A 330-360 GHz spectral survey of G34.3+0.15 - III. The outer halo

We have undertaken a 330-360 GHz molecular line survey of the halo gas surr ounding the hot core associated with G34.26+0.15. In contrast to our molecu lar line survey of the hot core itself, where 338 lines from at least 38 sp ecies were detected, only 18 lines from 9 species were detected in the halo . The lines are mainly single transitions of simple di atomic and triatomic molecules. Lower limits to their column densities have been evaluated by a n LTE method. In the case of methanol, where four transitions were detected , the rotation temperature and column density have been evaluated by the ro tation diagram technique. We have modified the previous depth-dependent che mical model developed in Paper II to calculate the column densities observe d along a general line of sight drawn through the model cloud. The model is also extended to produce beam-averaged column densities for better compari son with those observed. We compare the model column densities with those o bserved and make recommendations for future depth-dependent chemical modell ing of hot cores.