61 research outputs found
Deuterium fractionation on interstellar grains studied with the direct master equation approach
We have studied deuterium fractionation on interstellar grains with the use
of an exact method known as the direct master equation approach. We consider
conditions pertinent to dense clouds at late times when the hydrogen is mostly
in molecular form and a large portion of the gas-phase carbon has already been
converted to carbon monoxide. Hydrogen, oxygen and deuterium atoms, as well as
CO molecules, are allowed to accrete on to dust particles and react there to
produce various stable molecules. The surface abundances, as well as the
abundance ratios between deuterated and normal isotopomers, are compared with
those calculated with the Monte Carlo approach. We find that the agreement
between the Monte Carlo and the direct master equation methods can be made as
close as desired. Compared with previous examples of the use of the direct
master equation approach, our present method is much more efficient. It should
now be possible to run large-scale gas-grain models in which the diffusive dust
chemistry is handled `exactly'.Comment: 7 pages, 3 figure
On the master equation approach to diffusive grain-surface chemistry: the H, O, CO system
We have used the master equation approach to study a moderately complex
network of diffusive reactions occurring on the surfaces of interstellar dust
particles. This network is meant to apply to dense clouds in which a large
portion of the gas-phase carbon has already been converted to carbon monoxide.
Hydrogen atoms, oxygen atoms, and CO molecules are allowed to accrete onto dust
particles and their chemistry is followed. The stable molecules produced are
oxygen, hydrogen, water, carbon dioxide (CO2), formaldehyde (H2CO), and
methanol (CH3OH). The surface abundances calculated via the master equation
approach are in good agreement with those obtained via a Monte Carlo method but
can differ considerably from those obtained with standard rate equations.Comment: 13 pages, 5 figure
A new modified-rate approach for gas-grain chemical simulations
Understanding grain-surface processes is crucial to interpreting the
chemistry of the ISM. However, accurate surface chemistry models are
computationally expensive and are difficult to integrate with gas-phase
simulations. A new modified-rate method for solving grain-surface chemical
systems is presented. Its purpose is accurately to model highly complex systems
that can otherwise only be treated using the sometimes inadequate rate-equation
approach. In contrast to previous rate-modification techniques, the functional
form of the surface production rates was modified, and not simply the rate
coefficient. This form is appropriate to the extreme "small-grain" limit, and
can be verified using an analytical master-equation approach. Various further
modifications were made to this basic form, to account for competition between
processes, to improve estimates of surface occupation probabilities, and to
allow a switch-over to the normal rate equations where these are applicable.
The new method was tested against systems solved previously using exact
techniques. Even the simplest method is quite accurate, and a great improvement
over rate equations. Further modifications allow the master-equation results to
be reproduced exactly for the methanol-producing system, within computational
accuracy. Small discrepancies arise when non-zero activation energies are
assumed for the methanol system, which result from complex reaction-competition
processes that cannot be resolved easily without using exact methods.
Inaccuracies in computed abundances are never greater than a few tens of
percent, and typically of the order of one percent, in the most complex systems
tested. Implementation of the method in simple networks, including
hydrogen-only systems, is trivial, whilst the results are highly accurate.Comment: Accepted for publication in Astronomy & Astrophysics. 14 pages, 5
figures, 7 table
Chemical telemetry of OH observed to measure interstellar magnetic fields
We present models for the chemistry in gas moving towards the ionization
front of an HII region. When it is far from the ionization front, the gas is
highly depleted of elements more massive than helium. However, as it approaches
the ionization front, ices are destroyed and species formed on the grain
surfaces are injected into the gas phase. Photodissociation removes gas phase
molecular species as the gas flows towards the ionization front. We identify
models for which the OH column densities are comparable to those measured in
observations undertaken to study the magnetic fields in star forming regions
and give results for the column densities of other species that should be
abundant if the observed OH arises through a combination of the liberation of
H2O from surfaces and photodissociation. They include CH3OH, H2CO, and H2S.
Observations of these other species may help establish the nature of the OH
spatial distribution in the clouds, which is important for the interpretation
of the magnetic field results.Comment: 11 pages, 2 figures, accepted by Astrophysics and Space Scienc
Formation of CO2 on a carbonaceous surface: a quantum chemical study
The formation of CO2 in the gas phase and on a polyaromatic hydrocarbon surface (coronene) via three possible pathways is investigated with density functional theory. Calculations show that the coronene surface catalyses the formation of CO2 on model grain surfaces. The addition of O-3 to CO is activated by 2530 K in the gas phase. This barrier is lowered by 253 K for the Eley-Rideal mechanism and 952 K for the hot-atom mechanism on the surface of coronene. Alternative pathways for the formation of CO2 are the addition of O-3 to the HCO radical, followed by dissociation of the HCO2 intermediate. The O + HCO addition is barrierless in the gas phase and on the surface and is more than sufficiently exothermic to subsequently cleave the H-C bond. The third mechanism, OH + CO addition followed by H removal from the energized HOCO intermediate, has a gas-phase exit barrier that is 1160 K lower than the entrance barrier. On the coronene surface, however, both barriers are almost equal. Because the HOCO intermediate can also be stabilized by energy dissipation to the surface, it is anticipated that for the surface reaction the adsorbed HOCO could be a long-lived intermediate. In this case, the stabilized HOCO intermediate could react, in a barrierless manner, with a hydrogen atom to form H-2 + CO2, HCO2H, or H2O + CO
Exact results for hydrogen recombination on dust grain surfaces
The recombination of hydrogen in the interstellar medium, taking place on
surfaces of microscopic dust grains, is an essential process in the evolution
of chemical complexity in interstellar clouds. The H_2 formation process has
been studied theoretically, and in recent years also by laboratory experiments.
The experimental results were analyzed using a rate equation model. The
parameters of the surface, that are relevant to H_2 formation, were obtained
and used in order to calculate the recombination rate under interstellar
conditions. However, it turned out that due to the microscopic size of the dust
grains and the low density of H atoms, the rate equations may not always apply.
A master equation approach that provides a good description of the H_2
formation process was proposed. It takes into account both the discrete nature
of the H atoms and the fluctuations in the number of atoms on a grain. In this
paper we present a comprehensive analysis of the H_2 formation process, under
steady state conditions, using an exact solution of the master equation. This
solution provides an exact result for the hydrogen recombination rate and its
dependence on the flux, the surface temperature and the grain size. The results
are compared with those obtained from the rate equations. The relevant length
scales in the problem are identified and the parameter space is divided into
two domains. One domain, characterized by first order kinetics, exhibits high
efficiency of H_2 formation. In the other domain, characterized by second order
kinetics, the efficiency of H_2 formation is low. In each of these domains we
identify the range of parameters in which, the rate equations do not account
correctly for the recombination rate. and the master equation is needed.Comment: 23 pages + 8 figure
Circuit dissection of the role of somatostatin in itch and pain
Stimuli that elicit itch are detected by sensory neurons that innervate the skin. This information is processed by the spinal cord; however, the way in which this occurs is still poorly understood. Here we investigated the neuronal pathways for itch neurotransmission, particularly the contribution of the neuropeptide somatostatin. We find that in the periphery, somatostatin is exclusively expressed in Nppb+ neurons, and we demonstrate that Nppb+somatostatin+ cells function as pruriceptors. Employing chemogenetics, pharmacology and cell-specific ablation methods, we demonstrate that somatostatin potentiates itch by inhibiting inhibitory dynorphin neurons, which results in disinhibition of GRPR+ neurons. Furthermore, elimination of somatostatin from primary afferents and/or from spinal interneurons demonstrates differential involvement of the peptide released from these sources in itch and pain. Our results define the neural circuit underlying somatostatin-induced itch and characterize a contrasting antinociceptive role for the peptide
Grain Surface Models and Data for Astrochemistry
AbstractThe cross-disciplinary field of astrochemistry exists to understand the formation, destruction, and survival of molecules in astrophysical environments. Molecules in space are synthesized via a large variety of gas-phase reactions, and reactions on dust-grain surfaces, where the surface acts as a catalyst. A broad consensus has been reached in the astrochemistry community on how to suitably treat gas-phase processes in models, and also on how to present the necessary reaction data in databases; however, no such consensus has yet been reached for grain-surface processes. A team of ∼25 experts covering observational, laboratory and theoretical (astro)chemistry met in summer of 2014 at the Lorentz Center in Leiden with the aim to provide solutions for this problem and to review the current state-of-the-art of grain surface models, both in terms of technical implementation into models as well as the most up-to-date information available from experiments and chemical computations. This review builds on the results of this workshop and gives an outlook for future directions
Non-thermal desorption from interstellar dust grains via exothermic surface reactions
Aims: The gas-phase abundance of methanol in dark quiescent cores in the
interstellar medium cannot be explained by gas-phase chemistry. In fact, the
only possible synthesis of this species appears to be production on the
surfaces of dust grains followed by desorption into the gas. Yet, evaporation
is inefficient for heavy molecules such as methanol at the typical temperature
of 10 K. It is necessary then to consider non-thermal mechanisms for
desorption. But, if such mechanisms are considered for the production of
methanol, they must be considered for all surface species. Methods: Our
gas-grain network of reactions has been altered by the inclusion of a
non-thermal desorption mechanism in which the exothermicity of surface addition
reactions is utilized to break the bond between the product species and the
surface. Our estimated rate for this process derives from a simple version of
classical unimolecular rate theory with a variable parameter only loosely
contrained by theoretical work. Results: Our results show that the chemistry of
dark clouds is altered slightly at times up to 10^6 yr, mainly by the
enhancement in the gas-phase abundances of hydrogen-rich species such as
methanol that are formed on grain surfaces. At later times, however, there is a
rather strong change. Instead of the continuing accretion of most gas-phase
species onto dust particles, a steady-state is reached for both gas-phase and
grain-surface species, with significant abundances for the former.
Nevertheless, most of the carbon is contained in an undetermined assortment of
heavy surface hydrocarbons. Conclusions: The desorption mechanism discussed
here will be better constrained by observational data on pre-stellar cores,
where a significant accretion of species such as CO has already occurred.Comment: 14 pages, 9 figures. Accepted by Astronomy & Astrophysic
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