Optical long-slit spectroscopy and imaging of OH 231.8+4.2

We present optical long-slit spectra and complementary broad and narrow band images of the bipolar protoplanetary nebula OH231.8+4.2. Absolute J2000 coordinates have been calculated for our maps from the position of nearby stars. Our maps of the optical continuum show the spatial distribution of the starlight scattered by dust grains. This component is found to be highly elongated along the nebular axis, with a structure very similar to that of the molecular emission. Flux variations with time of the red continuum emission are detected. Our long-slit spectroscopy of H alpha and other atomic lines reveals wide spectral profiles and, in general, a complex spatial and spectral emission distribution. The emission arises from two broad lobes and is shifted toward shorter (north lobe) and longer wavelengths (south lobe), indicating that the gas is flowing outwards at high velocity. The clumpiness of the emission nebula is remarkable. A simple model has been used to describe the complex structure and kinematics of this source. Our model suggests that, in addition to the two extended, hollow lobes identified in the H alpha images, a smaller, bubble-like expanding structure should lie inside the south lobe. A comparison of the emission line spectrum with predictions of theoretical shock models confirms that the optical lines have been shock excited. We have estimated the electron density of the lobes and deduced the total ionized mass of the nebula, obtaining a low value of similar to 5 x 10(-4) M-circle dot. We interpret the shaping and evolution of OH231.8+4.2 in the wind interaction scenario for planetary nebulae formation. The peculiar structure and kinematics of the molecular outflow and the ionized envelope are explained in terms of a shock regime transition: the well collimated molecular outflow and the different components of the optical nebula would consist of circumstellar material swept-up by a unique shock in a radiative, and non-radiative regime, respectively. Finally, we briefly discuss the controversial evolutionary status of OH 231.8+4.2.