1,270 research outputs found
The origin of the molecular emission around the southern hemisphere Re 4 IRS - HH 188 region
We present SEST observations of the molecular environment ahead of the
southern Herbig-Haro object 188 (HH188), associated with the low-mass protostar
Re4 IRS. We have also used the SuperCosmos Halpha survey to search for Halpha
emission associated with the Re4 IRS - HH188 region. The aim of the present
work is to study the properties of the molecular gas and to better characterize
this southern star forming region. We mapped the HCO+ 3-2 and H13CO+ 1-0
emission around the YSO and took spectra of the CH3OH 2(0)-1(0)A+ and
2(-1)-1(-1)E and SO 6(5)-5(4) towards the central source. Column densities are
derived and different scenarios are considered to explain the origin of the
molecular emission. HCO+ arises from a relatively compact region around the
YSO; however, its peak emission is displaced to the south following the outflow
direction. Our chemical analysis indicates that a plausible scenario is that
most of the emission arises from the cold, illuminated dense gas ahead of the
HH188 object. We have also found that HH188, a high excitation object, seems to
be part of a parsec scale and highly collimated HH system. Re4 IRS is probably
a binary protostellar system, in the late Class 0 or Class I phase. One of the
protostars, invisible in the near-IR, seems to power the HH188 system.Comment: 9 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
Chemistry of dense clumps near moving Herbig-Haro objects
Localised regions of enhanced emission from HCO+, NH3 and other species near
Herbig-Haro objects (HHOs) have been interpreted as arising in a photochemistry
stimulated by the HHO radiation on high density quiescent clumps in molecular
clouds. Static models of this process have been successful in accounting for
the variety of molecular species arising ahead of the jet; however recent
observations show that the enhanced molecular emission is widespread along the
jet as well as ahead. Hence, a realistic model must take into account the
movement of the radiation field past the clump. It was previously unclear as to
whether the short interaction time between the clump and the HHO in a moving
source model would allow molecules such as HCO+ to reach high enough levels,
and to survive for long enough to be observed. In this work we model a moving
radiation source that approaches and passes a clump. The chemical picture is
qualitatively unchanged by the addition of the moving source, strengthening the
idea that enhancements are due to evaporation of molecules from dust grains. In
addition, in the case of several molecules, the enhanced emission regions are
longer-lived. Some photochemically-induced species, including methanol, are
expected to maintain high abundances for ~10,000 years.Comment: 7 pages, 3 figure
Modelling the ArH emission from the Crab Nebula
We have performed combined photoionization and photodissociation region (PDR)
modelling of a Crab Nebula filament subjected to the synchrotron radiation from
the central pulsar wind nebula, and to a high flux of charged particles; a
greatly enhanced cosmic ray ionization rate over the standard interstellar
value, , is required to account for the lack of detected [C I]
emission in published Herschel SPIRE FTS observations of the Crab Nebula. The
observed line surface brightness ratios of the OH and ArH transitions
seen in the SPIRE FTS frequency range can only be explained with both a high
cosmic ray ionization rate and a reduced ArH dissociative recombination
rate compared to that used by previous authors, although consistent with
experimental upper limits. We find that the ArH/OH line strengths and
the observed H vibration-rotation emission can be reproduced by model
filaments with cm,
and visual extinctions within the range found for dusty globules in the Crab
Nebula, although far-infrared emission from [O I] and [C II] is higher than the
observational constraints. Models with cm
underpredict the H surface brightness, but agree with the ArH and
OH surface brightnesses and predict [O I] and [C II] line ratios consistent
with observations. These models predict HeH rotational emission above
detection thresholds, but consideration of the formation timescale suggests
that the abundance of this molecule in the Crab Nebula should be lower than the
equilibrium values obtained in our analysis.Comment: Accepted by MNRAS. Author accepted manuscript. Accepted on
05/09/2017. Deposited on 05/09/1
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
High Framerate Imaging of Ultrasound Contrast Agents
Ultrasound contrast agents (UCAs) consists of a suspension of tiny gas bubbles that is injected into a patient's bloodstream to enhance the visualization of blood in ultrasound images. As UCAs respond differently to ultrasound pulses compared to the surrounding soft tissues and blood, it is possible to employ specialized techniques to identify and isolate UCAs in an ultrasound image. This is commonly referred to as Ultrasound Contrast Imaging.
This PhD thesis evaluates several traditional ultrasound contrast imaging strategies, based on scanning images through linear arrays; furthermore, innovative high frame rate strategies are explored, which are shown to be suited for high sensitivity tracking of even a single microbubble
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&
Desorption From Interstellar Ices
The desorption of molecular species from ice mantles back into the gas phase
in molecular clouds results from a variety of very poorly understood processes.
We have investigated three mechanisms; desorption resulting from H_2 formation
on grains, direct cosmic ray heating and cosmic ray induced photodesorption.
Whilst qualitative differences exist between these processes (essentially
deriving from the assumptions concerning the species-selectivity of the
desorption and the assumed threshold adsorption energies, E_t) all three
processes are found to be potentially very significant in dark cloud
conditions. It is therefore important that all three mechanisms should be
considered in studies of molecular clouds in which freeze-out and desorption
are believed to be important.
Employing a chemical model of a typical static molecular core and using
likely estimates for the quantum yields of the three processes we find that
desorption by H_2 formation probably dominates over the other two mechanisms.
However, the physics of the desorption processes and the nature of the dust
grains and ice mantles are very poorly constrained. We therefore conclude that
the best approach is to set empirical constraints on the desorption, based on
observed molecular depletions - rather than try to establish the desorption
efficiencies from purely theoretical considerations. Applying this method to
one such object (L1689B) yields upper limits to the desorption efficiencies
that are consistent with our understanding of these mechanisms.Comment: 11 pages, 5 figures, accepted by MNRAS subject to minor revision
which has been carried ou
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
History-independent tracers. Forgetful molecular probes of the physical conditions of the dense interstellar medium
Molecular line emission is a powerful probe of the physical conditions of astrophysical objects but can be complex to model, and it is often unclear which transitions would be the best targets for observers who wish to constrain a given parameter. We therefore produce a list of molecular species for which the gas history can be ignored, removing a major modelling complexity. We then determine the best of these species to observe when attempting to constrain various physical parameters. To achieve this, we use a large set of chemical models with different chemical histories to determine which species have abundances at 1 MYr that are insensitive to the initial conditions. We then use radiative transfer modelling to produce the intensity of every transition of these molecules. We finally compute the mutual information between the physical parameters and all transitions and transition ratios in order to rank their usefulness in determining the value of a given parameter. We find 48 species that are insensitive to the chemical history of the gas, 23 of which have collisional data available. We produce a ranked list of all the transitions and ratios of these species using their mutual information with various gas properties. We show mutual information is an adequate measure of how well a transition can constrain a physical parameter by recovering known probes and demonstrating that random forest regression models become more accurate predictors when high-scoring features are included. Therefore, this list can be used to select target transitions for observations in order to maximize knowledge about those physical parameters
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