5 Conclusions

  We have surveyed rotational transitions of molecular species towards 14 ultracompact HII regions, with the aims of observing the chemistry of molecular regions populated by species evaporated from grain mantles, for comparison with chemical models, looking in particular for any evidence of chemical evolution; and of determining the physical conditions of the molecular gas near a sample of UC HII regions, to identify common characteristics, and for comparison with models of UC HII expansion. The main conclusions are as follows:

1.

Eight out of fourteen sources - G5.89, G9.62, G10.47, G12.21, G29.96, G31.41, G34.26 and G75.78 - show high abundances of ice-evaporated species or their daughter products, confirming that grain mantle chemistry strongly influences the chemical composition of these star-forming regions. These sources are chemically rich and show many emission lines.

2.

The eight line-rich sources show evidence for a core-halo structure, with a hot, dense core surrounded by cooler ambient cloud material. In five of these sources, there is evidence that the hot material is a central condensation rather than scattered clumps. There is a gradual increase of density and temperature towards the core, with higher excitation molecular tracers showing smaller cores. Densities in the cores reach at least $10^8 \hbox{ cm}^{-3}$ and temperatures are over 80 K. Hot core sizes are of order 0.05 pc (depending on the tracer) corresponding to 2'' at 5 kpc. We did not find any evidence for hot cores in the line-poor sources - G10.30, G13.87, G43.89, G45.12, G45.45, and G45.47. Any cores in these sources must be low density or small (<1'').

3.

We find no correlation between UC HII region size or shape and core size, chemical evolution, ambient cloud density, or linewidth.

4.

Most of the measured column densities are satisfactorily predicted by our hot core chemical model, with the exception of CH₃CN, HCOOCH₃, and CH₃CCH.

This survey has gone some way towards determining the nature of high mass star formation regions associated with UC HII regions. Through observations of molecular lines we have measured the physical conditions and chemical conditions in 14 objects. The current generation of chemical models has proved quite successful in explaining many of the observed abundances in the hot cores and ambient clouds.

A number of outstanding questions remain about the structure and evolution of these regions. Are hot cores a necessary stage in the formation of high mass stars, or are they generated under special circumstances? What is the heating mechanism for hot cores? What are the interrelationships between hot cores, UC HII regions and outflows? How good is our understanding of the chemistry?

Further observations and modelling are needed to answer these questions, including interferometric observations which show the spatial relationships between hot cores, UC HII regions and outflows, and single-dish observations to measure excitation and abundances. Future developments in modelling need to take account of the variation in physical conditions which accompanies the chemical evolution on similar timescales, and could potentially include details of the grain surface processes.

Acknowledgements

This research was supported by the UK Particle Physics and Astronomy Research Council (PPARC) through grants to UMIST and the University of Kent. JH and MAT are grateful to PPARC for funding their postdoctoral position and studentship, respectively. We would like to thank the JCMT staff for their support during the observations.