1,982 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
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
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
Inverse condensation of adsorbed molecules with two conformations
Conventional gas-liquid phase transitions feature a coexistence line that has
a monotonic and positive slope in line with our intuition that cooling always
leads to condensation. Here we study the inverse phenomenon, condensation of
adsorbed organic molecules into dense domains upon heating. Our considerations
are motivated by recent experiments [Aeschlimann et al., Angew. Chem. (2021)],
which demonstrate the partial dissolution of an ordered molecular monolayer and
the mobilization of molecules upon cooling. We introduce a simple lattice model
in which each site can have three states corresponding to unoccupied and two
discernible molecular conformations. We investigate this model through Monte
Carlo simulations, mean-field theory, and exact results based on the analytical
solution of the Ising model in two dimensions. Our results should be broadly
applicable to molecules with distinct conformations that have sufficiently
different entropies or heat capacities
Fluctuation relations for heat engines in time-periodic steady states
A fluctuation relation for heat engines (FRHE) has been derived recently. In
the beginning, the system is in contact with the cooler bath. The system is
then coupled to the hotter bath and external parameters are changed cyclically,
eventually bringing the system back to its initial state, once the coupling
with the hot bath is switched off. In this work, we lift the condition of
initial thermal equilibrium and derive a new fluctuation relation for the
central system (heat engine) being in a time-periodic steady state (TPSS).
Carnot's inequality for classical thermodynamics follows as a direct
consequence of this fluctuation theorem even in TPSS. For the special cases of
the absence of hot bath and no extraction of work, we obtain the integral
fluctuation theorem for total entropy and the generalized exchange fluctuation
theorem, respectively. Recently microsized heat engines have been realized
experimentally in the TPSS. We numerically simulate the same model and verify
our proposed theorems.Comment: 9 page
Thermodynamic theory of epitaxial ferroelectric thin films with dense domain structures
A Landau-Ginsburg-Devonshire-type nonlinear phenomenological theory is
presented, which enables the thermodynamic description of dense laminar
polydomain states in epitaxial ferroelectric thin films. The theory explicitly
takes into account the mechanical substrate effect on the polarizations and
lattice strains in dissimilar elastic domains (twins). Numerical calculations
are performed for PbTiO3 and BaTiO3 films grown on (001)-oriented cubic
substrates. The "misfit strain-temperature" phase diagrams are developed for
these films, showing stability ranges of various possible polydomain and
single-domain states. Three types of polarization instabilities are revealed
for polydomain epitaxial ferroelectric films, which may lead to the formation
of new polydomain states forbidden in bulk crystals. The total dielectric and
piezoelectric small-signal responses of polydomain films are calculated,
resulting from both the volume and domain-wall contributions. For BaTiO3 films,
strong dielectric anomalies are predicted at room temperature near special
values of the misfit strain.Comment: 19 pages, 8 figure
Far-infrared imaging of post-AGB stars and (proto)-planetary nebulae with the AKARI Far-Infrared Surveyor
By tracing the distribution of cool dust in the extended envelopes of
post-AGB stars and (proto)-planetary nebulae ((P)PNe) we aim to recover, or
constrain, the mass loss history experienced by these stars in their recent
past. The Far-Infrared Surveyor (FIS) instrument on board the AKARI satellite
was used to obtain far-infrared maps for a selected sample of post-AGB stars
and (P)PNe. We derived flux densities (aperture photometry) for 13 post-AGB
stars and (P)PNe at four far-infrared wavelengths (60, 90, 140, and 160 um).
Radial (azimuthally averaged) profiles are used to investigate the presence of
extended emission from cool dust. No (detached) extended emission is detected
for any target in our sample at levels significant with respect to background
and cirrus emission. Only IRAS 21046+4739 reveals tentative excess emission
between 30 and 130". Estimates of the total dust and gas mass from the obtained
maps indicate that the envelope masses of these stars should be large in order
to be detected with the AKARI FIS. Imaging with higher sensitivity and higher
spatial resolution is needed to detect and resolve, if present, any cool
compact or extended emission associated with these evolved stars.Comment: accepted for publication in The Astronomical Journal (16 pages, 3
figures and 4 tables
The fluctuation-dissipation relation: how does one compare correlation functions and responses?
We discuss the well known Einstein and the Kubo Fluctuation Dissipation
Relations (FDRs) in the wider framework of a generalized FDR for systems with a
stationary probability distribution. A multi-variate linear Langevin model,
which includes dynamics with memory, is used as a treatable example to show how
the usual relations are recovered only in particular cases. This study brings
to the fore the ambiguities of a check of the FDR done without knowing the
significant degrees of freedom and their coupling. An analogous scenario
emerges in the dynamics of diluted shaken granular media. There, the
correlation between position and velocity of particles, due to spatial
inhomogeneities, induces violation of usual FDRs. The search for the
appropriate correlation function which could restore the FDR, can be more
insightful than a definition of ``non-equilibrium'' or ``effective
temperatures''.Comment: 22 pages, 9 figure
NâSubstituted Nipecotic Acids as (S )âSNAPâ5114 Analogues with Modified Lipophilic Domains
Potential mGAT4 inhibitors derived from the lead substance (S )âSNAPâ5114 have been synthesized and characterized for their inhibitory potency. Variations from the parent compound included the substitution of one of its aromatic 4âmethoxy and 4âmethoxyphenyl groups, respectively, with a more polar moiety, including a carboxylic acid, alcohol, nitrile, carboxamide, sulfonamide, aldehyde or ketone function, or amino acid partial structures. Furthermore, it was investigated how the substitution of more than one of the aromatic 4âmethoxy groups affects the potency and selectivity of the resulting compounds. Among the synthesized test substances (S )â1â{2â[(4âformylphenyl)bis(4âmethoxyphenyl)âmethoxy]ethyl}piperidineâ3âcarboxylic acid, that features a carbaldehyde function in place of one of the aromatic 4âmethoxy moieties of (S )âSNAPâ5114, was found to have a pIC50 value of 5.89±0.07, hence constituting a slightly more potent mGAT4 inhibitor than the parent substance while showing comparable subtype selectivity
The shape and composition of interstellar silicate grains
We investigate the composition and shape distribution of silicate dust grains
in the interstellar medium. The effect of the amount of magnesium in the
silicate lattice is studied. We fit the spectral shape of the interstellar 10
mu extinction feature as observed towards the galactic center. We use very
irregularly shaped coated and non-coated porous Gaussian Random Field particles
as well as a statistical approach to model shape effects. For the dust
materials we use amorphous and crystalline silicates with various composition
and SiC. The results of our analysis of the 10 mu feature are used to compute
the shape of the 20 mu silicate feature and to compare this with observations.
By using realistic particle shapes we are, for the first time, able to derive
the magnesium fraction in interstellar silicates. We find that the interstellar
silicates are highly magnesium rich (Mg/(Fe+Mg)>0.9) and that the stoichiometry
lies between pyroxene and olivine type silicates. This composition is not
consistent with that of the glassy material found in GEMS in interplanetary
dust particles indicating that these are, in general, not unprocessed remnants
from the interstellar medium. Also, we find a significant fraction of SiC
(~3%). We discuss the implications of our results for the formation and
evolutionary history of cometary and circumstellar dust. We argue that the fact
that crystalline silicates in cometary and circumstellar grains are almost
purely magnesium silicates is a natural consequence of our findings that the
amorphous silicates from which they were formed were already magnesium rich.Comment: Accepted for publication in A&
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