SANCHEZ CONTRERAS C., BUJARRABAL V. and ALCOLEA J.
Abstract (from CDS):
We have carried out maps of microwave lines of 8 different molecules (12CO(2-1 and 1-0), 13CO(2-1 and 1-0), SiO(5-4), HCO+(1-0), SO2(100,10-91,9), 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 emission. Confirming previous observations, the total molecular extent is comparable to the optical image and the lines show a total velocity range ~[-80:+250]km/s (LSR), due to a high-velocity flow in the axial direction. The observed transitions show a practically constant velocity gradient, ∼6km/s per arcsec, in the direction of the polar axis. All the observed molecular lines (except for HCO+) show similar emission features: an intense component in the velocity range [+10:+55] that comes from the nebula center, and weaker wing emission originating in the lobes, that appear fragmented in several 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 intensity ratio of the 13CO transitions, we have estimated that the CO excitation remains practically constant in the whole nebula, the rotational temperature showing a low value, ∼ 10K. We have also calculated the mass, momentum and molecular abundances in the different components of the nebula. We estimate a total molecular mass in the envelope of 0.5-1M☉, and at least 0.2M☉ are axially flowing at velocities (with respect to the systemic one) larger than 40km/s. It is argued that this material corresponds to a large fraction of the envelope ejected in the previous AGB phase, after being accelerated by interaction with the fast post-AGB jets. We stress that the high value of the measured axial momentum cannot be explained by radiation pressure, a different mechanism for the release of kinetic momentum by the star 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 south lobe, while SiO shows the opposite behavior, confirming the asymmetry of the source with respect to the equatorial plane. The HCO+ abundance 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 distribution. We suggest that this molecule (and probably SiO) is efficiently formed in the lobes of OH 231.8+4.2 by shock-induced reactions.