71 research outputs found
Grain Surface Chemistry: Modified Models
The rate equation approach to the chemistry occurring on grain surfaces in interstellar clouds has been criticized for not taking the discrete nature of grains into account. Indeed, investigations of simple models show that results obtained from rate equations can be significantly different from results obtained by a Monte Carlo procedure. Some modifications of the rate equations have been proposed that have the effect of eliminating most of the differences with the Monte Carlo procedure for simplified models of interstellar clouds at temperatures of 10 K and slightly above. In this study we investigate the use of the modified rate equations in more realistic chemical models of dark interstellar clouds with complex gas-grain interactions. Our results show some discrepancies between the results of models with unmodified and modified rate equations; these discrepancies are highly dependent, however, on the initial form of hydrogen chosen. If the initial form is mainly molecular, at early stages of cloud evolution there are some significant differences in calculated molecular abundances on grains, but at late times the two sets of results tend to converge for the main components of the grain mantles. If the initial form is atomic hydrogen, there are essentially no differences in results between models based on the unmodified rate equations and those based on the modified rate equations, except for the abundances on grains of some minor complex molecules. Thus, the major results of previous gas-grain models of cold, dark interstellar clouds remain at least partially intact
Moment equations for chemical reactions on interstellar dust grains
While most chemical reactions in the interstellar medium take place in the
gas phase, those occurring on the surfaces of dust grains play an essential
role. Chemical models based on rate equations including both gas phase and
grain surface reactions have been used in order to simulate the formation of
chemical complexity in interstellar clouds. For reactions in the gas phase and
on large grains, rate equations, which are highly efficient to simulate, are an
ideal tool. However, for small grains under low flux, the typical number of
atoms or molecules of certain reactive species on a grain may go down to order
one or less. In this case the discrete nature of the opulations of reactive
species as well as the fluctuations become dominant, thus the mean-field
approximation on which the rate equations are based does not apply. Recently, a
master equation approach, that provides a good description of chemical
reactions on interstellar dust grains, was proposed. Here we present a related
approach based on moment equations that can be obtained from the master
equation. These equations describe the time evolution of the moments of the
distribution of the population of the various chemical species on the grain. An
advantage of this approach is the fact that the production rates of molecular
species are expressed directly in terms of these moments. Here we use the
moment equations to calculate the rate of molecular hydrogen formation on small
grains. It is shown that the moment equation approach is efficient in this case
in which only a single reactive specie is involved. The set of equations for
the case of two species is presented and the difficulties in implementing this
approach for complex reaction networks involving multiple species are
discussed.Comment: 12 pages, submitted for publication in A&
NMA Survey of CO and HCN Emission from Nearby Active Galaxies
High resolution (a few arcseconds) observations of CO(1-0) and HCN(1-0)
emission from nearby Seyfert galaxies have been conducted with the Nobeyama
Millimeter Array. Based on the observed CO distributions and kinematics,we
suggest that a small scale (a few 100 pc - a few kpc) distortion of the
underlying potential seems to be necessary for Seyfert activity, although it is
not a sufficient condition. We also find that the Toomre's Q values in the
centers of Seyfert galaxies tend to be larger than unity, indicating the
circumnuclear molecular gas disks around Seyfert nuclei would be
gravitationally stable. The HCN/CO integrated intensity ratios (R_HCN/CO) range
over an order of magnitude, from 0.086 to 0.6. The Seyfert galaxies with high
R_HCN/CO may have an extended (r ~ 100 pc scale) envelope of obscuring
material. The presence of kpc scale jet/ outflow might be also related to the
extremely high R_HCN/CO.Comment: To appear in the Proceedings of the 3rd Cologne-Zermatt Symposium,
``The Physics and Chemistry of the Interstellar Medium'
Characterising the high-mass star forming region IRAS 18144-1723 through methanol maser observations
A Unified Monte Carlo Treatment of Gas-Grain Chemistry for Large Reaction Networks. I. Testing Validity of Rate Equations in Molecular Clouds
In this study we demonstrate for the first time that the unified Monte Carlo
approach can be applied to model gas-grain chemistry in large reaction
networks. Specifically, we build a time-dependent gas-grain chemical model of
the interstellar medium, involving about 6000 gas-phase and 200 grain surface
reactions. This model is used to test the validity of the standard and modified
rate equation methods in models of dense and translucent molecular clouds and
to specify under which conditions the use of the stochastic approach is
desirable.
We found that at temperatures 25--30 K gas-phase abundances of HO,
NH, CO and many other gas-phase and surface species in the stochastic model
differ from those in the deterministic models by more than an order of
magnitude, at least, when tunneling is accounted for and/or diffusion energies
are 3x lower than the binding energies. In this case, surface reactions,
involving light species, proceed faster than accretion of the same species. In
contrast, in the model without tunneling and with high binding energies, when
the typical timescale of a surface recombination is greater than the timescale
of accretion onto the grain, we obtain almost perfect agreement between results
of Monte Carlo and deterministic calculations in the same temperature range. At
lower temperatures ( K) gaseous and, in particular, surface abundances
of most important molecules are not much affected by stochastic processes.Comment: 33 pages, 9 figures, 1 table. Accepted for publication in Ap
Enhanced production of HD and D_2 molecules on small dust grains in diffuse clouds
Motivated by recent observations of deuterated molecules in the interstellar
medium, we examine the production of HD and D molecules on dust grain
surfaces. A mechanism for the enhanced production of these deuterated molecules
is studied. This mechanism applies on grain surfaces on which D atoms stick
more strongly than H atoms, under conditions of low flux and within a suitable
range of temperatures. It is shown that under these conditions the production
rates of HD and D are greatly enhanced (vs. the H production rate)
compared with the expected rates based on the adsorption of gas-phase atomic
abundances of D and H. The enhancement in the formation rate of HD is
comparable with the enhancement due to gas-phase ion-molecule reactions in
diffuse clouds.Comment: This is a preprint of an article accepted for publication in Monthly
Notices of The Royal Astromnomical Societ
Chemical sensitivity to the ratio of the cosmic-ray ionization rates of He and H2 in dense clouds
Aim: To determine whether or not gas-phase chemical models with homogeneous
and time-independent physical conditions explain the many observed molecular
abundances in astrophysical sources, it is crucial to estimate the
uncertainties in the calculated abundances and compare them with the observed
abundances and their uncertainties. Non linear amplification of the error and
bifurcation may limit the applicability of chemical models. Here we study such
effects on dense cloud chemistry. Method: Using a previously studied approach
to uncertainties based on the representation of rate coefficient errors as log
normal distributions, we attempted to apply our approach using as input a
variety of different elemental abundances from those studied previously. In
this approach, all rate coefficients are varied randomly within their log
normal (Gaussian) distribution, and the time-dependent chemistry calculated
anew many times so as to obtain good statistics for the uncertainties in the
calculated abundances. Results: Starting with so-called ``high-metal''
elemental abundances, we found bimodal rather than Gaussian like distributions
for the abundances of many species and traced these strange distributions to an
extreme sensitivity of the system to changes in the ratio of the cosmic ray
ionization rate zeta\_He for He and that for molecular hydrogen zeta\_H2. The
sensitivity can be so extreme as to cause a region of bistability, which was
subsequently found to be more extensive for another choice of elemental
abundances. To the best of our knowledge, the bistable solutions found in this
way are the same as found previously by other authors, but it is best to think
of the ratio zeta\_He/zeta\_H2 as a control parameter perpendicular to the
''standard'' control parameter zeta/n\_H.Comment: Accepted for publicatio
HCN J=5-4 Emission in APM08279+5255 at z=3.91
We detect HCN J=5-4 emission from the ultraluminous quasar APM08279+5255 at
z=3.911 using the IRAM Plateau de Bure interferometer. This object is strongly
gravitationally lensed, yet still thought to be one of the most intrinsically
luminous objects in the Universe. The new data imply a line luminosity
L'_HCN(J=5-4) = 4.0+/-0.5 x 10^(10) K km/s pc^2. The ~440 km/s full width half
maximum of the HCN J=5-4 line matches that of the previously observed high-J CO
lines in this object and suggests that the emission from both species emerges
from the same region: a warm, dense circumnuclear disk. Simple radiative
transfer models suggest an enhanced abundance of HCN relative to CO in the
nuclear region of APM08279+5255, perhaps due to increased ionization, or
possibly the selective depletion of oxygen. The ratio of far-infrared
luminosity to HCN luminosity is at the high end of the range found for nearby
star forming galaxies, but comparable to that observed in the few high redshift
objects detected in the HCN J=1-0 line. This is the first clear detection of
high-J HCN emission redshifted into the 3-millimeter atmospheric window.Comment: Accepted for publication in ApJ
The effect of grain size distribution on H formation rate in the interstellar medium
The formation of molecular hydrogen in the interstellar medium takes place on
the surfaces of dust grains. Hydrogen molecules play a role in gas-phase
reactions that produce other molecules, some of which serve as coolants during
gravitational collapse and star formation. Thus, the evaluation of the
roduction rate of hydrogen molecules and its dependence on the physical
conditions in the cloud are of great importance. Interstellar dust grains
exhibit a broad size distribution in which the small grains capture most of the
surface area. Recent studies have shown that the production efficiency strongly
depends on the grain composition and temperature as well as on its size. In
this paper we present a formula which provides the total production rate of
H per unit volume in the cloud, taking into account the grain composition
and temperature as well as the grain size distribution. The formula agrees very
well with the master equation results. It shows that for a physically relevant
range of grain temperatures, the production rate of H is significantly
enhanced due to their broad size distribution.Comment: to appear in MNRA
- …