892 research outputs found
Refit to numerically problematic UMIST reaction rate coefficients
Aims. Chemical databases such as the UMIST Database for Astrochemistry (UDFA)
are indispensable in the numerical modeling of astrochemical networks. Several
of the listed reactions in the UDFA have properties that are problematic in
numerical computations: Some are parametrized in a way that leads to extremely
divergent behavior for low kinetic temperatures. Other reactions possess
multiple entries that are each valid in a different temperature regime, but
have no smooth transition when switching from one to another. Numerically, this
introduces many difficulties.We present corrected parametrizations for these
sets of reactions in the UDFA06 database.
Methods. From the tabulated parametrization in UDFA, we created artificial
data points and used a Levenberg-Marquardt algorithm to find a set of improved
fit parameters without divergent behavior for low temperatures. For reactions
with multiple entries in the database that each possess a different temperature
regime, we present one joint parametrization that is designed to be valid over
the whole cumulative temperature range of all individual reactions.
Results. We show that it is possible to parametrize numerically problematic
reactions from UDFA in a form that avoids low temperature divergence.
Additionally, we demonstrate that it is possible to give a collective
parametrization for reaction rate coefficients of reactions with multiple
entries in UDFA. We present these new fitted values in tabulated form.Comment: accepted by A&
Carbon Fractionation in PDRs
We upgraded the chemical network from the UMIST Database for Astrochemistry
2006 to include isotopes such as ^{13}C and ^{18}O. This includes all
corresponding isotopologues, their chemical reactions and the properly scaled
reaction rate coefficients. We study the fractionation behavior of
astrochemically relevant species over a wide range of model parameters,
relevant for modelling of photo-dissociation regions (PDRs). We separately
analyze the fractionation of the local abundances, fractionation of the total
column densities, and fractionation visible in the emission line ratios. We
find that strong C^+ fractionation is possible in cool C^+ gas. Optical
thickness as well as excitation effects produce intensity ratios between 40 and
400. The fractionation of CO in PDRs is significantly different from the
diffuse interstellar medium. PDR model results never show a fractionation ratio
of the CO column density larger than the elemental ratio. Isotope-selective
photo-dissociation is always dominated by the isotope-selective chemistry in
dense PDR gas. The fractionation of C, CH, CH^+, and HCO^+ is studied in
detail, showing that the fractionation of C, CH and CH^+ is dominated by the
fractionation of their parental species. The light hydrides chemically derive
from C^+, and, consequently, their fractionation state is coupled to that of
C^+. The fractionation of C is a mixed case depending on whether formation from
CO or HCO^+ dominates. Ratios of the emission lines of [C II], [C I], ^{13}CO,
and H^{13}CO^+ provide individual diagnostics to the fractionation status of
C^+, C, and CO.Comment: to be published in A&
Metallicity Effects in PDRs
Almost all properties of a photodissociation region (PDR) depend on its
metallicity. The heating and cooling efficiencies that determine the
temperature of the gas and dust, the dust composition, as well as the elemental
abundances that influence the chemical structure of the PDR are just three
examples that demonstrate the importance of metallicity effects in PDRs. PDRs
are often associated with sites of star formation. If we want to understand the
star formation history of our own Galaxy and of distant low-metallicity objects
we need to understanding how metallicity acts on PDR physics and chemistry.Comment: 7 pages, 5 figures, to appear in proceedings of "Far-Infrared and
Submillimeter Emission of the Interstellar Medium", EAS Publication Series,
Bad Honnef, November 2007, Eds. C. Kramer, S. Aalto, R. Simo
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