178 research outputs found
CS Lines Profiles in Hot Cores
We present a theoretical study of CS line profiles in archetypal hot cores.
We provide estimates of line fluxes from the CS(1-0) to the CS(15-14)
transitions and present the temporal variation of these fluxes. We find that
\textit{i)} the CS(1-0) transition is a better tracer of the Envelope of the
hot core whereas the higher-J CS lines trace the ultra-compact core;
\textit{ii)} the peak temperature of the CS transitions is a good indicator of
the temperature inside the hot core; \textit{iii)} in the Envelope, the older
the hot core the stronger the self-absorption of CS; \textit{iv)} the
fractional abundance of CS is highest in the innermost parts of the
ultra-compact core, confirming the CS molecule as one of the best tracers of
very dense gas.Comment: 17 pages, 5 figures, 1 table, In press in Ap
Chemical tracers of high-metallicity environments
We present for the first time a detailed study of the properties of molecular
gas in metal-rich environments such as early-type galaxies (ETGs). We have
explored Photon-Dominated Region (PDR) chemistry for a wide range of physical
conditions likely to be appropriate for these sources. We derive fractional
abundances of the 20 most chemically reactive species as a function of the
metallicity, as a function of the optical depth and for various volume number
gas densities, Far-Ultra Violet (FUV) radiation fields and cosmic ray
ionisation rates. We also investigate the response of the chemistry to the
changes in element enhancement as seen in ETGs. We find that the
fractional abundances of CS, HS, HCS, HO, HO,
HCO and HCN seem invariant to an increase of metallicity whereas
C, CO, CH, CN, HCN, HNC and OCS appear to be the species most
sensitive to this change. The most sensitive species to the change in the
fractional abundance of elements are C, C, CN, HCN, HNC, SO,
SO, HO and CS. Finally, we provide line brightness ratios for the
most abundant species, especially in the range observable with ALMA. Discussion
of favorable line ratios to use for the estimation of super-solar metallicities
and -elements are also provided.Comment: 15 pages, 6 figures, 4 tables, Accepted for publication into MNRA
Mapping CS in Starburst Galaxies: Disentangling and Characterising Dense Gas
Aims. We observe the dense gas tracer CS in two nearby starburst galaxies to
determine how the conditions of the dense gas varies across the circumnuclear
regions in starburst galaxies. Methods. Using the IRAM-30m telescope, we mapped
the distribution of the CS(2-1) and CS(3-2) lines in the circumnuclear regions
of the nearby starburst galaxies NGC 3079 and NGC 6946. We also detected the
formaldehyde (H2CO) and methanol (CH3OH) in both galaxies. We marginally detect
the isotopologue C34S. Results. We calculate column densities under LTE
conditions for CS and CH3OH. Using the detections accumulated here to guide our
inputs, we link a time and depth dependent chemical model with a molecular line
radiative transfer model; we reproduce the observations, showing how conditions
where CS is present are likely to vary away from the galactic centres.
Conclusions. Using the rotational diagram method for CH3OH, we obtain a lower
limit temperature of 14 K. In addition to this, by comparing the chemical and
radiative transfer models to observations, we determine the properties of the
dense gas as traced by CS (and CH3OH). We also estimate the quantity of the
dense gas. We find that, provided that there are a between 10^5 and 10^6 dense
cores in our beam, for both target galaxies, emission of CS from warm (T = 100
- 400 K), dense (n(H2) = 10^5-6 cm-3) cores, possibly with a high cosmic ray
ionisation rate (zeta = 100 zeta0) best describes conditions for our central
pointing. In NGC 6946, conditions are generally cooler and/or less dense
further from the centre, whereas in NGC 3079, conditions are more uniform. The
inclusion of shocks allows for more efficient CS formation, leading to an order
of magnitude less dense gas being required to replicate observations in some
cases.Comment: 14 pages, 10 figures, accepted to A&
Deuterated species in extragalactic star-forming regions
We present a theoretical study of the deuterated species detectability in
various types of extragalactic star-forming regions based on our predictions of
chemical abundances. This work is motivated by the past and current attempts at
observing deuterated species in external galaxies such as NGC~253, IC~342 and
the LMC. Here, we investigate the influence of the density, the temperature,
the FUV radiation field, the cosmic ray ionisation, and the metallicity on the
fractional abundances and D/H abundance ratios of about 20 deuterated species.
Without modelling any particular source, we determined how the deuterium
chemistry behaves in different physical environments such as starburst,
cosmic-rays enhanced environments, low metallicity and high redshift galaxies.
In general, our predicted column densities seem in good agreement with those
derived from the current limited dataset of observations in external galaxies.
We provide, for the first time, a list of key deuterated species whose
abundances are high enough to be possibly detectable by the Atacama Large
Millimeter Array (ALMA) and Herschel, as a function of galactic nuclear
activity and redshift.Comment: 19 pages, 6 figures, 5 tables, In press in Ap
Extragalactic CS survey
We present a coherent and homogeneous multi-line study of the CS molecule in
nearby (D10Mpc) galaxies. We include, from the literature, all the available
observations from the to the transitions towards NGC 253, NGC
1068, IC 342, Henize~2-10, M~82, the Antennae Galaxies and M~83. We have, for
the first time, detected the CS(7-6) line in NGC 253, M~82 (both in the
North-East and South-West molecular lobes), NGC 4038, M~83 and tentatively in
NGC 1068, IC 342 and Henize~2-10. We use the CS molecule as a tracer of the
densest gas component of the ISM in extragalactic star-forming regions,
following previous theoretical and observational studies by Bayet et al.
(2008a,b and 2009). In this first paper out of a series, we analyze the CS data
sample under both Local Thermodynamical Equilibrium (LTE) and non-LTE (Large
Velocity Gradient-LVG) approximations. We show that except for M~83 and Overlap
(a shifted gas-rich position from the nucleus NGC 4039 in the Antennae
Galaxies), the observations in NGC 253, IC 342, M~82-NE, M~82-SW and NGC 4038
are not well reproduced by a single set of gas component properties and that,
at least, two gas components are required. For each gas component, we provide
estimates of the corresponding kinetic temperature, total CS column density and
gas density.Comment: 17 pages, 16 figures, 3 tables, Accepted to Ap
The influence of cosmic rays in the circumnuclear molecular gas of NGC1068
We surveyed the circumnuclear disk of the Seyfert galaxy NGC1068 between the
frequencies 86.2 GHz and 115.6 GHz, and identified 17 different molecules.
Using a time and depth dependent chemical model we reproduced the observational
results, and show that the column densities of most of the species are better
reproduced if the molecular gas is heavily pervaded by a high cosmic ray
ionization rate of about 1000 times that of the Milky Way. We discuss how
molecules in the NGC1068 nucleus may be influenced by this external radiation,
as well as by UV radiation fields.Comment: 6 pages. Conference proceeding for the workshop on "Cosmic-ray
induced phenomenology in star-forming environments" held in Sant Cugat,
Spain, on April 16-19, 201
Chemical complexity in NGC1068
We aimed to study the chemistry of the circumnuclear molecular gas of
NGC1068, and to compare it with those of the starburst galaxies M82 and NGC253.
Using the IRAM-30m telescope, we observed the inner 2 kpc of NGC1068 between
86.2 GHz and 115.6 GHz. We identified 35 spectral features, corresponding to 24
different molecular species. Among them, HC3N, SO, N2H+, CH3CN, NS, 13CN, and
HN13C are detected for the first time in NGC1068. Assuming local thermodynamic
equilibrium (LTE), we calculated the column densities of the detected
molecules, as well as the upper limits to the column densities of some
undetected species. The comparison among the chemistries of NGC1068, M82, and
NGC253, suggests that, apart from X-rays, shocks also determine the chemistry
of NGC1068. We propose the column density ratio between CH3CCH and HC3N as a
prime indicator of the imprints of starburst and AGN environments in the
circumnuclear interstellar medium. This ratio is, at least, 64 times larger in
M82 than in NGC1068, and, at least, 4 times larger in NGC253 than in NGC1068.
Finally, we used the UCL_CHEM and UCL_PDR chemical codes to constrain the
origin of the species, as well as to test the influence of UV radiation fields
and cosmic rays on the observed abundances.Comment: 8 pages, 2 figures, 3 tables. Proceedings of the "The Central
Kiloparsec in Galactic Nuclei", 29 August - 2 September 2011, Bad Honnef,
German
Molecular tracers of PDR-dominated galaxies
Photon-dominated regions (PDRs) are powerful molecular line emitters in
external galaxies. They are expected in galaxies with high rates of massive
star formation due to either starburst (SB) events or starburst coupled with
active galactic nuclei (AGN) events. We have explored the PDR chemistry for a
range of physical conditions representing a variety of galaxy types. Our main
result is a demonstration of the sensitivity of the chemistry to changes in the
physical conditions. We adopt crude estimates of relevant physical parameters
for several galaxy types and use our models to predict suitable molecular
tracers of those conditions. The set of recommended molecular tracers differs
from that which we recommended for use in galaxies with embedded massive stars.
Thus, molecular observations can in principle be used to distinguish between
excitation by starburst and by SB+AGN in distant galaxies. Our recommendations
are intended to be useful in preparing Herschel and ALMA proposals to identify
sources of excitation in galaxies.Comment: 18 pages, 6 figures, Accepted in Ap
Chemistry in Cosmic-Ray Dominated Regions (CRDRs)
Molecular line observations may serve as diagnostics of the degree to which
the number density of cosmic ray protons, having energies of 10s to 100s of
MeVs each, is enhanced in starburst galaxies and galaxies with active nuclei.
Results, obtained with the UCL\_PDR code, for the fractional abundances of
molecules as functions of the cosmic-ray induced ionisation rate, , are
presented. The aim is not to model any particular external galaxies. Rather, it
is to identify characteristics of the dependencies of molecular abundances on
, in part to enable the development of suitable observational programmes
for cosmic ray dominated regions (CRDRs) which will then stimulate detailed
modelling. For a number density of hydrogen nuclei of of cm, and
high visual extinction, the fractional abundances of some species increase as
increases to s, but for much higher values of
the fractional abundances of all molecular species are significantly below
their peak values. We show in particular that OH, HO, H,
HO and OH attain large fractional abundances () for as large as s. HCO is a poor
tracer of CRDRs when s. Sulphur-bearing species may
be useful tracers of CRDRs gas in which s. Ammonia
has a large fractional abundance for s and
nitrogen appears in CN-bearing species at significant levels as
increases, even up to s. In this paper, we also discuss
our model predictions, comparing them to recent detections in both galactic and
extragalactic sources. We show that they agree well, to a first approximation,
with the observational constraints.Comment: 11 pages, 4 figures, 2 tables, Accepted to MNRAS publicatio
Are CO lines good indicators of the star formation rate in galaxies?
In this paper, we investigate the relevance of using the CO line
emissions as indicators of star formation rates (SFR). For the first time, we
present this study for a relatively large number of CO transitions (12)
as well as over a large interval in redshift (from z0 to z6). For
the nearby sources (D10 Mpc), we have used homogeneous sample of
CO data provided by Bayet et al. (2004, 2006), mixing observational and
modelled line intensities. For higher-z sources (z 1), we have collected
CO observations from various papers and have completed the data set of
line intensities with model predictions which we also present in this paper.
Finally, for increasing the statistics, we have included recent CO(1-0)
and CO(3-2) observations of intermediate-z sources. Linear regressions
have been calculated for identifying the tightest SFR-CO line luminosity
relationships. We show that the \emph{total} CO, the CO(5-4), the
CO(6-5) and the CO(7-6) luminosities are the best indicators of
SFR (as measured by the far-infrared luminosity). Comparisons with theoretical
approaches from Krumholz and Thompson (2007) and Narayanan et al. (2008) are
also performed in this paper. Although in general agreement, the predictions
made by these authors and the observational results we present here show small
and interesting discrepancies. In particular, the slope of the linear
regressions, for J 4 CO lines are not similar between
theoretical studies and observations. On one hand, a larger high-J CO
data set of observations might help to better agree with models, increasing the
statistics. On the other hand, theoretical studies extended to high redshift
sources might also reduce such discrepancies.Comment: 10 pages, 3 figures and 4 tables, Accepted in MNRA
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