5 Summary and conclusions

We have performed a 330-360 GHz molecular line survey of the molecular gas associated with the UC HII region G5.89-0.39. The lines detected in the survey can be split into two types; those with narrow line profiles which probe the dense molecular envelope surrounding the UC HII region and those with broad line wings which probably arise from the massive outflow known to be associated with G5.89-0.39. The species which exhibit high-velocity emission are consistent with those seen toward outflows from low-mass YSOs, indicating that similar processes are responsible for the chemical evolution of outflowing gas. The rotation temperature of the species detected in the survey is 60-80 K and the column densities of most species are in the range 10¹⁴-10¹⁵ cm^-2.

No heavy organic species (e.g. CH₃OCH₃, CH₂CHCN) were detected. The spectrum is dominated by lines of sulphur-bearing species; over 50$\%$ of the identified lines originate from sulphur-bearing species. The dominant molecule in terms of integrated intensity is CO, which has twice the integrated intensity of SO₂. This is the opposite of the situation in Orion-KL (Schilke et al. 1997b) where SO₂ has twice the integrated intensity of CO. Beam dilution is the most likely cause.

The chemistry of G5.89 has features that are similar to hot core and shock-driven models. The broad features of the envelope chemistry are similar to those of hot cores with the caveats that the density and temperature are lower than in other hot core sources and that there is no evidence of the heavy organic molecules seen in the hot core surveys of Macdonald et al. (1996), Schilke et al. (1997b) and Hatchell et al. (1998a). It is possible that the envelope of G5.89 is evolved much further than the dynamical ages of both the UC HII region and outflow suggest. We intend to subject the survey data to detailed physical and chemical analysis in a following paper, in order to derive the physical parameters of the molecular gas and to model the chemistry of the envelope and outflow to determine whether the chemical evolution can be better described by hot core or shock-driven models.

Interferometric observations would also be extremely useful in determining the locations of the sulphur-bearing species and to confirm whether shock-driven chemistry or hot core chemistry is the prime cause. Acord et al. (1997) remark upon the similarity of the line profiles of SiO, NH₃ and CO at high velocities, perhaps suggesting that these three species are coexistent in the outflow. Interferometric observation of these species would confirm this and allow the refinement of models of the chemistry and physics of outflow shocks.

Acknowledgements

MAT would like to thank PPARC for their support via a research studentship and the JCMT staff for all their assistance during the observations.