AMMONIA AND METHYL CYANIDE IN HOT CORES

We have used the IRAM 30 telescope to observe the J=6-5, J=8-7, and J= 12-11 rotational transitions of methyl cyanide (CH3CN) towards 11 ultr acompact HII regions in the inner galaxy. The sources observed were ta ken from a recent study of high-excitation ammonia lines by Cesaroni e t al. (1992). All of the sources in our list we-re detected. We have a nalysed 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 allo wing for the effects of optical thickness using a large velocity gradi ent statistical equilibrium program. We find in this way methyl cyanid e column densities in the range 3 10(15)-8 10(16) cm-2, and kinetic te mperatures in the range 85-160 K. We also present results of a small s urvey of similar sources which we have observed in NH3(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 h ave ammonia absorption lines red-shifted relative to emission or vice- versa. Comparison with these and earlier ammonia (NH3) results of Cesa roni et al. (1992) show that the inferred rotation temperatures are si milar below 50 K, but there are large deviations between temperature e stimates from the two molecules at higher temperatures. The abundance ratio [CH3CN]/[NH3] which we derive varies between 2 10(-4) and 8 10(- 3).