scholarly journals Models of hot molecular cores

1997 ◽  
Vol 178 ◽  
pp. 75-88 ◽  
Author(s):  
T.J. Millar
Keyword(s):  
Gas Phase ◽  
Theoretical Modelling ◽  
Specific Reference ◽  
Future Directions ◽  
Chemical Modelling

We review the properties of hot molecular cores with specific reference to chemical modelling. While most theoretical modelling has been generic, we discuss models for two specific sources, G34.3+0.15 and W3 (H2O). We show that the hot core gas in both these regions has an age of 5 103 — 104 yrs and that the abundances of many molecules are consistent with formation in the gas-phase following evaporation of simple molecular ices from the grains. We end with a discussion of current problems and future directions.

scholarly journals The Epidemiology Study in Multiple Sclerosis - Relevance to Natural History

2020 ◽  
Vol 7 (1) ◽  
Author(s):  
Jeffrey Sloka
Keyword(s):  
Multiple Sclerosis ◽  
Natural History ◽  
Search Strategy ◽  
Demyelinating Disease ◽  
Epidemiological Studies ◽  
Reference Points ◽  
Specific Reference ◽  
Future Directions ◽  
History Of ◽  
The Central Nervous System

Multiple sclerosis (MS) is a chronic, demyelinating disease of the central nervous system white matter that has been extensively studied using the epidemiological approach, and yet an etiology for the disease remains elusive. This paper presents a review of past publications that have made suggestions toward the design of epidemiological studies in MS. A formal search strategy is described, and a short summary of these papers is provided. A natural history of MS based on previous studies is proposed as a framework for describing future directions in the neuroepidemiology of the disease, and categorization based on the clinical forms of MS is described. Within the context of a proposed natural history, suggestions are made on the use of sub-regionalization in cluster studies across different domains, as well as on the use of specific reference points in a patient's lifetime in the analysis of clusters.

scholarly journals The ALMA-PILS survey: propyne (CH3CCH) in IRAS 16293–2422

2019 ◽  
Vol 631 ◽  
pp. A137 ◽  
Author(s):  
H. Calcutt ◽  
E. R. Willis ◽  
J. K. Jørgensen ◽  
P. Bjerkeli ◽  
N. F. W. Ligterink ◽  
...  
Keyword(s):  
Gas Phase ◽  
Spatial Scales ◽  
Excitation Temperature ◽  
Gas Phase Reactions ◽  
Star Forming ◽  
Physical Conditions ◽  
Star Forming Regions ◽  
Chemical Modelling ◽  
Low Mass ◽  
Grain Surface

Context. Propyne (CH3CCH), also known as methyl acetylene, has been detected in a variety of environments, from Galactic star-forming regions to extragalactic sources. These molecules are excellent tracers of the physical conditions in star-forming regions, allowing the temperature and density conditions surrounding a forming star to be determined. Aims. This study explores the emission of CH3CCH in the low-mass protostellar binary, IRAS 16293–2422, and examines the spatial scales traced by this molecule, as well as its formation and destruction pathways. Methods. Atacama Large Millimeter/submillimeter Array (ALMA) observations from the Protostellar Interferometric Line Survey (PILS) were used to determine the abundances and excitation temperatures of CH3CCH towards both protostars. This data allows us to explore spatial scales from 70 to 2400 au. This data is also compared with the three-phase chemical kinetics model MAGICKAL, to explore the chemical reactions of this molecule. Results. CH3CCH is detected towards both IRAS 16293A and IRAS 16293B, and is found the hot corino components, one around each source, in the PILS dataset. Eighteen transitions above 3σ are detected, enabling robust excitation temperatures and column densities to be determined in each source. In IRAS 16293A, an excitation temperature of 90 K and a column density of 7.8 × 1015 cm−2 best fits the spectra. In IRAS 16293B, an excitation temperature of 100 K and 6.8 × 1015 cm−2 best fits the spectra. The chemical modelling finds that in order to reproduce the observed abundances, both gas-phase and grain-surface reactions are needed. The gas-phase reactions are particularly sensitive to the temperature at which CH4 desorbs from the grains. Conclusions. CH3CCH is a molecule whose brightness and abundance in many different regions can be utilised to provide a benchmark of molecular variation with the physical properties of star-forming regions. It is essential when making such comparisons, that the abundances are determined with a good understanding of the spatial scale of the emitting region, to ensure that accurate abundances are derived.

Chemical processes in the interstellar medium: on the nature of the carrier of the diffuse interstellar bands

1981 ◽  
Vol 303 (1480) ◽  
pp. 503-511 ◽  
Keyword(s):  
Observational Study ◽  
Gas Phase ◽  
Transition Metal Ions ◽  
Chemical Processes ◽  
Focused Attention ◽  
The Past ◽  
Diffuse Interstellar Bands ◽  
Great Progress ◽  
Quantitative Basis ◽  
Chemical Modelling

The great progress of the past decade in molecular astronomy, primarily by millimetre radio observations, has focused attention upon the synthesis of observed gas-phase species. We briefly discuss chemical modelling of the gas phase. The major portion of this article is devoted to the discussion of the nature of the carrier of the diffuse interstellar bands. The detailed observational study by G. H. Herbig ( Astrophys. J. 196, 129 (1975)) provides a quantitative basis against which suggestions for the nature of the carrier of the diffuse bands may be tested. The present paper suggests that the carrier is transition metal ions interacting with the polysulphide ions S 2 and S 3 in oxidic lattices.

scholarly journals Methanol ice co-desorption as a mechanism to explain cold methanol in the gas-phase

2018 ◽  
Vol 612 ◽  
pp. A88 ◽  
Author(s):  
N. F. W. Ligterink ◽  
C. Walsh ◽  
R. G. Bhuin ◽  
S. Vissapragada ◽  
J. Terwisscha van Scheltinga ◽  
...  
Keyword(s):  
Gas Phase ◽  
Thermal Desorption ◽  
High Vacuum ◽  
Protoplanetary Disk ◽  
Ultra High Vacuum ◽  
Indirect Pathway ◽  
Cold Environments ◽  
Chemical Modelling ◽  
Phase Chemistry ◽  
The Impact

Context. Methanol is formed via surface reactions on icy dust grains. Methanol is also detected in the gas-phase at temperatures below its thermal desorption temperature and at levels higher than can be explained by pure gas-phase chemistry. The process that controls the transition from solid state to gas-phase methanol in cold environments is not understood. Aims. The goal of this work is to investigate whether thermal CO desorption provides an indirect pathway for methanol to co-desorb at low temperatures. Methods. Mixed CH3OH:CO/CH4 ices were heated under ultra-high vacuum conditions and ice contents are traced using RAIRS (reflection absorption IR spectroscopy), while desorbing species were detected mass spectrometrically. An updated gas-grain chemical network was used to test the impact of the results of these experiments. The physical model used is applicable for TW Hya, a protoplanetary disk in which cold gas-phase methanol has recently been detected. Results. Methanol release together with thermal CO desorption is found to be an ineffective process in the experiments, resulting in an upper limit of ≤ 7.3 × 10−7 CH3OH molecules per CO molecule over all ice mixtures considered. Chemical modelling based on the upper limits shows that co-desorption rates as low as 10−6 CH3OH molecules per CO molecule are high enough to release substantial amounts of methanol to the gas-phase at and around the location of the CO thermal desorption front in a protoplanetary disk. The impact of thermal co-desorption of CH3OH with CO as a grain-gas bridge mechanism is compared with that of UV induced photodesorption and chemisorption.

scholarly journals Chemical modelling of dust–gas chemistry within AGB outflows – III. Photoprocessing of the ice and return to the ISM

2020 ◽  
Vol 501 (1) ◽  
pp. 491-506
Author(s):  
M Van de Sande ◽  
C Walsh ◽  
T J Millar
Keyword(s):  
Gas Phase ◽  
Organic Material ◽  
Interstellar Dust ◽  
Agb Stars ◽  
Chemical Modelling ◽  
Minor Part ◽  
A Minor ◽  
Almost All ◽  
Grain Surface ◽  
Stellar Dust

ABSTRACT To explain the properties of dust in the interstellar medium (ISM), the presence of a refractory organic mantle is necessary. The outflows of AGB stars are among the main contributors of stellar dust to the ISM. We present the first study of the refractory organic contribution of AGB stars to the ISM. Based on laboratory experiments, we included a new reaction in our extended chemical kinetics model: the photoprocessing of volatile complex ices into inert refractory organic material. The refractory organic feedback of AGB outflows to the ISM is estimated using observationally motivated parent species and grids of models of C-rich and O-rich outflows. Refractory organic material is mainly inherited from the gas phase through accretion on to the dust and subsequent photoprocessing. Grain-surface chemistry, initiated by photodissociation of ices, produces only a minor part and takes place in a sub-monolayer regime in almost all outflows. The formation of refractory organic material increases with outflow density and depends on the initial gas-phase composition. While O-rich dust is negligibly covered by refractory organics, C-rich dust has an average coverage of $3\!-\!9{{\ \rm per\ cent}}$, but can be as high as $8\!-\!22{{\ \rm per\ cent}}$. Although C-rich dust does not enter the ISM bare, its average coverage is too low to influence its evolution in the ISM or significantly contribute to the coverage of interstellar dust. This study opens up questions on the coverage of other dust-producing environments. It highlights the need for an improved understanding of dust formation and for models specific to density structures within the outflow.

scholarly journals The structure of chromophore-grafted amyloid-β12–28 dimers in the gas-phase: FRET-experiment guided modelling

2016 ◽  
Vol 18 (13) ◽  
pp. 9061-9069 ◽  
Author(s):  
Alexander Kulesza ◽  
Steven Daly ◽  
Chang Min Choi ◽  
Anne-Laure Simon ◽  
Fabien Chirot ◽  
...  
Keyword(s):  
Gas Phase ◽  
Ion Mobility Spectrometry ◽  
Ion Mobility ◽  
Amyloid Β ◽  
Theoretical Modelling

Theoretical modelling, ion mobility spectrometry and action-FRET experiments are combined to an experiment guided approach and used to elucidate the structure of chromophore-grafted amyloid-β12–28 dimers in the gas-phase.

Shining light on CO2: from materials discovery to photocatalyst, photoreactor and process engineering

2020 ◽  
Vol 49 (16) ◽  
pp. 5648-5663 ◽  
Author(s):  
Yuchan Dong ◽  
Paul Duchesne ◽  
Abhinav Mohan ◽  
Kulbir Kaur Ghuman ◽  
Paul Kant ◽  
...  
Keyword(s):  
Gas Phase ◽  
Indium Oxide ◽  
Process Development ◽  
Process Engineering ◽  
Theoretical Modelling ◽  
Oxide Systems ◽  
Materials Engineering

Materials engineering, theoretical modelling, reactor engineering and process development of gas-phase photocatalytic CO2 reduction exemplified by indium oxide systems.

scholarly journals Conformer-Dependent VUV Photodynamics and Chiral Asymmetries in Pure Enantiomers of Gas Phase Proline

2020 ◽  
Author(s):  
Rim Hadidi ◽  
Dusan Bozanic ◽  
Hassan Ganjitabar ◽  
Gustavo Garcia ◽  
Ivan Powis ◽  
...  
Keyword(s):  
Gas Phase ◽  
Energy Difference ◽  
Molecular Structures ◽  
Theoretical Modelling ◽  
Pyrrolidine Ring ◽  
Conformational Landscape ◽  
Vuv Photoionization ◽  
Molecular Electronic Structure ◽  
Unique Amino Acid ◽  
Structure Calculations

Abstract Proline is a unique amino-acid, with a secondary amine fixed within a pyrrolidine ring providing specific structural properties to proline-rich biopolymers. Gas-phase proline possesses four main H-bond stabilized conformers differing by the ring puckering and carboxylic acid orientation. The latter defines two classes of conformation, whose large ionization energy difference allows a unique conformer-class tagging via electron spectroscopy. Photoelectron circular dichroism (PECD) is an intense chiroptical effect sensitive to molecular structures, hence theorized to be highly conformation-dependent. Here, besides a conformer-dependant cation fragmentation behaviour, we present experimental evidence of an intense and striking conformer-specific PECD, measured in the VUV photoionization of proline. This finding, combined with theoretical modelling, allows a refinement of the conformational landscape and energetic ordering, that proves inaccessible to current molecular electronic structure calculations. Additionally, astrochemical implications regarding a possible link of PECD to the origin of life’s homochirality are considered in terms of plausible temperature constraints.

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