Isotopic fractionation of carbon, deuterium and nitrogen : a full chemical study

Context. The increased sensitivity and high spectral resolution of millimeter telescopes allow the detection of an increasing number of isotopically substituted molecules in the interstellar medium. The 14N/ 15N ratio is difficult to measure directly for carbon containing molecules. Aims. We want to check the underlying hypothesis that the 13C/ 12C ratio of nitriles and isonitriles is equal to the elemental value via a chemical time dependent gas phase chemical model. Methods. We have built a chemical network containing D, 13C and 15N molecular species after a careful check of the possible fractionation reactions at work in the gas phase. Results. Model results obtained for 2 different physical conditions corresponding respectively to a moderately dense cloud in an early evolutionary stage and a dense depleted pre-stellar core tend to show that ammonia and its singly deuterated form are somewhat enriched in 15N, in agreement with observations. The 14N/ 15N ratio in N2H+ is found to be close to the elemental value, in contrast to previous models which obtain a significant enrichment, as we found that the fractionation reaction between 15N and N2H+ has a barrier in the entrance channel. The large values of the N2H+/15NNH+ and N2H+/ N15NH+ ratios derived in L1544 cannot be reproduced in our model. Finally we find that nitriles and isonitriles are in fact significantly depleted in 13C, questioning previous interpretations of observed C15N, HC15N and H15NC abundances from 13C containing isotopologues.Comment: 21 pages, 9 figures in the text, 3 Figures in the appendices. 7 tables in the text, 4 tables in the appendices. Accepted for publication by Astronomy Astrophysic

Cosmology with liquid mirror telescopes

Liquid mirrors provide an exciting means to obtain large optical telescopes for substantially lower costs than conventional technologies. The liquid mirror concept has been demonstrated in the lab with the construction of a diffraction limited 1.5 m mirror. The mirror surface, using liquid mercury, forms a perfect parabolic shape when the mirror cell is rotated at a uniform velocity. A liquid mirror must be able to support a heavy mercury load with minimal flexure and have a fundamental resonant frequency that is as high as possible, to suppress the amplitude of surface waves caused by small vibrations transmitted to the mirror. To minimize the transmission of vibrations to the liquid surface, the entire mirror rests on an air bearing. This necessitates the mirror cell being lightweight, due to the limited load capabilities of the air bearing. The mirror components must also have physical characteristics which minimize the effects of thermal expansion with ambient temperature fluctuations in the observatory. In addition, the 2.7 m mirror construction is designed so that the techniques used may be readily extended to the construction of large mirrors. To attain the goals of a lightweight, rigid mirror, a composite laminant construction was used. The mirror consists of a foam core cut to the desired parabolic shape, with an accuracy of a few mm. An aluminum hub serves as an anchor for the foam and skin, and allows precise centering of the mirror on the air bearing and drive system. Several plys of Kevlar, covered in an epoxy matrix, are then applied to the foam. A final layer of pure epoxy is formed by spin casting. This final layer is parabolic to within a fraction of a mm. An aluminum ring bonded to the circumference of the mirror retains the mercury, and incorporates stainless-steel hard-points for the attachment of balance weights

Multi-filter spectrophotometry simulations

To complement both the multi-filter observations of quasar environments described in these proceedings, as well as the proposed UBC 2.7 m Liquid Mirror Telescope (LMT) redshift survey, we have initiated a program of simulated multi-filter spectrophotometry. The goal of this work, still very much in progress, is a better quantitative assessment of the multiband technique as a viable mechanism for obtaining useful redshift and morphological class information from large scale multi-filter surveys

Multi-filter spectrophotometry of quasar environments

A many-filter photometric technique for determining redshifts and morphological types, by fitting spectral templates to spectral energy distributions, has good potential for application in surveys. Despite success in studies performed on simulated data, the results have not been fully reliable when applied to real, low signal-to-noise data. We are investigating techniques to improve the fitting process

Rate constants and Arrhenius parameters for the reactions of OH radicals and Cl atoms with CF3CH2OCHF2, CF3CHClOCHF2 and CF3CH2OCClF2, using the discharge-flow/resonance fluorescence method

Rate constants have been determined for the reactions of OH radicals and Cl atoms with the three partially halogenated methyl-ethyl ethers, CF$_3$CH$_2$OCHF$_2$, CF$_3$CHClOCHF$_2$ and CF$_3$CH$_2$OCClF$_2$, using discharge-flow techniques to generate the OH radicals and the Cl atoms and resonance fluorescence to observe changes in their relative concentrations in the presence of added ether. For each combination of radical and ether, experiments were carried out at three temperatures between 292 and 410 K, yielding the following Arrhenius expressions for the rate constants within this range of temperature: OH + CF$_3$CH$_2$OCHF$_2$: $k$ = (2.0$\pm$0.8) $\times$ 10$^{-11}$ exp( – 2110 $\pm$ 150 K / T) cm$^3$ molecule$^{-1}$ s$^{-1}$ OH + CF$_3$CHClOCHF$_2$: $k$ = (4.5 $\pm$ 1.3) $\times$ 10$^{-13}$ exp( – 940 $\pm$ 100 K / T) cm$^3$ molecule$^{-1}$ s$^{-1}$ OH + CF$_3$CH$_2$OCClF$_2$: $k$ = (1.6 $\pm$ 0.6) $\times$ 10$^{-12}$ exp( – 1100 $\pm$ 125 K / T) cm$^3$ molecule$^{-1}$ s$^{-1}$ Cl + CF$_3$CH$_2$OCHF$_2$: $k$ = (6.1 $\pm$ 1.4) $\times$ 10$^{-12}$ exp( – 1830 $\pm$ 90 K / T) cm$^3$ molecule$^{-1}$ s$^{-1}$ Cl + CF$_3$CHClOCHF$_2$: $k$ = (7.8 $\pm$ 2.6) $\times$ 10$^{-11}$ exp( – 2980 $\pm$ 130 K / T) cm$^3$ molecule$^{-1}$ s$^{-1}$ Cl + CF$_3$CH$_2$OCClF$_2$: $k$ = (2.2 $\pm$ 0.2) $\times$ 10$^{-11}$ exp( – 2700 $\pm$ 40 K / T) cm$^3$ molecule$^{-1}$ s$^{-1}$ The results are compared with those obtained previously for the same and related reactions of OH radicals and Cl atoms, and the atmospheric implications of the results are considered briefly

Roles of the Bloom's syndrome helicase in the maintenance of genome stability

The RecQ family of DNA helicases is highly conserved in evolution from bacteria to humans. Of the five known human RecQ family members, three (BLM, WRN and RECQ4, which cause Bloom's syndrome, Werner's syndrome and Rothmund-Thomson syndrome respectively) are mutated in distinct clinical disorders associated with cancer predisposition and/or premature aging. BLM forms part of a multienzyme complex including topoisomerase IIIalpha, replication protein A and a newly identified factor called BLAP75. Together, these proteins play a role in the resolution of DNA structures that arise during the process of homologous recombination repair. In the absence of BLM, cells show genomic instability and a high incidence of sister-chromatid exchanges. In addition to a DNA structure-specific helicase activity, BLM also catalyses Holliday-junction branch migration and the annealing of complementary single-stranded DNA molecules

A Kinetic Study of the Gas-Phase O( 1 D) + CH3OH and O( 1 D) + CH3CN Reactions. Low Temperature Rate Constants and Atomic Hydrogen Product Yields

Atomic oxygen in its first excited singlet state, O(1 D), is an important species in the photochemistry of several planetary atmospheres and has been predicted to be a potentially important reactive species on interstellar ices. Here, we report the results of a kinetic study of the reactions of O(1 D) with methanol, CH3OH, and acetonitrile, CH3CN, over the 50-296 K temperature range. A continuous supersonic flow reactor was used to attain these low temperatures coupled with pulsed laser photolysis and pulsed laser induced fluorescence to generate and monitor O(1 D) atoms respectively. Secondary experiments examining the atomic hydrogen product channels of these reactions were also performed, through laser induced fluorescence measurements of H(2 S) atom formation. On the kinetics side, the rate constants for these reactions were seen to be large (> 2 x 10-10 cm 3 s-1) and consistent with barrierless reactions, although they display contrasting dependences as a function of temperature. On the product formation side, both reactions are seen to yield non-negligible quantities of atomic hydrogen. For the O(1 D) + CH3OH reaction, the derived yields are in good agreement with the conclusions of previous experimental and theoretical work. For the O(1 D) + CH3CN reaction, whose H-atom formation channels had not previously been investigated, electronic structure calculations of several new product formation channels were performed to explain the observed H-atom yields. These calculations demonstrate the barrierless and exothermic nature of the relevant exit channels, confirming that atomic hydrogen is also an important product of the O(1 D) + CH3CN reaction

Internal radionuclide dosimetry of model and patient based voxelised phantoms using the GATE toolkit

The desire for realistic patient specific dosimetry estimates of internally distributed radioactivity are realised by using Monte Carlo simulations of voxelised phantoms. The purpose of this thesis was to validate the GATE Monte Carlo package as a dosimetry tool and to investigate the accurate application of model and patient specific voxelised phantoms. Validation of the GATE Monte Carlo package was performed by simulating the absorbed fractions of simple geometric spheres of uniform radioactivity compared to accepted values. Voxelised spheres have also been simulated and it was found that the GATE Low Energy physics package was the most suitable for simulations of voxelised phantoms. The simulation of the scalable XCAT voxelised phantom has been performed to evaluate the effect of voxel size and patient organ mass on the calculation of dose factors. It was found that for organ self-irradiation significantly small voxels are required to ensure that insufficient voxel sampling does not effect the absorbed dose calculation. A retrospective absorbed dose calculation of true patient images was then performed with a correction for insufficient voxel sampling. In this work the scalable XCAT phantom has also been used to show that a voxel size of 2 mm or less is suitable for accurate calculations of organ cross dose. By comparing the scaled XCAT phantoms with patient and traditional phantoms it was concluded that considerable care is required when adapting model-based phantom results to individual patients. As differences in patient anatomy contribute significant variability to the dosimetry calculation, it is therefore recommended that where available individual patient specific dosimetry should be calculated using direct Monte Carlo simulation in favor of organ mass scaling