THE RICH MOLECULAR CONTENT OF OH-231.8+4.2

We have carried out maps of microwave lines of 8 different molecules ( (CO)-C-12(2-1 and 1-0), (CO)-C-13(2-1 and 1-0), SiO(5-4), HCO+(1-0), S O2(10(0),(10)-9(1,9)), CS(5-4), HCN(1-0) and HNC(1-0)) in OH 231.8+4.2 , a protoplanetary nebula that shows a particularly rich molecular emi ssion. Confirming previous observations, the total molecular extent is comparable to the optical image and the lines show a total velocity r ange similar to [-80:+250] km s(-1) (LSR), due to a high-velocity flow in the axial direction. The observed transitions show a practically c onstant velocity gradient, similar to 6 km s(-1) per arcsec, in the di rection of the polar axis. All the observed molecular lines (except fo r HCO+) show similar emission features: an intense component in the ve locity range [+10:+55] that comes from the nebula center, and weaker w ing emission originating in the lobes, that appear fragmented in sever al gas components flowing at high velocity in the axial direction. HCO +, remarkably, does not show a dominant central feature, its emission being dominated by the contribution of the fast clumps. From the inten sity ratio of the (CO)-C-13 transitions, we have estimated that the CO excitation remains practically constant in the whole nebula, the rota tional temperature showing a low value, similar to 10 K. We have also calculated the mass, momentum and molecular abundances in the differen t components of the nebula. We estimate a total molecular mass in the envelope of 0.5-1 M., and at least 0.2 M. are axially flowing at veloc ities (with respect to the systemic one) larger than 40 km s(-1). It i s argued that this material corresponds to a large fraction of the env elope ejected in the previous AGE phase, after being accelerated by in teraction with the fast post-AGE jets. We stress that the high value o f the measured axial momentum cannot be explained by radiation pressur e, a different mechanism for the release of kinetic momentum by the st ar must be at work. The abundances of CS, HNC and HCN are found to be practically constant across the nebula. SO2 is more abundant in the so uth lobe, while SiO shows the opposite behavior, confirming the asymme try of the source with respect to the equatorial plane. The HCO+ abund ance is found to be much higher in the axial flow than in the central component of the nebula, as expected in view of its intensity distribu tion. We suggest that this molecule (and probably SiO) is efficiently formed in the lobes of OH 231.8+4.2 by shock-induced reactions.