31 research outputs found
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&
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
Polycylcic Aromatic Hydrocarbons (PAH's) in dense cloud chemistry
Virtually all detailed gas-phase models of the chemistry of dense
interstellar clouds exclude polycyclic aromatic hydrocarbons (PAH's). This
omission is unfortunate because from the few studies that have been done on the
subject, it is known that the inclusion of PAH's can affect the gas-phase
chemistry strongly. We have added PAH's to our network to determine the role
they play in the chemistry of cold dense cores. In the models presented here,
we include radiative attachment to form PAH-, mutual neutralization between PAH
anions and small positively-charged ions, and photodetachment. We also test the
sensitivity of our results to changes in the size and abundance of the PAH's.
Our results confirm that the inclusion of PAH's changes many of the calculated
abundances of smaller species considerably. In TMC-1, the general agreement
with observations is significantly improved contrary to L134N. This may
indicate a difference in PAH properties between the two regions. With the
inclusion of PAH's in dense cloud chemistry, high-metal elemental abundances
give a satisfactory agreement with observations. As a result, we do not need to
decrease the observed elemental abundances of all metals and we do not need to
vary the elemental C/O ratio in order to produce large abundances of carbon
species in TMC-1 (CP).Comment: Accepted to ApJ. Astrophysical Journal (2008) accepte
A multi-transition HCN and HCO+ study of 12 nearby active galaxies: AGN versus SB environments
Recent studies have indicated that the HCN-to-CO(J=1-0) and
HCO+-to-HCN(J=1-0) ratios are significantly different between galaxies with AGN
(active galactic nucleus) and SB (starburst) signatures. In order to study the
molecular gas properties in active galaxies and search for differences between
AGN and SB environments, we observed the HCN(J=1-0), (J=2-1), (J=3-2),
HCO+(J=1-0) and HCO+(J=3-2), emission with the IRAM 30m in the centre of 12
nearby active galaxies which either exhibit nuclear SB and/or AGN signatures.
Consistent with previous results, we find a significant difference of the
HCN(J=2-1)-to-HCN(J=1-0), HCN(J=3-2)-to-HCN(J=1-0), HCO+(J=3-2)-to-HCO+(J=3-2),
and HCO+-to-HCN intensity ratios between the sources dominated by an AGN and
those with an additional or pure central SB: the HCN, HCO+ and HCO+-to-HCN
intensity ratios tend to be higher in the galaxies of our sample with a central
SB as opposed to the pure AGN cases which show rather low intensity ratios.
Based on an LVG analysis of these data, i.e., assuming purely collisional
excitation, the (average) molecular gas densities in the SB dominated sources
of our sample seem to be systematically higher than in the AGN sources. The LVG
analysis seems to further support systematically higher HCN and/or lower HCO+
abundances as well as similar or higher gas temperatures in AGN compared to the
SB sources of our sample. Also, we find that the HCN-to-CO ratios decrease with
increasing rotational number J for the AGN while they stay mostly constant for
the SB sources.Comment: accepted for publication in ApJ; 20 pages, 7 figures; in emulateApJ
forma
Dense Molecular Gas Associated with the Circumnuclear Star Forming Ring in the Barred Spiral Galaxy NGC 6951
We present high resolution (3" - 5") observations of CO(1-0) and HCN(1-0)
emission from the circumnuclear star forming ring in the barred spiral galaxy
NGC 6951, a host of a type-2 Seyfert, using the Nobeyama Millimeter Array and
45 m telescope. We find that most of the HCN emission is associated with the
circumnuclear ring, where vigorous star formation occurs. The HCN to CO
integrated intensity ratio is also enhanced in the star forming ring; the peak
value of HCN/CO ratio is 0.18, which is comparable to the ratio in the
starbursts NGC 253 and M82. The formation mechanism of dense molecular gas has
been investigated. We find that the shocks along the orbit crowding do not
promote the formation of the dense molecular gas effectively but enhance the
presence of low density GMCs. Instead, gravitational instabilities of the gas
can account for the dense molecular gas formation. The HCN/CO ratio toward the
Seyfert nucleus of NGC 6951 is a rather normal value (0.086), in contrast with
other Seyferts NGC 1068 and M51 where extremely high HCN/CO value of ~ 0.5 have
been reported.Comment: 33 pages, 17 figures, to appear in the Astrophysical Journa
Incorporation of stochastic chemistry on dust grains in the PDR code using moment equations
Unlike gas-phase reactions, chemical reactions taking place on interstellar
dust grain surfaces cannot always be modeled by rate equations. Due to the
small grain sizes and low flux,these reactions may exhibit large fluctuations
and thus require stochastic methods such as the moment equations.
We evaluate the formation rates of H2, HD and D2 molecules on dust grain
surfaces and their abundances in the gas phase under interstellar conditions.
We incorporate the moment equations into the Meudon PDR code and compare the
results with those obtained from the rate equations. We find that within the
experimental constraints on the energy barriers for diffusion and desorption
and for the density of adsorption sites on the grain surface, H2, HD and D2
molecules can be formed efficiently on dust grains.
Under a broad range of conditions, the moment equation results coincide with
those obtained from the rate equations. However, in a range of relatively high
grain temperatures, there are significant deviations. In this range, the rate
equations fail while the moment equations provide accurate results. The
incorporation of the moment equations into the PDR code can be extended to
other reactions taking place on grain surfaces
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
The effect of the initial elemental abundance on gas-grain chemical models
For any chemical modeling, it is important to recognize that the
adopted set of
initial elemental abundances is a crucial parameter.
The effect of initial abundance variation has been investigated.
Using the most recent observations and theoretical grain models,
we have set some constraints
upon the set of the initial elemental abundances. Both gas-phase and
gas-grain chemical models are used in this study.
At early-time stages less than 1 Myr, there is little difference
between results with different initial [C]/[O] ratios. This holds
for gas-phase and gas-grain models. At a later evolutionary time
or in the steady state, the result of the gas-grain model shows
little or no dependence on the initial [C]/[O] ratios. By
contrast, at late or steady-state times, the abundances of
chemical species using gas-phase models are very sensitive to any
variation of the initial [C]/[O] ratios. Sulfur depletion is
needed for both gas-phase and gas-grain models to reproduce the
observed sulfur-bearing molecules.
Our main conclusion is that the gas-grain interaction processes
such as accretion, surface reactions, and desorption minimize the
vital role of the initial set of elemental abundance in
gas-grain chemical models