Engineering topics

Charnley, S. B.

Publications and source records attributed to Charnley, S. B..

43 records · Page 3

NASA NTRS1999

Summary of Research

In protostellar cores where the dust temperature has been raised above 100K and subsequently allowed to fall below the condensation temperature of methanol, recondensation on to cooling grains removes methanol molecules from the gas at rates.faster (about 1000 times) than those of chemical reactions. Molecular recondensation can have a profound effect on the chemical composition of hot cores. The methanol chemistry of hot cores is solved analytically and the trend in molecule binding energies required is compared with theoretical and experimental values. It is demonstrated, through a model calculation incorporating recondensation, that it yields a consistent explanation of the similarity of the CH3OH , CH3OCH3 and HCOOCH3 abundances measured in G34.3 and W3(H2O). These observations suggest that the latter molecules could not be derived from CH3OH through gas phase reactions. The manner in which molecular recondensation could affect the interpretation of hot core chemistry in general, particularly of organic molecules, is briefly discussed.

dePater, I.↗

NASA NTRS1995

Interstellar Alcohols

We have investigated the gas-phase chemistry in dense cores where ice mantles containing ethanol and other alcohols have been evaporated. Model calculations show that methanol, ethanol, propanol, and butanol drive a chemistry leading to the formation of several large ethers and esters. Of these molecules, methyl ethyl ether (CH3OC2H5) and diethyl ether (C2H5)2O attain the highest abundances and should be present in detectable quantities within cores rich in ethanol and methanol. Gas-phase reactions act to destroy evaporated ethanol and a low observed abundance of gas-phase C,H,OH does not rule out a high solid-phase abundance. Grain surface formation mechanisms and other possible gas-phase reactions driven by alcohols are discussed, as are observing strategies for the detection of these large interstellar molecules.

Charnley, S. B.↗

NASA NTRS1995

Organic Molecules in Oxygen-Rich Circumstellar Envelopes: Methanol and Hydrocarbons

The existence of anomalously high abundances of gaseous CH4 has been invoked to explain the unexpectedly high abundances of the carbon-bearing molecules HCN and H2CO in the outflows from O-rich red giants. We have modelled the chemistry that proceeds in the outer envelope when CH4 is injected from the inner envelope. We find that photolysis by the interstellar radiation field drives an ion-neutral chemistry which produces several organic molecules. The calculated abundances of CH3OH, C2H and C2 can be comparable to those calculated for H2CO and HCN. Species such as C2H4, C2H2 and CH3CN can also be abundant. A search for CH3OH and C2H in several O-rich outflows known to exhibit strong HCN emission is needed. As it derives entirely from the CH4 photochain, is insensitive to the envelope temperature distribution, and has accessible transitions at millimetre wavelengths, the detection of the C2H radical would provide further indirect support for the presence of the hypothesized methane.

Charnley, S. B.↗

NASA NTRS1994

Chemistry of Star-Forming Cores

Chemical effects arising from the exchange of molecules between dust and gas in dense clumps are described. Selective desorption of CO and N2 from grains in cool cores can account for the presence of ammonia in several cores in the NGC 2024 cloud. Evaporation of ices containing methanol and ethanol can lead to detectable abundances of (C2H5)(sub 2)O and CH3OC2H5. Results are presented for the hot core chemistries of sulphur and phosphorus which are initiated by evaporated hydrogen sulphide and phosphine. The implications of these studies for understanding the nature of molecular mantles, the evolution of molecular complexity in the gas phase, and the presence of small-scale abundance gradients in star-forming regions, are briefly discussed.

Charnley, S. B.↗

NASA NTRS1992

On the molecular complexity of the hot cores in Orion A - Grain surface chemistry as 'The last refuge of the scoundrel'

We have modeled the gas phase chemistry of warm molecular material around protostars that is seeded with evaporating grain mantles. We show that the release of simple molecules into the gas drives ion-molecule and neutral chemistries which can account for many of the complex O-bearing and N-bearing molecules observed in hot cores. Initial grain mantle components and secondary product molecules are identified, and the observational consequences are discussed.

Charnley, S. B.↗

NASA NTRS1991

The formation of oxygen-containing organic molecules in the Orion compact ridge

Following a suggestion of Blake et al. (1987), an attempt was made to account for the unusually large abundances of selected oxygen-containing organic molecules in the so-called 'compact ridge' source directed toward Orion KL by a gas-phase chemical model in which large amounts of water are injected into the source from the IRc2 outflow. Although quantitative model results show that the calculated abundances of methanol, methyl formate, and dimethyl ether can be enhanced relative to their values in the absence of water injection, the enhancements fall far short of explaining the very large observed abundances of these species. Models in which methanol is injected rather than water are more successful, although the source of the methanol is unclear.

Millar, T. J.↗

NASA NTRS1991

Chemical models of interstellar gas-grain processes. II - The effect of grain-catalysed methane on gas phase evolution

The effects on gas phase chemistry which result from the continuous desorption of methane molecules from grain surfaces are studied. Significant and sustained enhancements in the abundances of several complex hydrocarbon molecules are found, in good agreement with their observed values in TMC-1. The overall agreement is, however, just as good for the case of zero CH4 desorption efficiency. It is thus impossible to determine from the models whether or not the grain-surface production of methane is responsible for the observed abundances of some hydrocarbon molecules.

Brown, Paul D.↗