1,193 research outputs found
Chemical chronology of the Southern Coalsack
We demonstrate how the observed H2O ice column densities toward three dense
globules in the Southern Coalsack could be used to constrain the ages of these
sources. We derive ages of ~10^5 yr, in agreement with dynamical studies of
these objects. We have modelled the chemical evolution of the globules, and
show how the molecular abundances are controlled by both the gas density and
the initial chemical conditions as the globules formed. Based on our derived
ages, we predict the column densities of several species of interest. These
predictions should be straightforward to test by performing molecular line
observationsComment: 10 pages, 4 figures, in press at MNRA
A new modified-rate approach for gas-grain chemical simulations
Understanding grain-surface processes is crucial to interpreting the
chemistry of the ISM. However, accurate surface chemistry models are
computationally expensive and are difficult to integrate with gas-phase
simulations. A new modified-rate method for solving grain-surface chemical
systems is presented. Its purpose is accurately to model highly complex systems
that can otherwise only be treated using the sometimes inadequate rate-equation
approach. In contrast to previous rate-modification techniques, the functional
form of the surface production rates was modified, and not simply the rate
coefficient. This form is appropriate to the extreme "small-grain" limit, and
can be verified using an analytical master-equation approach. Various further
modifications were made to this basic form, to account for competition between
processes, to improve estimates of surface occupation probabilities, and to
allow a switch-over to the normal rate equations where these are applicable.
The new method was tested against systems solved previously using exact
techniques. Even the simplest method is quite accurate, and a great improvement
over rate equations. Further modifications allow the master-equation results to
be reproduced exactly for the methanol-producing system, within computational
accuracy. Small discrepancies arise when non-zero activation energies are
assumed for the methanol system, which result from complex reaction-competition
processes that cannot be resolved easily without using exact methods.
Inaccuracies in computed abundances are never greater than a few tens of
percent, and typically of the order of one percent, in the most complex systems
tested. Implementation of the method in simple networks, including
hydrogen-only systems, is trivial, whilst the results are highly accurate.Comment: Accepted for publication in Astronomy & Astrophysics. 14 pages, 5
figures, 7 table
A Dynamical Study of the Non-Star Forming Translucent Molecular Cloud MBM16: Evidence for Shear Driven Turbulence in the Interstellar Medium
We present the results of a velocity correlation study of the high latitude
cloud MBM16 using a fully sampled CO map, supplemented by new CO
data. We find a correlation length of 0.4 pc. This is similar in size to the
formaldehyde clumps described in our previous study. We associate this
correlated motion with coherent structures within the turbulent flow. Such
structures are generated by free shear flows. Their presence in this non-star
forming cloud indicates that kinetic energy is being supplied to the internal
turbulence by an external shear flow. Such large scale driving over long times
is a possible solution to the dissipation problem for molecular cloud
turbulence.Comment: Uses AAS aasms4.sty macros. Accepted for publication in Ap
Family History of Cancer and Tobacco Exposure in Index Cases of Pancreatic Ductal Adenocarcinoma
Aim. To examine interaction between history of cancer in first-degree relatives and tobacco smoking in index patients of pancreatic adenocarcinoma.
Methods. We carried out a case-control involving 113 patients with pancreatic adenocarcinoma and 110 controls over a 12-month period at the Freeman Hospital, Newcastle upon Tyne, UK. They were all administered a detailed tobacco exposure questionnaire and a family history questionnaire. We calculated cumulative tobacco exposure and risk for pancreas cancer.
Results. Both smokers (OR 3.01 (95% CI: 1.73 to 5.24)) and those with a family history of malignancy (OR 1.98 (95% CI: 1.15–3.38)) were more likely to develop pancreatic cancer. Having more than one first-degree relative with cancer did not significantly further increase the risk of pancreatic cancer. Amongst pancreatic cancer cases, cumulative tobacco exposure was significantly decreased (P = .032) in the group of smokers (current and ex-smokers) who had a family history of malignancy [mean (SD): 30.00 (24.77) pack-years versus 44.69 (28.47) pack-years with no such history].
Conclusions. Individuals with a family history of malignancy are at an increased risk of pancreatic cancer. Furthermore, individuals with a family history of malignancy and who smoke appear to require a lesser degree of tobacco exposure for the development of pancreatic cancer
Exact results for hydrogen recombination on dust grain surfaces
The recombination of hydrogen in the interstellar medium, taking place on
surfaces of microscopic dust grains, is an essential process in the evolution
of chemical complexity in interstellar clouds. The H_2 formation process has
been studied theoretically, and in recent years also by laboratory experiments.
The experimental results were analyzed using a rate equation model. The
parameters of the surface, that are relevant to H_2 formation, were obtained
and used in order to calculate the recombination rate under interstellar
conditions. However, it turned out that due to the microscopic size of the dust
grains and the low density of H atoms, the rate equations may not always apply.
A master equation approach that provides a good description of the H_2
formation process was proposed. It takes into account both the discrete nature
of the H atoms and the fluctuations in the number of atoms on a grain. In this
paper we present a comprehensive analysis of the H_2 formation process, under
steady state conditions, using an exact solution of the master equation. This
solution provides an exact result for the hydrogen recombination rate and its
dependence on the flux, the surface temperature and the grain size. The results
are compared with those obtained from the rate equations. The relevant length
scales in the problem are identified and the parameter space is divided into
two domains. One domain, characterized by first order kinetics, exhibits high
efficiency of H_2 formation. In the other domain, characterized by second order
kinetics, the efficiency of H_2 formation is low. In each of these domains we
identify the range of parameters in which, the rate equations do not account
correctly for the recombination rate. and the master equation is needed.Comment: 23 pages + 8 figure
On the nature of the enigmatic object IRAS 19312+1950: A rare phase of massive star formation?
IRAS 19312+1950 is a peculiar object that has eluded firm characterization
since its discovery, with combined maser properties similar to an evolved star
and a young stellar object (YSO). To help determine its true nature, we
obtained infrared spectra of IRAS 19312+1950 in the range 5-550 m using
the Herschel and Spitzer space observatories. The Herschel PACS maps exhibit a
compact, slightly asymmetric continuum source at 170 m, indicative of a
large, dusty circumstellar envelope. The far-IR CO emission line spectrum
reveals two gas temperature components: of material at
K, and of material at K. The OI 63
m line is detected on-source but no significant emission from atomic ions
was found. The HIFI observations display shocked, high-velocity gas with
outflow speeds up to 90 km s along the line of sight. From Spitzer
spectroscopy, we identify ice absorption bands due to HO at 5.8 m and
CO at 15 m. The spectral energy distribution is consistent with a
massive, luminous () central source surrounded by a
dense, warm circumstellar disk and envelope of total mass
-, with large bipolar outflow cavities. The combination
of distinctive far-IR spectral features suggest that IRAS 19312+1950 should be
classified as an accreting high-mass YSO rather than an evolved star. In light
of this reclassification, IRAS 19312+1950 becomes only the 5th high-mass
protostar known to exhibit SiO maser activity, and demonstrates that 18 cm OH
maser line ratios may not be reliable observational discriminators between
evolved stars and YSOs.Comment: 16 pages. Accepted for publication in Ap
Comparative chemistry of diffuse clouds III: sulfur-bearing molecules
Using data from IRAM's Plateau de Bure Interferometer and 30 m Telescope, we
discuss the mm-wave absorption lines of CS, SO, H2S and HCS+ which arise in
diffuse clouds occulting several extragalactic continuum sources. Typical
relative abundances are X(CS)/X(HCO+) ~ 2, X(CS)/X(SO) ~ 2, X(CS)/X(H2S) ~ 6
and X(CS)/X(HCS+) ~ 13.Comment: Accepted by A&A 2002-Jan-1
Soluble E-Cadherin: An Early Marker of Severity in Acute Pancreatitis
Background/Aims. At present, there is no simple test for predicting severity in acute pancreatitis. We investigated the use of an assay of soluble E-cadherin (sE-cadherin). Methods. Concentrations of sE-cadherin, from 19 patients with mild acute pancreatitis, 7 patients with severe acute pancreatitis, 11 patients with other acute gastrointestinal pathologies, and 12 healthy subjects were measured using a commercially available sandwich ELISA kit based on two monoclonal antibodies specific to the extracellular fragment of human E-cadherin. Measurements were made at 12 hours or less from onset of pain and also at 24 and 48 hours after onset of pain. Results. Mean (standard deviation) concentration of sE-cadherin in patients with severe acute pancreatitis at <12 hours was 17780 ng/mL (7853), significantly higher than that of healthy volunteers 5180 ng/mL (1350), P = .0039, patients with other gastrointestinal pathologies 7358 ng/mL (6655), P = .0073, and also significantly higher than that of patients with mild pancreatitis, 7332 ng/mL (2843), P = .0019. Discussion. Serum sE-cadherin could be an early (within 12 hours) objective marker of severity in acute pancreatitis. This molecule warrants further investigation in the form of a large multicentre trial
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