HST observations of the protoplanetary nebula OH 231.8+4.2: The structure of the jets and shocks

We present high-resolution images obtained with the WFPC2, on board the HST, of the protoplanetary nebula (PPN) OH231.8+4.2. Halpha and NII line emission and scattered light in the continuum at 6750 and 7910 Angstrom were observed. We also discuss NIR NICMOS images from the HST archive. The images show with high accuracy the shape and excitation state of the shocks developed in the nebula. Our high-resolution images (and data from other works) allow a very detailed and quantitative description of the different nebular components and of the physical conditions in them. We interpret specific structures identified in our images using existing models of shock interaction. In the center of the nebula, there is a dense torus- or disk-like condensation continued by an hourglass-like structure, with relatively high densities (similar to10(5) -10(6) cm(-3)) and temperatures (similar to30 K). Inside this torus we have identified the location of the central star, from SiO maser observations. Two shock regions are detected from the optical line emission images, respectively in the north and south lobes. In both regions, a forward and a backward shock are identified. The densities of this hot gas vary between 40 and 250 cm(-3), with the densest clumps being placed in the reverse shocks. The total mass of the shocked hot gas is similar to2 x 10(-3) M., both lobes showing similar masses in spite of their different extents. The relatively collimated jet that impinges on an originally slow shell, so producing the shocks, is identified from the scattered light images and in CO maps. This flow is significantly denser and cooler than the shocked Halpha regions. Its density decreases with the distance to the star, with typical values (similar to10(5)-10(4) cm(-3)), and its temperature ranges between about 25 and 8 K. We explain the high Halpha emission of the backward shock assuming that it propagates in a diffuse gas component, entrained by the observed collimated flow and sharing its axial movement. The existence of shocks also in the collimated densest flow is suggested by the high abundance of some molecules like HCO+ and its structure and kinematics in certain regions, but they are not seen in Halpha emission, probably because of the absence of (well developed) hot components in this dense flow. We think that the exceptionally detailed and quantitative image derived for the wind interaction regions in OH231.8+4.2 is a challenge to check and improve hydrodynamical models of wind interaction in PPNe.