1,485 research outputs found
First evidence for molecular interfaces between outflows and ambient clouds in high-mass star-forming regions?
We present new observations of the Cep A East region of massive star formation and describe an extended and dynamically distinct feature not previously recognized. This feature is present in emission from H2CS, OCS, CH3OH, and HDO at −5.5 km s−1 but is not traced by the conventional tracers of star-forming regions, H2S, SO2, SO, and CS. The feature is extended up to at least 0.1 pc. We show that the feature is neither a hot core nor a shocked outflow. However, the chemistry of the feature is consistent with predictions from a model of an eroding interface between a fast wind and a dense core; mixing between the two media occurs in the interface on a timescale of 10–50 yr. If these observations are confirmed by detailed maps and by detections in species also predicted to be abundant (e.g., HCO+, H2CO, and NH3), this feature would be the first detection of such an interface in regions of massive star formation. An important implication of the model is that a significant reservoir of sulfur in grain mantles is required to be in the form of OCS
The molecular condensations ahead of Herbig-Haro objects. II: a theoretical investigation of the HH 2 condensation
Clumps of enhanced molecular emission are present close to a number of Herbig-Haro (HH) objects. These enhancements may be the consequence of an active photochemistry driven by the UV radiation originating from the shock front of the HH object. On the basis of this picture and as a follow up to a molecular line survey toward the quiescent molecular clump ahead of the HH object, HH 2 (Girart et al. 2002), we present a detailed time and depth dependent chemical model of the observed clump. Despite several difficulties in matching the observations, we constrain some of the physical and chemical parameters of the clump ahead of HH 2. In particular, we find that the clump is best described by more than one density component with a peak density of 3 × 105 cm-3 and a visual extinction of ≤3.5 mag; its lifetime can not be much higher than 100 years and the impinging radiation is enhanced with respect to the ambient one by probably no more than 3 orders of magnitude. Our models also indicate that carbon-bearing species should not completely hydrogenate as methane when freezing out on grains during the formation of the clump
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&
Modelling the sulphur chemistry evolution in Orion KL
We study the sulphur chemistry evolution in the Orion KL along the gas and
grain phases of the cloud. We investigate the processes that dominate the
sulphur chemistry and to determine how physical and chemical parameters, such
as the final star mass and the initial elemental abundances, influence the
evolution of the hot core and of the surrounding outflows and shocked gas (the
plateau). We independently modelled the chemistry evolution of both components
using the time-dependent gas-grain model UCL_CHEM and considering two different
phase calculations. Phase I starts with the collapsing cloud and the depletion
of atoms and molecules onto grain surfaces. Phase II starts when a central
protostar is formed and the evaporation from grains takes place. We show how
the gas density, the gas depletion efficiency, the initial sulphur abundance,
the shocked gas temperature and the different chemical paths on the grains
leading to different reservoirs of sulphur on the mantles affect
sulphur-bearing molecules at different evolutionary stages. We also compare the
predicted column densities with those inferred from observations of the species
SO, SO2, CS, OCS, H2S and H2CS. The models that reproduce the observations of
the largest number of sulphur-bearing species are those with an initial sulphur
abundance of 0.1 times the sulphur solar abundance and a density of at least
n_H=5x10^6 cm^-3 in the shocked gas region. We conclude that most of the
sulphur atoms were ionised during Phase I, consistent with an inhomogeneous and
clumpy region where the UV interstellar radiation penetrates leading to sulphur
ionisation. We also conclude that the main sulphur reservoir on the ice mantles
was H2S. In addition, we deduce that a chemical transition currently takes
place in the shocked gas, where SO and SO2 gas-phase formation reactions change
from being dominated by O2 to being dominated by OH.Comment: 14 pages, 28 figures, 6 table
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 clumpiness of molecular clouds: HCO+ (3--2) survey near Herbig-Haro objects
Some well-studied Herbig Haro objects have associated with them one or more
cold, dense, and quiescent clumps of gas. We propose that such clumps near an
HH object can be used as a general measure of clumpiness in the molecular cloud
that contains that HH object. Our aim is to make a survey of clumps around a
sample of HH objects, and to use the results to make an estimate of the
clumpiness in molecular clouds. All known cold, dense, and quiescent clumps
near HH objects are anomalously strong HCO+ emitters. Our method is, therefore,
to search for strong HCO+ emission as an indicator of a clump near to an HH
object. The searches were made using JCMT and SEST in the HCO+ 3-2 and also
H13CO+ 1-0 lines, with some additional searches for methanol and sulphur
monoxide lines. The sources selected were a sample of 22 HH objects in which no
previous HCO+ emission had been detected. We find that half of the HH objects
have clumps detected in the HCO+ 3-2 line and that all searches in H13CO$+ 1-0
lines show evidence of clumpiness. All condensations have narrow linewidths and
are evidently unaffected dynamically by the HH jet shock. We conclude that the
molecular clouds in which these HH objects are found must be highly
heterogeneous on scales of less than 0.1 pc. An approximate calculation based
on these results suggests that the area filling factor of clumps affected by HH
objects is on the order of 10%. These clumps have gas number densities larger
than 3e4 cm-2.Comment: 11 pages, 14 figures. Accepted for publication in Astronomy and
Astrophysic
An Alternative Accurate Tracer of Molecular Clouds: The "-Factor"
We explore the utility of CI as an alternative high-fidelity gas mass tracer
for Galactic molecular clouds. We evaluate the X-factor for the 609
m carbon line, the analog of the CO X-factor, which is the ratio of the
H column density to the integrated CO(1-0) line intensity. We use
3D-PDR to post-process hydrodynamic simulations of turbulent, star-forming
clouds. We compare the emission of CI and CO for model clouds irradiated by 1
and 10 times the average background and demonstrate that CI is a comparable or
superior tracer of the molecular gas distribution for column densities up to cm. Our results hold for both reduced and full chemical
networks. For our fiducial Galactic cloud we derive an average of
cmKkms and of cmKkms.Comment: 5 pages, 4 figures, 1 table, accepted to MNRAS Letter
The Molecular Condensations Ahead of Herbig-Haro Objects. III. Radiative and dynamical perturbations of the HH 2 condensation
We have carried out an extensive observational study (from BIMA data) and
made a preliminary theoretical investigation of the molecular gas around HH2.
The molecular maps show a very complex morphological, kinematical and chemical
structure. The overall main conclusion of this work confirms the findings of
Paper I and II, by demonstrating that in addition to the strong photochemical
effects caused by penetration of the UV photons from HH2 into molecular cloud,
a range of complex radiative and dynamical interactions occur. Thus, despite
the apparent `quiescent' nature of the molecular cloud ahead of HH2, the
kinematical properties observed within the field of view suggest that it is
possibly being driven out by powerful winds from the VLA 1 protostar.Comment: 20 pages. Accepted for publication to Astronomy & Astrophysic
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