40 research outputs found
Chemical sensitivity to the ratio of the cosmic-ray ionization rates of He and H2 in dense clouds
Aim: To determine whether or not gas-phase chemical models with homogeneous
and time-independent physical conditions explain the many observed molecular
abundances in astrophysical sources, it is crucial to estimate the
uncertainties in the calculated abundances and compare them with the observed
abundances and their uncertainties. Non linear amplification of the error and
bifurcation may limit the applicability of chemical models. Here we study such
effects on dense cloud chemistry. Method: Using a previously studied approach
to uncertainties based on the representation of rate coefficient errors as log
normal distributions, we attempted to apply our approach using as input a
variety of different elemental abundances from those studied previously. In
this approach, all rate coefficients are varied randomly within their log
normal (Gaussian) distribution, and the time-dependent chemistry calculated
anew many times so as to obtain good statistics for the uncertainties in the
calculated abundances. Results: Starting with so-called ``high-metal''
elemental abundances, we found bimodal rather than Gaussian like distributions
for the abundances of many species and traced these strange distributions to an
extreme sensitivity of the system to changes in the ratio of the cosmic ray
ionization rate zeta\_He for He and that for molecular hydrogen zeta\_H2. The
sensitivity can be so extreme as to cause a region of bistability, which was
subsequently found to be more extensive for another choice of elemental
abundances. To the best of our knowledge, the bistable solutions found in this
way are the same as found previously by other authors, but it is best to think
of the ratio zeta\_He/zeta\_H2 as a control parameter perpendicular to the
''standard'' control parameter zeta/n\_H.Comment: Accepted for publicatio
Maple procedures for the coupling of angular momenta. VI. LS-jj transformations
Transformation matrices between different coupling schemes are required, if a
reliable classification of the level structure is to be obtained for open-shell
atoms and ions. While, for instance, relativistic computations are
traditionally carried out in jj-coupling, a LSJ coupling notation often occurs
much more appropriate for classifying the valence-shell structure of atoms.
Apart from the (known) transformation of single open shells, however, further
demand on proper transformation coefficients has recently arose from the study
of open d- and f-shell elements, the analysis of multiple--excited levels, or
the investigation on inner-shell phenomena. Therefore, in order to facilitate a
simple access to LS jj transformation matrices, here we present an
extension to the Racah program for the set-up and the transformation of
symmetry-adapted functions. A flexible notation is introduced for defining and
for manipulating open-shell configurations at different level of complexity
which can be extended also to other coupling schemes and, hence, may help
determine an optimum classification of atomic levels and processes in the
future
Identification of IMDC intermediate-risk subgroups in patients with metastatic clear-cell renal cell carcinoma (ccRCC).
e16577Background: Majority of patients (pts) with ccRCC at first line (1L) treatment are classified in the IR subgroup according to International Metastatic Renal Cell Carcinoma Database Consortium..
SiO line emission from C-type shock waves : interstellar jets and outflows
We study the production of SiO in the gas phase of molecular outflows,
through the sputtering of Si--bearing material in refractory grain cores, which
are taken to be olivine; we calculate also the rotational line spectrum of the
SiO. The sputtering is driven by neutral particle impact on charged grains, in
steady--state C-type shock waves, at the speed of ambipolar diffusion. The
emission of the SiO molecule is calculated by means of an LVG code. A grid of
models has been generated. We compare our results with those of an earlier
study (Schilke et al. 1997). Improvements in the treatment of the coupling
between the charged grains and the neutral fluid lead to narrower shock waves
and lower fractions of Si being released into the gas phase. More realistic
assumptions concerning the initial fractional abundance of O2 lead to SiO
formation being delayed, so that it occurs in the cool, dense postshock flow.
Good agreement is obtained with recent observations of SiO line intensities in
the L1157 and L1448 molecular outflows. The inferred temperature, opacity, and
SiO column density in the emission region differ significantly from those
estimated by means of LVG `slab' models. The fractional abundance of SiO is
deduced. Observed line profiles are wider than predicted and imply multiple,
unresolved shock regions within the beam.Comment: 1 tex doc, 19 figure
On the Influence of Uncertainties in Chemical Reaction Rates on Results of the Astrochemical Modelling
With the chemical reaction rate database UMIST95 (Millar et al. 1997) we
analyze how uncertainties in rate constants of gas-phase chemical reactions
influence the modelling of molecular abundances in the interstellar medium.
Random variations are introduced into the rate constants to estimate the
scatter in theoretical abundances. Calculations are performed for dark and
translucent molecular clouds where gas phase chemistry is adequate. Similar
approach was used by Pineau des Forets & Roueff (2000) for the study of
chemical bistability. All the species are divided into 6 sensitivity groups
according to the value of the scatter in their model abundances computed with
varied rate constants. It is shown that the distribution of species within
these groups depends on the number of atoms in a molecule and on the adopted
physical conditions. The simple method is suggested which allows to single out
reactions that are most important for the evolution of a given species.Comment: 4 pages. To appear in the proceedings of the 4th Cologne-Bonn Zermatt
Symposiu
Using ordinal logistic regression to evaluate the performance of laser-Doppler predictions of burn-healing time
Background
Laser-Doppler imaging (LDI) of cutaneous blood flow is beginning to be used by burn surgeons to predict the healing time of burn wounds; predicted healing time is used to determine wound treatment as either dressings or surgery. In this paper, we do a statistical analysis of the performance of the technique.
Methods
We used data from a study carried out by five burn centers: LDI was done once between days 2 to 5 post burn, and healing was assessed at both 14 days and 21 days post burn. Random-effects ordinal logistic regression and other models such as the continuation ratio model were used to model healing-time as a function of the LDI data, and of demographic and wound history variables. Statistical methods were also used to study the false-color palette, which enables the laser-Doppler imager to be used by clinicians as a decision-support tool.
Results
Overall performance is that diagnoses are over 90% correct. Related questions addressed were what was the best blood flow summary statistic and whether, given the blood flow measurements, demographic and observational variables had any additional predictive power (age, sex, race, % total body surface area burned (%TBSA), site and cause of burn, day of LDI scan, burn center). It was found that mean laser-Doppler flux over a wound area was the best statistic, and that, given the same mean flux, women recover slightly more slowly than men. Further, the likely degradation in predictive performance on moving to a patient group with larger %TBSA than those in the data sample was studied, and shown to be small.
Conclusion
Modeling healing time is a complex statistical problem, with random effects due to multiple burn areas per individual, and censoring caused by patients missing hospital visits and undergoing surgery. This analysis applies state-of-the art statistical methods such as the bootstrap and permutation tests to a medical problem of topical interest. New medical findings are that age and %TBSA are not important predictors of healing time when the LDI results are known, whereas gender does influence recovery time, even when blood flow is controlled for.
The conclusion regarding the palette is that an optimum three-color palette can be chosen 'automatically', but the optimum choice of a 5-color palette cannot be made solely by optimizing the percentage of correct diagnoses
The optical spectroscopy of extraterrestrial molecules
The ongoing quest to identify molecules in the interstellar medium by their
electronic spectra in the visible region is reviewed. Identification of
molecular absorption is described in the context of the elucidation of the
carriers of the unidentified diffuse interstellar bands while molecular
emission is discussed with reference to the unidentified Red Rectangle bands.
The experimental techniques employed in undertaking studies on the optical
spectroscopy of extraterrestrial molecules are described and critiqued in the
context of their application.Comment: 21 pages, 9 figures, Invited review Australian Journal of Chemistry,
accepted for publicatio
Metastatic chromophobe renal cell carcinoma treated with targeted therapies: A Renal Cross Channel Group study
Treatment of non–clear cell renal cell carcinoma (RCC) remains controversial despite several recent prospective studies of targeted therapies (TT). Often Vascular Endothelial growth Factor (VEGF) and Mammalian Target of Rapamycin (mTOR) inhibitors are used, extrapolating the data from use of these agents in clear cell RCC.
We performed a retrospective data analysis within the Renal Cross Channel Group to determine metastatic chromophobe RCC (mChRCC) outcomes in the TT era. The end-points were overall response, overall survival (OS) and time to treatment failure (TTF). The two latter were estimated using the Kaplan–Meier method.
91 mChRCC patients from 26 centres were included. Median follow-up from the date of first metastasis was 6.1 years (range: 0–13.9). Median OS was 37.9 months (95% confidence interval [CI]: 21.4–46.8) from the diagnosis of metastatic disease. Among the 61 patients who received TT, 50 (82%) were treated with anti-angiogenic (AA) and 11 with mTOR inhibitors. Median TTF and OS in patients receiving a first line of AA was 8.7 months (95% CI: 5.2–10.9) and 22.9 months (95% CI: 17.8–49.2) versus 1.9 months (95% CI: 1.0–6.0) and 3.2 months (95% CI: 2.3–not evaluable) with mTOR inhibitors, respectively. A stratified log-rank test was used to compare AA and mTOR inhibitors TT, while controlling the effect of the International Metastatic RCC Database Consortium risk group and no significant difference between AA and mTOR inhibitors was observed for TTF (p = 0.26) or for OS (p = 0.55).
We report the largest retrospective cohort of patients with mChRCC treated with TT and no significant difference between AA and mTOR inhibitors was observed for TTF and OS