346 research outputs found
Isotopic fractionation of carbon, deuterium and nitrogen : a full chemical study
Context. The increased sensitivity and high spectral resolution of millimeter
telescopes allow the detection of an increasing number of isotopically
substituted molecules in the interstellar medium. The 14N/ 15N ratio is
difficult to measure directly for carbon containing molecules. Aims. We want to
check the underlying hypothesis that the 13C/ 12C ratio of nitriles and
isonitriles is equal to the elemental value via a chemical time dependent gas
phase chemical model. Methods. We have built a chemical network containing D,
13C and 15N molecular species after a careful check of the possible
fractionation reactions at work in the gas phase. Results. Model results
obtained for 2 different physical conditions corresponding respectively to a
moderately dense cloud in an early evolutionary stage and a dense depleted
pre-stellar core tend to show that ammonia and its singly deuterated form are
somewhat enriched in 15N, in agreement with observations. The 14N/ 15N ratio in
N2H+ is found to be close to the elemental value, in contrast to previous
models which obtain a significant enrichment, as we found that the
fractionation reaction between 15N and N2H+ has a barrier in the entrance
channel. The large values of the N2H+/15NNH+ and N2H+/ N15NH+ ratios derived in
L1544 cannot be reproduced in our model. Finally we find that nitriles and
isonitriles are in fact significantly depleted in 13C, questioning previous
interpretations of observed C15N, HC15N and H15NC abundances from 13C
containing isotopologues.Comment: 21 pages, 9 figures in the text, 3 Figures in the appendices. 7
tables in the text, 4 tables in the appendices. Accepted for publication by
Astronomy Astrophysic
Pores in Bilayer Membranes of Amphiphilic Molecules: Coarse-Grained Molecular Dynamics Simulations Compared with Simple Mesoscopic Models
We investigate pores in fluid membranes by molecular dynamics simulations of
an amphiphile-solvent mixture, using a molecular coarse-grained model. The
amphiphilic membranes self-assemble into a lamellar stack of amphiphilic
bilayers separated by solvent layers. We focus on the particular case of
tension less membranes, in which pores spontaneously appear because of thermal
fluctuations. Their spatial distribution is similar to that of a random set of
repulsive hard discs. The size and shape distribution of individual pores can
be described satisfactorily by a simple mesoscopic model, which accounts only
for a pore independent core energy and a line tension penalty at the pore
edges. In particular, the pores are not circular: their shapes are fractal and
have the same characteristics as those of two dimensional ring polymers.
Finally, we study the size-fluctuation dynamics of the pores, and compare the
time evolution of their contour length to a random walk in a linear potential
The critical behavior of frustrated spin models with noncollinear order
We study the critical behavior of frustrated spin models with noncollinear
order, including stacked triangular antiferromagnets and helimagnets. For this
purpose we compute the field-theoretic expansions at fixed dimension to six
loops and determine their large-order behavior. For the physically relevant
cases of two and three components, we show the existence of a new stable fixed
point that corresponds to the conjectured chiral universality class. This
contradicts previous three-loop field-theoretical results but is in agreement
with experiments.Comment: 4 pages, RevTe
Reaction Networks For Interstellar Chemical Modelling: Improvements and Challenges
We survey the current situation regarding chemical modelling of the synthesis
of molecules in the interstellar medium. The present state of knowledge
concerning the rate coefficients and their uncertainties for the major
gas-phase processes -- ion-neutral reactions, neutral-neutral reactions,
radiative association, and dissociative recombination -- is reviewed. Emphasis
is placed on those reactions that have been identified, by sensitivity
analyses, as 'crucial' in determining the predicted abundances of the species
observed in the interstellar medium. These sensitivity analyses have been
carried out for gas-phase models of three representative, molecule-rich,
astronomical sources: the cold dense molecular clouds TMC-1 and L134N, and the
expanding circumstellar envelope IRC +10216. Our review has led to the proposal
of new values and uncertainties for the rate coefficients of many of the key
reactions. The impact of these new data on the predicted abundances in TMC-1
and L134N is reported. Interstellar dust particles also influence the observed
abundances of molecules in the interstellar medium. Their role is included in
gas-grain, as distinct from gas-phase only, models. We review the methods for
incorporating both accretion onto, and reactions on, the surfaces of grains in
such models, as well as describing some recent experimental efforts to simulate
and examine relevant processes in the laboratory. These efforts include
experiments on the surface-catalysed recombination of hydrogen atoms, on
chemical processing on and in the ices that are known to exist on the surface
of interstellar grains, and on desorption processes, which may enable species
formed on grains to return to the gas-phase.Comment: Accepted for publication in Space Science Review
Finite-Temperature Transition in the Spin-Dimer Antiferromagnet BaCuSi2O6
We consider a classical XY-like Hamiltonian on a body-centered tetragonal
lattice, focusing on the role of interlayer frustration. A three-dimensional
(3D) ordered phase is realized via thermal fluctuations, breaking the
mirror-image reflection symmetry in addition to the XY symmetry. A heuristic
field-theoretical model of the transition has a decoupled fixed point in the 3D
XY universality, and our Monte Carlo simulation suggests that there is such a
temperature region where long-wavelength fluctuations can be described by this
fixed point. However, it is shown using scaling arguments that the decoupled
fixed point is unstable against a fluctuation-induced biquadratic interaction,
indicating that a crossover to nontrivial critical phenomena with different
exponents appears as one approaches the critical point beyond the transient
temperature region. This new scenario clearly contradicts the previous notion
of the 3D XY universality.Comment: 16 pages, 7 figure
Detection of ethanol, acetone, and propanal in TMC-1: New O-bearing complex organics in cold sources
We present the detection of ethanol (C2H5OH), acetone (CH3COCH3), and
propanal (C2H5CHO) toward the cyanopolyyne peak of TMC-1. These three O-bearing
complex organic molecules are known to be present in warm interstellar clouds,
but had never been observed in a starless core. The addition of these three new
pieces to the puzzle of complex organic molecules in cold interstellar clouds
stresses the rich chemical diversity of cold dense cores in stages prior to the
onset of star formation. The detections of ethanol, acetone, and propanal were
made in the framework of QUIJOTE, a deep line survey of TMC-1 in the Q band
that is being carried out with the Yebes 40m telescope. We derive column
densities of (1.1 +/- 0.3)e12 cm-2 for C2H5OH, (1.4 +/- 0.6)e11 cm-2 for
CH3COCH3, and (1.9 +/- 0.7)e11 cm-2 for C2H5CHO. The formation of these three
O-bearing complex organic molecules is investigated with the aid of a detailed
chemical model which includes gas and ice chemistry. The calculated abundances
at a time around 2e5 yr are in reasonable agreement with the values derived
from the observations. The formation mechanisms of these molecules in our
chemical model are as follows. Ethanol is formed on grains by addition of
atomic carbon on methanol followed by hydrogenation and non-thermal desorption.
Acetone and propanal are produced by the gas-phase reaction between atomic
oxygen and two different isomers of the C3H7 radical, where the latter follows
from the hydrogenation of C3 on grains followed by non-thermal desorption. A
gas-phase route involving the formation of (CH3)2COH+ through several
ion-neutral reactions followed by its dissociative recombination with electrons
do also contribute to the formation of acetone.Comment: Accepted for publication in A&
Astrochemical models of interstellar ices: History matters
Ice is ubiquitous in the interstellar medium. We model the formation of the
main constituents of interstellar ices, including H2O, CO2 , CO, and CH3 OH. We
strive to understand what physical or chemical parameters influence the final
composition of the ice and how they benchmark to what has already been
observed, with the aim of applying these models to the preparation and analysis
of JWST observations. We used the Nautilus gas-grain model, which computes the
gas and ice composition as a function of time for a set of physical conditions,
starting from an initial gas phase composition. All important processes
(gas-phase reactions, gas-grain interactions, and grain surface processes) are
included and solved with the rate equation approximation. We first ran an
astrochemical code for fixed conditions of temperature and density mapped in
the cold core L429-C to benchmark the chemistry. One key parameter was revealed
to be the dust temperature. When the dust temperature is higher than 12 K, CO2
will form efficiently at the expense of H2O, while at temperatures below 12 K,
it will not form. Whatever hypothesis we assumed for the chemistry (within
realistic conditions), the static simulations failed to reproduce the observed
trends of interstellar ices in our target core. In a second step, we simulated
the chemical evolution of parcels of gas undergoing different physical and
chemical situations throughout the molecular cloud evolution and starting a few
1e7 yr prior to the core formation (dynamical simulations). Our dynamical
simulations satisfactorily reproduce the main trends already observed for
interstellar ices. Moreover, we predict that the apparent constant ratio of
CO2/H2O observed to date is probably not true for regions of low AV , and that
the history of the evolution of clouds plays an essential role, even prior to
their formation.Comment: Accepted for publication in A&
The ALMA-PILS survey: First tentative detection of 3-hydroxypropenal (HOCHCHCHO) in the interstellar medium and chemical modeling of the CHO isomers
Characterizing the molecular composition of solar-type protostars is useful
for improving our understanding of the physico-chemical conditions under which
the Sun and its planets formed. In this work, we analyzed the Atacama Large
Millimeter/submillimeter Array (ALMA) data of the Protostellar Interferometric
Line Survey (PILS), an unbiased spectral survey of the solar-type protostar
IRAS~16293--2422, and we tentatively detected 3-hydroxypropenal (HOCHCHCHO) for
the first time in the interstellar medium towards source B. Based on the
observed line intensities and assuming local thermodynamic equilibrium, its
column density is constrained to be 10 cm, corresponding to
an abundance of 10 relative to methanol, CHOH. Additional
spectroscopic studies are needed to constrain the excitation temperature of
this molecule. We included HOCHCHCHO and five of its isomers in the chemical
network presented in Manigand et al. (2021) and we predicted their chemical
evolution with the Nautilus code. The model reproduces the abundance of
HOCHCHCHO within the uncertainties. This species is mainly formed through the
grain surface reaction CHCHO + HCO HCOCHCHO, followed by
the tautomerization of HCOCHCHO into HOCHCHCHO. Two isomers, CHCOCHO
and CHCOHCHO, are predicted to be even more abundant than HOCHCHCHO.
Spectroscopic studies of these molecules are essential in searching for them in
IRAS~16293--2422 and other astrophysical sources.Comment: Accepted in A&A Letter
Gromov-Hausdorff-like distance function defined in the aspect of Riemannian submanifold theory
In this paper, we discuss how a Gromov-Hausdorff-like distance function over
the space of all isometric classes of compact -Riemannian manifolds should
be defined in the aspect of the Riemannan submanifold theory, where .
The most important fact in this discussion is as follows. The Hausdorff
distance function between two spheres of mutually distinct radii isometrically
embedded into the hypebolic space of curvature converges to zero as
. The key in the construction of the Gromov-Hausdorff-like
distance function given in this paper is to define the distance of two
-isometric embeddings of distinct compact -Riemannian manifolds
into a higher dimensional Riemannian manifold by using the Hausdorff distance
function in the tangent bundle of order equipped with the Sasaki metric.
Furthermore, we show that the convergence of a sequence of compact Riemannian
manifolds with respect to this distance function coincides with the convergence
in the sense of R. S. Hamilton.Comment: 14 page
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