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

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Sánchez Contreras, C.
Bujarrabal, V.
Miranda, L. F.
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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_α 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_α 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 ∼5 x 10^-4 M_⨀. 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.
The Isaac Newton and Jacobus Kapteyn Telescopes are operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. We are grateful to J. Zamorano and A. Gil de Paz for the images obtained with the JKT, and to J. Sanz Forcada for its help in the spectroscopic data reduction process with IRAF. We also thank J. Alcolea for critically reading this paper. C. Sánchez Contreras acknowledges the support of the Universidad Complutense de Madrid through a predoctoral fellowship. C. Sánchez Contreras is also grateful to the staff of the Instituto de Astrofísica de Andalucía for their hospitality during a part of the preparation of this paper. This work has been partially supported by the DGES, project numbers PB96-0104, PB95-0066, APC1998-0031, and the Junta de Andalucía.
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