The methyl cyanide hot and warm cores in Orion - Statistical equilibrium excitation models of a symmetric-top molecule

Astronomy and Astrophysics – Astronomy

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Cyanides, Interstellar Matter, Methyl Compounds, Molecular Excitation, Orion Nebula, Radio Sources (Astronomy), Abundance, Acetonitrile, High Resolution, Line Spectra, Molecular Spectra, Statistical Analysis

Scientific paper

High-velocity resolution line profiles have been obtained for 50 lines of the top-symmetric-top molecule CH3CN, from J = 6-5 up to J = 16-15 with components from K = O to K = 7 in the vicinity of OMC-1. The profiles reveal two distinct regions of both broad and narrow emission at 5.1 km/s, and 8.7 km/s, respectively. The kinetic temperatures for these regions are 275 (+ or -25) K and 95 (+ or -8) K respectively. It is found that CH3CN abundance is enhanced in the hot core by a factor of 10 relative to the quiescent cloud. The excitation of statistical equilibrium (SE) models fitted to the resolved line profiles is compared to that predicted by the rotational equilibrium (RTE) analytic method. It is found that the RTE method can accurately predict kinetic temperature for both of the nearly thermalized regions of OMC-1 when rotational temperature is not determined from levels which span a large range of J, but not by Hollis's (1982) method based on adjacent J levels. For high temperature regions the RTE method did not correctly predict kinetic temperature in the range of densities from 10 to the 2.5 per cu cm to 10 to the 6th per cu cm which are common in molecular clouds.

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