Ammonia and methyl cyanide in hot cores

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Session 16 -- Molecular Clouds
Display presentation, Wednesday, January 12, 9:30-6:45, Salons I/II Room (Crystal Gateway)

[16.06] Ammonia and methyl cyanide in hot cores

L. Olmi, R. Cesaroni, C.M. Walmsley (N.A.I.C. Arecibo Observatory, Osservatorio di Arcetri, MPIfR)

We present the results of a work which used the IRAM 30-m telescope to observe the $J$=6--5, $J$=8--7, and $J$=12--11 rotational transitions of methyl cyanide (CH$_{3}$CN) towards 11 ultracompact H{\small II} regions in the inner galaxy. The sources observed were taken from a recent study of high-excitation ammonia lines by Cesaroni et al. (1992). All of the sources in our list were detected. We have analysed the data assuming the observed lines to be optically thin and have derived ``rotation temperatures'' and column densities. For four of the sources, we have carried out a more sophisticated analysis allowing for the effects of optical thickness using a large velocity gradient statistical equilibrium program. We find in this way methyl cyanide column densities in the range $3\,10^{15}$--$8\,10^{16}$~cm$^{-2}$, and kinetic temperatures in the range $85$--$160$~K. We also present results of a small survey of similar sources which we have observed in NH$_{3}$ (4,4) and (5,5) using the Bonn 100-m telescope. An interesting feature of the new 100-m results is the discovery of two new sources showing high excitation ammonia in absorption. There is no obvious preference for sources to have ammonia absorption lines red-shifted relative to emission or vice-versa. Comparison with these and earlier ammonia results of Cesaroni et al. (1992) show that the inferred rotation temperatures are similar below 50 K, but there are large deviations between temperature estimates from the two molecules at higher temperatures. The abundance ratio [CH$_{3}$CN]/[NH$_{3}$] which we derive varies between $2\,10^{-4}$ and $8\,10^{-3}$.

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