HIGH-RESOLUTION MOLECULAR OBSERVATIONS OF NGC 7538-IRS-1

High-resolution observations (3″) of the J=1-0 transitions of HCO+ and HCN toward NGC 7538 IRS 1 have been made with the Berkeley-Illinois-Maryland Array and the NRAO 12 m telescope. The HCO+ emission traces the high-velocity outflow which appears to originate about 15″ south of IRS 1, at the center of an elongated structure which is seen at higher velocities. The axis of the outflow is at a position angle of 145° (east of north), and only the blue lobe of the outflow is seen. The direction of the high-density outflow agrees very well with the direction of the CO outflow, although the CO outflow is more extended. The general structure of the HCN and HCO+ emission at the quiescent velocity of the source is very similar. Both molecules trace a density enhancement which outlines a cavity, of diameter 0.09 pc, in the cloud around the H II region. The cavity is interpreted as a lack of high-density material which is caused by the expansion of the H II region. The expansion also creates the density enhancement in the material surrounding the cavity, which is clearly traced by the HCN and HCO+ emission. The high-resolution observations show that this density enhancement is very clumpy. A detailed comparison between individual HCN and HCO+ emission peaks shows a definite anticorrelation, which is an indication of the different mechanisms which excite the two molecules. The molecular cloud is also seen to consist of high-density clumps. The distribution of the deconvolved clump sizes shows that the HCO+ clumps are resolved by the 3″ beam and that the distribution is Gaussian with a peak at a clump size of 0.12 pc (assuming a distance of 3.5 kpc to the source). The HCN clump size distribution peaks at 0.08 pc. The existence of HCN clumps at the limit of the resolution could be an indication of smaller clumps in the cloud.