A SURVEY OF MOLECULAR LINE EMISSION TOWARDS ULTRACOMPACT HII-REGIONS

We have used the JCMT to survey molecular line emission towards 14 ult racompact HII regions (G5.89, G9.62, G10.30, G10.47, G12.21, G13.87, G 29.96, G31.41, G34.26, G43.89, G45.12, G45.45, G45.47, and G75.78). Fo r each source, we observed up to ten 1 GHz bands between 200 and 350 G Hz, covering lines of more than 30 species including multiple transiti ons of CO isotopes, CH3OH, CH3CCH, CH3CN and HCOOCH3, and sulphuretted molecules. The number of transitions detected varied by a factor of 2 0 between sources; which were chosen following observations of high-ex citation ammonia (Cesaroni et al. 1994a) and methyl cyanide (Olmi et a l. 1993). In half our sample (the line-poor sources), only (CO)-O-17: (CO)-O-18, SO, (CS)-S-34 and CH3OH were detected. In the line-rich sou rces, we detected over 150 lines, including high excitation lines of C H3CN, HCOOCH3; C2H5CN, CH3OH, and CH3CCH. We have calculated the physi cal conditions of the molecular gas. To reproduce the emission from th e line-rich sources requires both a hot, dense compact core and an amb ient cloud consisting of less dense, cooler gas. The hot cores, which are less than 0.1 pc in size; reach densities of at least 10(8) cm(-3) and temperatures of more than 80 K. The line-poor sources can be mode lled without a hot core by a 20-30 K, 10(5) cm(-3) cloud. We find no c orrelation between the size of the HII region and the current physical conditions in the molecular environment. A comparison with chemical m odels (Millar et al. 1997) confirms that grain surface chemistry is im portant in hot cores.