373 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
Modelling clumpy PDRs in 3D - Understanding the Orion Bar stratification
Context. Models of photon-dominated regions (PDRs) still fail to fully
reproduce some of the observed properties, in particular the combination of the
intensities of different PDR cooling lines together with the chemical
stratification, as observed e.g. for the Orion Bar PDR. Aims. We aim to
construct a numerical PDR model, KOSMA-\tau 3D, to simulate full spectral cubes
of line emission from arbitrary PDRs in three dimensions (3D). The model is to
reproduce the intensity of the main cooling lines from the Orion Bar PDR and
the observed layered structure of the different transitions. Methods. We build
up a 3D compound, made of voxels ("3D pixels") that contain a discrete mass
distribution of spherical "clumpy" structures, approximating the fractal ISM.
To analyse each individual clump the new code is combined with the KOSMA-\tau
PDR model. Probabilistic algorithms are used to calculate the local FUV flux
for each voxel as well as the voxel-averaged line emissivities and optical
depths, based on the properties of the individual clumps. Finally, the
computation of the radiative transfer through the compound provides full
spectral cubes. To test the new model we try to simulate the structure of the
Orion Bar PDR and compare the results to observations from HIFI/Herschel and
from the Caltech Submillimetre Observatory (CSO). In this context new Herschel
data from the HEXOS guaranteed-time key program is presented. Results. Our
model is able to reproduce the line integrated intensities within a factor 2.5
and the observed stratification pattern within 0.016 pc for the [Cii] 158 \mu m
and different 12/13 CO and HCO+ transitions, based on the representation of the
Orion Bar PDR by a clumpy edge-on cavity wall. In the cavity wall, a large
fraction of the total mass needs to be contained in clumps. The mass of the
interclump medium is constrained by the FUV penetration. Furthermore, ...Comment: Major changes compared to v1. Also several references have been adde
Modeling and Analysis Generic Interface for eXternal numerical codes (MAGIX)
The modeling and analysis generic interface for external numerical codes
(MAGIX) is a model optimizer developed under the framework of the coherent set
of astrophysical tools for spectroscopy (CATS) project. The MAGIX package
provides a framework of an easy interface between existing codes and an
iterating engine that attempts to minimize deviations of the model results from
available observational data, constraining the values of the model parameters
and providing corresponding error estimates. Many models (and, in principle,
not only astrophysical models) can be plugged into MAGIX to explore their
parameter space and find the set of parameter values that best fits
observational/experimental data. MAGIX complies with the data structures and
reduction tools of ALMA (Atacama Large Millimeter Array), but can be used with
other astronomical and with non-astronomical data.Comment: 12 pages, 15 figures, 2 tables, paper is also available at
http://www.aanda.org/articles/aa/pdf/forth/aa20063-12.pd
Diagnostics of the Molecular Component of PDRs with Mechanical Heating
Context. Multitransition CO observations of galaxy centers have revealed that
significant fractions of the dense circumnuclear gas have high kinetic
temperatures, which are hard to explain by pure photon excitation, but may be
caused by dissipation of turbulent energy.
Aims. We aim to determine to what extent mechanical heating should be taken
into account while modelling PDRs. To this end, the effect of dissipated
turbulence on the thermal and chemical properties of PDRs is explored. Methods.
Clouds are modelled as 1D semi-infinite slabs whose thermal and chemical
equilibrium is solved for using the Leiden PDR-XDR code.
Results. In a steady-state treatment, mechanical heating seems to play an
important role in determining the kinetic temperature of the gas in molecular
clouds. Particularly in high-energy environments such as starburst galaxies and
galaxy centers, model gas temperatures are underestimated by at least a factor
of two if mechanical heating is ignored. The models also show that CO, HCN and
H2 O column densities increase as a function of mechanical heating. The HNC/HCN
integrated column density ratio shows a decrease by a factor of at least two in
high density regions with n \sim 105 cm-3, whereas that of HCN/HCO+ shows a
strong dependence on mechanical heating for this same density range, with
boosts of up to three orders of magnitude.
Conclusions. The effects of mechanical heating cannot be ignored in studies
of the molecular gas excitation whenever the ratio of the star formation rate
to the gas density is close to, or exceeds, 7 \times 10-6 M yr-1 cm4.5 . If
mechanical heating is not included, predicted column densities are
underestimated, sometimes even by a few orders of magnitude. As a lower bound
to its importance, we determined that it has non-negligible effects already
when mechanical heating is as little as 1% of the UV heating in a PDR.Comment: 26 pages, 14 figures in the text and 13 figures as supplementary
material. Accepted for publication in A&
Study of Photon Dominated Regions in Cepheus B
Aim: The aim of the paper is to understand the emission from the photon
dominated regions in Cepheus B, estimate the column densities of neutral carbon
in bulk of the gas in Cepheus B and to derive constraints on the factors which
determine the abundance of neutral carbon relative to CO. Methods: This paper
presents 15'x15' fully sampled maps of CI at 492 GHz and 12CO 4-3 observed with
KOSMA at 1' resolution. The new observations have been combined with the FCRAO
12CO 1-0, IRAM-30m 13CO 2-1 and C18O 1-0 data, and far-infrared continuum data
from HIRES/IRAS. The KOSMA-tau spherical PDR model has been used to understand
the CI and CO emission from the PDRs in Cepheus B and to explain the observed
variation of the relative abundances of both C^0 and CO. Results: The emission
from the PDR associated with Cepheus B is primarily at V_LSR between -14 and
-11 km s^-1. We estimate about 23% of the observed CII emission from the
molecular hotspot is due to the ionized gas in the HII region. Over bulk of the
material the C^0 column density does not change significantly, (2.0+-1.4)x10^17
cm^-2, although the CO column density changes by an order of magnitude. The
observed \cbyco abundance ratio varies between 0.06 and 4 in Cepheus B. We find
an anti-correlation of the observed C/CO abundance ratio with the observed
hydrogen column density, which holds even when all previous observations
providing C/CO ratios are included. Here we show that this observed variation
of C/CO abundance with total column density can be explained only by clumpy
PDRs consisting of an ensemble of clumps. At high H2 column densities high mass
clumps, which exhibit low C/CO abundance, dominate, while at low column
densities, low mass clumps with high C/CO abundance dominate.Comment: 12 pages, 10 figures, Accepted for publication in A&
The Photon Dominated Region in the IC 348 molecular cloud
In this paper we discuss the physical conditions of clumpy nature in the IC
348 molecular cloud.
We combine new observations of fully sampled maps in [C I] at 492 GHz and
12CO 4--3, taken with the KOSMA 3 m telescope at about 1' resolution, with
FCRAO data of 12CO 1--0, 13CO 1--0 and far-infrared continuum data observed by
HIRES/IRAS. To derive the physical parameters of the region we analyze the
three different line ratios. A first rough estimate of abundance is obtained
from an LTE analysis. To understand the [C I] and CO emission from the PDRs in
IC 348, we use a clumpy PDR model. With an ensemble of identical clumps, we
constrain the total mass from the observed absolute intensities. Then we apply
a more realistic clump distribution model with a power law index of 1.8 for
clump-mass spectrum and a power law index of 2.3 for mass-size relation.
We provide detailed fits to observations at seven representative positions in
the cloud, revealing clump densities between 4 10 cm and 4
10 cm and C/CO column density ratios between 0.02 and 0.26. The
derived FUV flux from the model fit is consistent with the field calculated
from FIR continuum data, varying between 2 and 100 Draine units across the
cloud. We find that both an ensemble of identical clumps and an ensemble with a
power law clump mass distribution produce line intensities which are in good
agreement (within a factor ~ 2) with the observed intensities. The models
confirm the anti-correlation between the C/CO abundance ratio and the hydrogen
column density found in many regions.Comment: 11 pages, 8 figures, accepted by A&
A clumpy-cloud PDR model of the global far-infrared line emission of the Milky Way
The fractal structure of the interstellar medium suggests that the
interaction of UV radiation with the ISM as described in the context of
photon-dominated regions (PDR) dominates most of the physical and chemical
conditions, and hence the far-infrared and submm emission from the ISM in the
Milky Way. We investigate to what extent the Galactic FIR line emission of the
important species CO, C, C+, and O, as observed by the Cosmic Background
Explorer (COBE) satellite can be modeled in the framework of a clumpy,
UV-penetrated cloud scenario. The far-infrared line emission of the Milky Way
is modeled as the emission from an ensemble of clumps with a power law clump
mass spectrum and mass-size relation with power-law indices consistent with the
observed ISM structure. The individual clump line intensities are calculated
using the KOSMA-tau PDR-model for spherical clumps. The model parameters for
the cylindrically symmetric Galactic distribution of the mass density and
volume filling factor are determined by the observed radial distributions. A
constant FUV intensity, in which the clumps are embedded, is assumed. We show
that this scenario can explain, without any further assumptions and within a
factor of about 2, the absolute FIR-line intensities and their distribution
with Galactic longitude as observed by COBE.Comment: 14 pages, 13 figures, accepted by A&A at the 7th of July, 200
Photon Dominated Regions in NGC 3603
Aims: We aim at deriving the excitation conditions of the interstellar gas as
well as the local FUV intensities in the molecular cloud surrounding NGC 3603
to get a coherent picture of how the gas is energized by the central stars.
Methods: The NANTEN2-4m submillimeter antenna is used to map the [CI] 1-0, 2-1
and CO 4-3, 7-6 lines in a 2' x 2' region around the young OB cluster NGC 3603
YC. These data are combined with C18O 2-1 data, HIRES-processed IRAS 60 and 100
micron maps of the FIR continuum, and Spitzer/IRAC maps. Results: The NANTEN2
observations show the presence of two molecular clumps located south-east and
south-west of the cluster and confirm the overall structure already found by
previous CS and C18O observations. We find a slight position offset of the peak
intensity of CO and [CI], and the atomic carbon appears to be further extended
compared to the molecular material. We used the HIRES far-infrared dust data to
derive a map of the FUV field heating the dust. We constrain the FUV field to
values of \chi = 3 - 6 \times 10^3 in units of the Draine field across the
clouds. Approximately 0.2 to 0.3 % of the total FUV energy is re-emitted in the
[CII] 158 {\mu}m cooling line observed by ISO. Applying LTE and escape
probability calculations, we derive temperatures (TMM1 = 43 K, TMM2 = 47 K),
column densities (N(MM1) = 0.9 \times 10^22 cm^-2, N(MM2) = 2.5 \times 10^22
cm^-2) and densities (n(MM1) = 3 \times 10^3 cm^-3, n(MM2) = 10^3 -10^4 cm^-3)
for the two observed molecular clumps MM1 and MM2. Conclusions: The cluster is
strongly interacting with the ambient molecular cloud, governing its structure
and physical conditions. A stability analysis shows the existence of
gravitationally collapsing gas clumps which should lead to star formation.
Embedded IR sources have already been observed in the outskirts of the
molecular cloud and seem to support our conclusions.Comment: 13 pages, 10 figures, accepted for publication by A&
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