The molecular condensations ahead of Herbig-Haro objects - III. Radiative and dynamical perturbations of the HH 2 condensation

We have carried out an extensive observational study ( from BIMA data) and made a preliminary theoretical investigation of the molecular gas around HH 2. The molecular maps show a very complex morphological, kinematical and chemical structure. For clarity we divided the observed region in four subregions: ( 1) The Ahead Core, located ahead of HH 2: its chemistry may be a consequence of a weak UV field originating in HH 2. The chemical structure within the Ahead Core suggests that it is not homogeneous but probably composed of small clumps; ( 2) The SO2 Clump, which is a molecular component within the Ahead Core that is more exposed to the UV radiation from HH 2. An increase of density and relative molecular abundances is observed towards HH 2. The UV radiation is possibly the source of molecular enhancement. Our chemical analysis confirms that this clump must have substructure within it; ( 3) the West Core, which is surrounded by a ring structure of shocked ionized gas and mid-IR emission. The ring structure is likely a consequence of the fact that the core is in the foreground with respect to the shocked and hot component. The chemistry of this core can be best explained as arising from a combination of an old photo-processed dense clump and a PDR, with or without a warm interface created in the interaction of the outflow with the core; ( 4) The High Velocity Region, associated with HH 2, is traced by HCO+ but not by other molecular shock tracers. The chemistry can be accounted for by the interaction of the VLA 1 outflow with a dense clump via non-dissociative shocks and by the presence of a very strong UV field. The overall main conclusion of this work confirms the findings of Papers I and II, by demonstrating that in addition to the strong photochemical effects caused by penetration of the UV photons from HH 2 into molecular cloud, a range of complex radiative and dynamical interactions occur. Thus, despite the apparent "quiescent" nature of the molecular cloud ahead of HH 2, the kinematical properties observed within the field of view suggest that it is possibly being driven out by the powerful winds from the VLA 1 protostar.