2,453 research outputs found

    The interstellar gas-phase chemistry of HCN and HNC

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    We review the reactions involving HCN and HNC in dark molecular clouds to elucidate new chemical sources and sinks of these isomers. We find that the most important reactions for the HCN-HNC system are Dissociative Recombination (DR) reactions of HCNH+ (HCNH+ + e-), the ionic CN + H3+, HCN + C+, HCN and HNC reactions with H+/He+/H3+/H3O+/HCO+, the N + CH2 reaction and two new reactions: H + CCN and C + HNC. We test the effect of the new rate constants and branching ratios on the predictions of gas-grain chemical models for dark cloud conditions. The rapid C + HNC reaction keeps the HCN/HNC ratio significantly above one as long as the carbon atom abundance remains high. However, the reaction of HCN with H3+ followed by DR of HCNH+ acts to isomerize HCN into HNC when carbon atoms and CO are depleted leading to a HCN/HNC ratio close to or slightly greater than 1. This agrees well with observations in TMC-1 and L134N taking into consideration the overestimation of HNC abundances through the use of the same rotational excitation rate constants for HNC as for HCN in many radiative transfer models.Comment: Accepted for publication in MNRA

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

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    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

    Nutritional interventions in sarcopenia: a critical review.

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    The aim of the present paper is to critically review the details of the published nutrition intervention trials, with and without exercise, targeting sarcopenia. Sarcopenia is the loss of muscle mass, strength and/or performance with age. Since amino acids and energy are required for muscle synthesis it is possible that nutritional intake influences sarcopenia. Nutritional studies are challenging to carry out because of the complexity of modulating dietary intake. It is very difficult to change one nutrient without influencing many others, which means that many of the published studies are problematic to interpret. The studies included evaluate whole protein, essential amino acids and β-hydroxyl β-methylbutyrate (HMB). Whole-protein supplementation failed to show a consistent effect on muscle mass, strength or function. This can be explained by the variations in study design, composition of the protein supplement and the failure to monitor voluntary food intake, adherence and baseline nutritional status. Essential amino-acid supplements showed an inconsistent effect but there are only two trials that have significant differences in methodology and the supplement used. The HMB studies are suggestive of a beneficial effect on older adults, but larger well-controlled studies are required that measure outcomes relevant to sarcopenia, ideally in sarcopenic populations. The issues of timing and distribution of protein intake, and increased splanchnic amino-acid sequestration are discussed, and recommendations for future trials are made

    NASAs Orbital Debris JAO/ES-MCAT Optical Telescope Facility on Ascension Island

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    The NASA Orbital Debris Program Office has a long-standing optical program begun over three and a half decades ago in 1984, designed to observe the Earth-orbiting environment with optical telescopes. Photometrically calibrated optical data provides a statistical sample for input to NASA models of the debris population for understanding the current and future debris environment around the Earth. Tracked objects and orbits allow for analysis of break-up events. Both known (correlated target in the SSN catalogue, or CT) and unknown (uncorrelated target, or UCT) objects are of interest to better understand how to protect current spacecraft and design more robust future operational satellites, and advise on how policies and practices can lead to protecting the environment itself for future generations. In 2015, a joint NASA JSC Air Force Research Labs (AFRL) project culminated in the installation of the 1.3-meter Eugene Stansbery Meter Class Autonomous Telescope, ES-MCAT (a.k.a. MCAT) on Ascension Island. This DFM Engineering designed telescope provides nearly five-times greater light-collecting power than its predecessor, the 0.6-m MODEST telescope, and faster tracking capabilities by both the telescope and the 7-m ObservaDome. This allows for all orbital regimes to be easily within reach, ranging from low Earth to geosynchronous orbits. Extensive testing and commissioning activities of this custom system led to successfully reaching Initial Operational Capability in 2018, and the facility is currently on track to reach Full Operational Capability. The John Africano Observatory (JAO) comprises the primary 1.3-m ES-MCAT facility, the adjacent tower platform with a 0.4-m telescope, a sophisticated suite of weather instruments, and custom software by Euclid Research for autonomously running the entire system, including monitoring the weather and hardware, tasking all components, and collecting, processing, and analyzing the data. The mission of JAO and MCAT will be discussed, including survey and tracking tasking, a full discussion of data calibration, and both optics and weather-dependent performance

    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

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    Rate constants have been determined for the reactions of OH radicals and Cl atoms with the three partially halogenated methyl-ethyl ethers, CF3_3CH2_2OCHF2_2, CF3_3CHClOCHF2_2 and CF3_3CH2_2OCClF2_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 + CF3_3CH2_2OCHF2_2: kk = (2.0±\pm0.8) ×\times 10−11^{-11} exp( – 2110 ±\pm 150 K / T) cm3^3 molecule−1^{-1} s−1^{-1} OH + CF3_3CHClOCHF2_2: kk = (4.5 ±\pm 1.3) ×\times 10−13^{-13} exp( – 940 ±\pm 100 K / T) cm3^3 molecule−1^{-1} s−1^{-1} OH + CF3_3CH2_2OCClF2_2: kk = (1.6 ±\pm 0.6) ×\times 10−12^{-12} exp( – 1100 ±\pm 125 K / T) cm3^3 molecule−1^{-1} s−1^{-1} Cl + CF3_3CH2_2OCHF2_2: kk = (6.1 ±\pm 1.4) ×\times 10−12^{-12} exp( – 1830 ±\pm 90 K / T) cm3^3 molecule−1^{-1} s−1^{-1} Cl + CF3_3CHClOCHF2_2: kk = (7.8 ±\pm 2.6) ×\times 10−11^{-11} exp( – 2980 ±\pm 130 K / T) cm3^3 molecule−1^{-1} s−1^{-1} Cl + CF3_3CH2_2OCClF2_2: kk = (2.2 ±\pm 0.2) ×\times 10−11^{-11} exp( – 2700 ±\pm 40 K / T) cm3^3 molecule−1^{-1} s−1^{-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

    The gas-phase chemistry of carbon chains in dark cloud chemical models

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    We review the reactions between carbon chain molecules and radicals, namely Cn, CnH, CnH2, C2n+1O, CnN, HC2n+1N, with C, N and O atoms. Rate constants and branching ratios for these processes have been re-evaluated using experimental and theoretical literature data. In total 8 new species have been introduced, 41 new reactions have been proposed and 122 rate coefficients from kida.uva.2011 (Wakelam et al. 2012) have been modified. We test the effect of the new rate constants and branching ratios on the predictions of gas-grain chemical models for dark cloud conditions using two different C/O elemental ratios. We show that the new rate constants produce large differences in the predicted abundances of carbon chains since the formation of long chains is less effective. The general agreement between the model predictions and observed abundances in the dark cloud TMC-1 (CP) is improved by the new network and we find that C/O ratios of 0.7 and 0.95 both produce a similar agreement for different times. The general agreement for L134N (N) is not significantly changed. The current work specifically highlights the importance of O + CnH and N + CnH reactions. As there are very few experimental or theoretical data for the rate constants of these reactions we highlight the need for experimental studies of the O + CnH and N + CnH reactions, particularly at low temperature.Comment: Accepted for publication in MNRA

    Impact of Sodium Layer variations on the performance of the E-ELT MCAO module

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    Multi-Conjugate Adaptive Optics systems based on sodium Laser Guide Stars may exploit Natural Guide Stars to solve intrinsic limitations of artificial beacons (tip-tilt indetermination and anisoplanatism) and to mitigate the impact of the sodium layer structure and variability. The sodium layer may also have transverse structures leading to differential effects among Laser Guide Stars. Starting from the analysis of the input perturbations related to the Sodium Layer variability, modeled directly on measured sodium layer profiles, we analyze, through a simplified end-to-end simulation code, the impact of the low/medium orders induced on global performance of the European Extremely Large Telescope Multi-Conjugate Adaptive Optics module MAORY.Comment: 7 pages, 5 figures, SPIE conference Proceedin

    London counts: HIV prevention needs and interventions among gay men in 16 London Health Authorities

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    Duration: December 1998 - February 2001 Commissioned by the sixteen London Health Authority Commissioners, the Link Evaluation included work by both Sigma Research, and the Thomas Coram Research Unit at the Institute of Education, University of London. The LINK Evaluation aimed to identify whether and why HIV prevention interventions are successful for Gay men in London. Objectives of the study included: Identification and agreement of health promotion aims with all commissioners and prevention agencies within the partnership. Collation of data and information from related studies of Gay men’s HIV health promotion needs and comparable programme evaluations. Annual health promotion ‘Activity Map' which described all HIV health promotion interventions planned for London for each year. Annual ‘Needs Map' report concerning the needs of Gay men in London. Examination of the collective impact of the activities of collaborating agencies on the population of Gay men in London. Four examinations of the performance and impact of individual interventions or groups of interventions sharing methodological characteristics. The LINK Evaluation was a rolling programme of utilisation-focussed outcome evaluation and needs assessment of HIV health promotion intended to reduce the incidence of HIV through sex between men resident in London. A range of research methods were employed including: self-completion questionnaires, face-to-face interviews and workshops. Sample sizes varied according to the specific programme activity: 1500 men were recruited to a rolling cohort to examine their changing HIV health promotion needs, a further 200 were recruited for more in-depth needs and experiences mapping and over 100 agencies were recruited to examine the health promotion activity being delivered in London. Findings were useful in answering questions such as: i) What needs required addressing - the Needs Map was used to prioritise health promotion aims and population groups, and to guide intervention selection and the development of services within a broad programme planning approach. ii) Which interventions best address identified need - findings regarding the performance and impact of specific interventions were used in order to maximise cost-efficiency. iii) Where interventions were being implemented, and where they were not - the Activity Maps were used to gauge intended impact, identify gaps in provision and to facilitate collaborative planning. iv) Whether the programme of HIV health promotion was working - the programme evaluation findings were used to identify the overall impact of a strategic and collaborative London-wide programme and to inform future developments. v) Why the programme of HIV health promotion was working - the in-depth data from those exposed to interventions was used to identify the key features of a strategic programme which contribute to change. Two reports are available: London counts: HIV prevention needs and interventions among gay and bisexual men in 16 London Health Authorities and HIV health promotion activity map for Greater London 1999-2000
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