to metallic contact between two surfaces does
not always involve a jump, but can be smooth. We have observed that the
configuration and material composition of the electrodes before contact largely
determines the presence or absence of a jump. Moreover, when jumps are found
preferential values of conductance have been identified. Through combination of
experiments, molecular dynamics, and first-principles transport calculations
these conductance values are identified with atomic contacts of either
monomers, dimers or double-bond contacts.Comment: 4 pages, 5 figure
Coherence and synchronization in diode-laser arrays with delayed global coupling
The dynamics of a semiconductor-laser array whose individual elements are
coupled in a global way through an external mirror is numerically analysed. A
coherent in-phase solution is seen to be preferred by the system at
intermediate values of the feedback coupling strength. At low values of this
parameter, a strong amplification of the spontaneous emission noise is
observed. A tendency towards chaos synchronization is also observed at large
values of the feedback strength.Comment: 8 pages, LaTeX, 6 PS figures, to appear in International Journal of
Bifurcation and Chao
Structure of the stationary state of the asymmetric target process
We introduce a novel migration process, the target process. This process is
dual to the zero-range process (ZRP) in the sense that, while for the ZRP the
rate of transfer of a particle only depends on the occupation of the departure
site, it only depends on the occupation of the arrival site for the target
process. More precisely, duality associates to a given ZRP a unique target
process, and vice-versa. If the dynamics is symmetric, i.e., in the absence of
a bias, both processes have the same stationary-state product measure. In this
work we focus our interest on the situation where the latter measure exhibits a
continuous condensation transition at some finite critical density ,
irrespective of the dimensionality. The novelty comes from the case of
asymmetric dynamics, where the target process has a nontrivial fluctuating
stationary state, whose characteristics depend on the dimensionality. In one
dimension, the system remains homogeneous at any finite density. An alternating
scenario however prevails in the high-density regime: typical configurations
consist of long alternating sequences of highly occupied and less occupied
sites. The local density of the latter is equal to and their
occupation distribution is critical. In dimension two and above, the asymmetric
target process exhibits a phase transition at a threshold density much
larger than . The system is homogeneous at any density below ,
whereas for higher densities it exhibits an extended condensate elongated along
the direction of the mean current, on top of a critical background with density
.Comment: 30 pages, 16 figure
Fast ignition driven by quasi-monoenergetic ions: Optimal ion type and reduction of ignition energies with an ion beam array
Fast ignition of inertial fusion targets driven by quasi-monoenergetic ion
beams is investigated by means of numerical simulations. Light and intermediate
ions such as lithium, carbon, aluminium and vanadium have been considered.
Simulations show that the minimum ignition energies of an ideal configuration
of compressed Deuterium-Tritium are almost independent on the ion atomic
number. However, they are obtained for increasing ion energies, which scale,
approximately, as Z^2, where Z is the ion atomic number. Assuming that the ion
beam can be focused into 10 {\mu}m spots, a new irradiation scheme is proposed
to reduce the ignition energies. The combination of intermediate Z ions, such
as 5.5 GeV vanadium, and the new irradiation scheme allows a reduction of the
number of ions required for ignition by, roughly, three orders of magnitude
when compared with the standard proton fast ignition scheme
Career progression and formal versus on-the-job training
We model the choice of individuals to follow or not apprenticeship training and their subsequent career. We use German administrative data, which records education, labour market transitions and wages to estimate a dynamic discrete choice model of training choice, employment and wage growth. The model allows for returns to experience and tenure, match specific effects, job mobility and search frictions. We show how apprenticeship training affects labour market careers and we quantify its benefits, relative to the overall costs. We then use our model to show how two welfare reforms change life-cycle decisions and human capital accumulation: One is the introduction of an Earned Income Tax Credit in Germany, and the other is a reform to Unemployment Insurance. In both reforms we find very significant impacts of the policy on training choices and on the value of realized matches,
demonstrating the importance of considering such longer term implications
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