2,532 research outputs found
The SiC problem: astronomical and meteoritic evidence
Pre-solar grains of silicon carbide found in meteorites and interpreted as
having had an origin around carbon stars from their isotopic composition, have
all been found to be of the beta-SiC polytype. Yet to date fits to the 11.3
microns SiC emission band of carbon stars had been obtained only for alpha-SiC
grains. We present thin film infrared (IR) absorption spectra measured in a
diamond anvil cell for both the alpha- and beta- polymorphs of synthetic SiC
and compare the results with previously published spectra taken using the KBr
matrix method. We find that our thin film spectra have positions nearly
identical to those obtained previously from finely ground samples in KBr.
Hence, we show that this discrepancy has arisen from inappropriate `KBr
corrections' having been made to laboratory spectra of SiC particles dispersed
in KBr matrices. We re-fit a sample of carbon star mid-IR spectra, using
laboratory data with no KBr correction applied, and show that beta-SiC grains
fit the observations, while alpha-SiC grains do not. The discrepancy between
meteoritic and astronomical identifications of the SiC-type is therefore
removed. This work shows that the diamond anvil cell thin film method can be
used to produce mineral spectra applicable to cosmic environments without
further manipulation.Comment: to be published in Astrophysical Journal Letter 4 pages, 3 figure
Fluctuation-Dissipation Theorem in Nonequilibrium Steady States
In equilibrium, the fluctuation-dissipation theorem (FDT) expresses the
response of an observable to a small perturbation by a correlation function of
this variable with another one that is conjugate to the perturbation with
respect to \emph{energy}. For a nonequilibrium steady state (NESS), the
corresponding FDT is shown to involve in the correlation function a variable
that is conjugate with respect to \emph{entropy}. By splitting up entropy
production into one of the system and one of the medium, it is shown that for
systems with a genuine equilibrium state the FDT of the NESS differs from its
equilibrium form by an additive term involving \emph{total} entropy production.
A related variant of the FDT not requiring explicit knowledge of the stationary
state is particularly useful for coupled Langevin systems. The \emph{a priori}
surprising freedom apparently involved in different forms of the FDT in a NESS
is clarified.Comment: 6 pages; EPL, in pres
On the hyperbolicity and causality of the relativistic Euler system under the kinetic equation of state
We show that a pair of conjectures raised in [11] concerning the construction
of normal solutions to the relativistic Boltzmann equation are valid. This
ensures that the results in [11] hold for any range of positive temperatures
and that the relativistic Euler system under the kinetic equation of state is
hyperbolic and the speed of sound cannot overcome .Comment: 6 pages. Abridged version; full version to appear in Commun. Pure
Appl. Ana
The Curious Conundrum Regarding Sulfur Abundances In Planetary Nebulae
Sulfur abundances derived from optical emission line measurements and
ionization correction factors in planetary nebulae are systematically lower
than expected for the objects' metallicities. We have carefully considered a
large range of explanations for this "sulfur anomaly", including: (1)
correlations between the size of the sulfur deficit and numerous nebular and
central star properties; (2) ionization correction factors which under-correct
for unobserved ions; (3) effects of dielectronic recombination on the sulfur
ionization balance; (4) sequestering of S into dust and/or molecules; and (5)
excessive destruction of S or production of O by AGB stars. It appears that all
but the second scenario can be ruled out. However, we find evidence that the
sulfur deficit is generally reduced but not eliminated when S^+3 abundances
determined directly from IR measurements are used in place of the customary
sulfur ionization correction factor. We tentatively conclude that the sulfur
anomaly is caused by the inability of commonly used ICFs to properly correct
for populations of ionization stages higher than S^+2.Comment: 40 pages, 14 figures, 3 tables. Accepted for publication in the
Astrophysical Journa
Measurement of Stochastic Entropy Production
Using fluorescence spectroscopy we directly measure entropy production of a
single two-level system realized experimentally as an optically driven defect
center in diamond. We exploit a recent suggestion to define entropy on the
level of a single stochastic trajectory (Seifert, Phys. Rev. Lett. {\bf 95},
040602 (2005)). Entropy production can then be split into one of the system
itself and one of the surrounding medium. We demonstrate that the total entropy
production obeys various exact relations for finite time trajectories.Comment: Phys. Rev. Lett., in pres
Irreversible effects of memory
The steady state of a Langevin equation with short ranged memory and coloured
noise is analyzed. When the fluctuation-dissipation theorem of second kind is
not satisfied, the dynamics is irreversible, i.e. detailed balance is violated.
We show that the entropy production rate for this system should include the
power injected by ``memory forces''. With this additional contribution, the
Fluctuation Relation is fairly verified in simulations. Both dynamics with
inertia and overdamped dynamics yield the same expression for this additional
power. The role of ``memory forces'' within the fluctuation-dissipation
relation of first kind is also discussed.Comment: 6 pages, 1 figure, publishe
Probability density functions of work and heat near the stochastic resonance of a colloidal particle
We study experimentally and theoretically the probability density functions
of the injected and dissipated energy in a system of a colloidal particle
trapped in a double well potential periodically modulated by an external
perturbation. The work done by the external force and the dissipated energy are
measured close to the stochastic resonance where the injected power is maximum.
We show a good agreement between the probability density functions exactly
computed from a Langevin dynamics and the measured ones. The probability
density function of the work done on the particle satisfies the fluctuation
theorem
Mobility and Diffusion of a Tagged Particle in a Driven Colloidal Suspension
We study numerically the influence of density and strain rate on the
diffusion and mobility of a single tagged particle in a sheared colloidal
suspension. We determine independently the time-dependent velocity
autocorrelation functions and, through a novel method, the response functions
with respect to a small force. While both the diffusion coefficient and the
mobility depend on the strain rate the latter exhibits a rather weak
dependency. Somewhat surprisingly, we find that the initial decay of response
and correlation functions coincide, allowing for an interpretation in terms of
an 'effective temperature'. Such a phenomenological effective temperature
recovers the Einstein relation in nonequilibrium. We show that our data is well
described by two expansions to lowest order in the strain rate.Comment: submitted to EP
Violation of the Einstein relation in Granular Fluids: the role of correlations
We study the linear response in different models of driven granular gases. In
some situations, even if the the velocity statistics can be strongly
non-Gaussian, we do not observe appreciable violations of the Einstein formula
for diffusion versus mobility. The situation changes when strong correlations
between velocities and density are present: in this case, although a form of
fluctuation-dissipation relation holds, the differential velocity response of a
particle and its velocity self-correlation are no more proportional. This
happens at high densities and strong inelasticities, but still in the
fluid-like (and ergodic) regime.Comment: 18 pages, 6 figures, submitted for publicatio
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