Astron. Astrophys. Suppl. Ser. 133, 29-49

A survey of molecular line emission towards ultracompact HII regions^,

J. Hatchell¹ - M.A. Thompson² - T.J. Millar¹ - G.H. Macdonald²

Send offprint request: J. Hatchell

1 - Department of Physics, UMIST, P.O. Box 88, Manchester M60 1QD, UK
2 - Electronic Engineering Laboratory, University of Kent, Canterbury, Kent CT2 7NT, UK

Received August 18, 1997; accepted May 13, 1998

Abstract:

We have used the JCMT to survey molecular line emission towards 14 ultracompact HII regions (G5.89, G9.62, G10.30, G10.47, G12.21, G13.87, G29.96, G31.41, G34.26, G43.89, G45.12, G45.45, G45.47, and G75.78). For each source, we observed up to ten 1 GHz bands between 200 and 350 GHz, covering lines of more than 30 species including multiple transitions of CO isotopes, CH₃OH, CH₃CCH, CH₃CN and HCOOCH₃, and sulphuretted molecules. The number of transitions detected varied by a factor of 20 between sources, which were chosen following observations of high-excitation ammonia (Cesaroni et al. 1994a) and methyl cyanide (Olmi et al. 1993). In half our sample (the line-poor sources), only C¹⁷O, C¹⁸O, SO, C³⁴S and CH₃OH were detected. In the line-rich sources, we detected over 150 lines, including high excitation lines of CH₃CN, HCOOCH₃, C₂H₅CN, CH₃OH, and CH₃CCH. We have calculated the physical conditions of the molecular gas. To reproduce the emission from the line-rich sources requires both a hot, dense compact core and an ambient cloud consisting of less dense, cooler gas. The hot cores, which are less than 0.1 pc in size, reach densities of at least $10^8 \hbox{ cm}^{-3}$ and temperatures of more than 80 K. The line-poor sources can be modelled without a hot core by a 20-30 K, $10^5 \hbox{ cm}^{-3}$ cloud. We find no correlation between the size of the HII region and the current physical conditions in the molecular environment. A comparison with chemical models (Millar et al. 1997) confirms that grain surface chemistry is important in hot cores.

Key words: ISM: molecules -- ISM: clouds -- ISM: HII regions -- radio lines: ISM -- stars: formation -- molecular processes