3,465 research outputs found
Effective destruction of CO by cosmic rays: implications for tracing H gas in the Universe
We report on the effects of cosmic rays (CRs) on the abundance of CO in clouds under conditions typical for star-forming galaxies in the Universe.
We discover that this most important molecule for tracing H gas is very
effectively destroyed in ISM environments with CR energy densities , a range expected in numerous
star-forming systems throughout the Universe. This density-dependent effect
operates volumetrically rather than only on molecular cloud surfaces (i.e.
unlike FUV radiation that also destroys CO), and is facilitated by: a) the
direct destruction of CO by CRs, and b) a reaction channel activated by
CR-produced He. The effect we uncover is strong enough to render
Milky-Way type Giant Molecular Clouds (GMCs) very CO-poor (and thus
CO-untraceable), even in ISM environments with rather modestly enhanced average
CR energy densities of . We conclude
that the CR-induced destruction of CO in molecular clouds, unhindered by dust
absorption, is perhaps the single most important factor controlling the
CO-visibility of molecular gas in vigorously star-forming galaxies. We
anticipate that a second order effect of this CO destruction mechanism will be
to make the H distribution in the gas-rich disks of such galaxies appear
much clumpier in CO =1--0, 2--1 line emission than it actually is. Finally
we give an analytical approximation of the CO/H abundance ratio as a
function of gas density and CR energy density for use in galaxy-size or
cosmological hydrodynamical simulations, and propose some key observational
tests.Comment: Accepted for publication in ApJ, 29 page
Backreaction in Acoustic Black Holes
The backreaction equations for the linearized quantum fluctuations in an
acoustic black hole are given. The solution near the horizon, obtained within a
dimensional reduction, indicates that acoustic black holes, unlike
Schwarzschild ones, get cooler as they radiate phonons. They show remarkable
analogies with near-extremal Reissner-Nordstrom black holes.Comment: 4 pages, revtex, 1 figure. revised version, published in pr
Statistical analysis of Ni nanowires breaking processes: a numerical simulation study
A statistical analysis of the breaking behavior of Ni nanowires is presented.
Using molecular dynamic simulations, we have determined the time evolution of
both the nanowire atomic structure and its minimum cross section (Sm(t)).
Accumulating thousands of independent breaking events, Sm histograms are built
and used to study the influence of the temperature, the crystalline stretching
direction and the initial nanowire size. The proportion of monomers, dimers and
more complex structures at the latest stages of the breaking process are
calculated, finding important differences among results obtained for different
nanowire orientations and sizes. Three main cases have been observed. (A) [111]
stretching direction and large nanowire sizes: the wire evolves from more
complex structures to monomers and dimers prior its rupture; well ordered
structures is presented during the breaking process. (B) Large nanowires
stretched along the [100] and [110] directions: the system mainly breaks from
complex structures (low probability of finding monomers and dimers), having
disordered regions during their breakage; at room temperature, a huge histogram
peak around Sm=5 appears, showing the presence of long staggered pentagonal Ni
wires with ...-5-1-5-... structure. (C) Initial wire size is small: strong size
effects independently on the temperature and stretching direction. Finally, the
local structure around monomers and dimmers do not depend on the stretching
direction. These configurations differ from those usually chosen in static
studies of conductance.Comment: 18 pages, 13 figure
Real space investigation of structural changes at the metal-insulator transition in VO2
Synchrotron X-ray total scattering studies of structural changes in rutile
VO2 at the metal-insulator transition temperature of 340 K reveal that
monoclinic and tetragonal phases of VO2 coexist in equilibrium, as expected for
a first-order phase transition. No evidence for any distinct intermediate phase
is seen. Unbiased local structure studies of the changes in V--V distances
through the phase transition, using reverse Monte Carlo methods, support the
idea of phase coexistence and point to the high degree of correlation in the
dimerized low-temperature structure. No evidence for short range V--V
correlations that would be suggestive of local dimers is found in the metallic
phase.Comment: 4 pages, 5 figure
Interplay between multiple scattering and optical nonlinearity in liquid crystals
We discuss the role played by time-dependent scattering on light propagation in liquid crystals. In the linear regime, the effects of the molecular disorder accumulate in propagation, yielding a monotonic decrease in the beam spatial coherence. In the nonlinear case, despite the disorder-imposed Brownian-like motion to the self-guided waves, self-focusing increases the spatial coherence of the beam by inducing spatial localization. Eventually, a strong enhancement in the beam oscillations occurs when power is strong enough to induce self-steering, i.e. in the non-perturbative regime.SCOPUS: ar.jinfo:eu-repo/semantics/publishe
Scattering lengths and universality in superdiffusive L\'evy materials
We study the effects of scattering lengths on L\'evy walks in quenched
one-dimensional random and fractal quasi-lattices, with scatterers spaced
according to a long-tailed distribution. By analyzing the scaling properties of
the random-walk probability distribution, we show that the effect of the
varying scattering length can be reabsorbed in the multiplicative coefficient
of the scaling length. This leads to a superscaling behavior, where the
dynamical exponents and also the scaling functions do not depend on the value
of the scattering length. Within the scaling framework, we obtain an exact
expression for the multiplicative coefficient as a function of the scattering
length both in the annealed and in the quenched random and fractal cases. Our
analytic results are compared with numerical simulations, with excellent
agreement, and are supposed to hold also in higher dimensionsComment: 6 pages, 8 figure
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