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Astron. Astrophys. 357, 651-660 (2000)

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1. Introduction

OH 231.8+4.2 is a remarkable bipolar nebula that surrounds an M 9 III Mira star (Reipurth, 1987; Cohen, 1981; Kastner et al., 1998). The optical nebula consists of two extended, inflated lobes, oriented at a position angle (PA)[FORMULA] 21o, which are flowing at high velocity from the nebula center. Its optical emission-line spectrum indicates that the gas in the lobes has been excited by shock waves (Cohen et al., 1985; Sánchez Contreras et al., 2000 - Paper I).

The neutral envelope of OH 231.8+4.2, that has been studied by means of its CO emission (Alcolea et al., 1996; Sánchez Contreras et al., 1997; Alcolea et al., in preparation - hereafter Paper III), is found to be very cold ([FORMULA] 10 K) and massive ([FORMULA] 1[FORMULA]). This envelope is expanding at low velocity ([FORMULA] 10 km s-1) near the equator, while at higher latitudes is affected by a strong axial expansion: deprojected velocities as high as [FORMULA] 375 km s-1 (relative to the nebula center) are reached at the end of the southern lobe. In contrast to the atomic material, the molecular gas is highly restricted to the symmetry axis of the nebula, showing a high length/width ratio of [FORMULA] 10-15. Emission of a large number of molecular species has been detected in OH 231.8+4.2 (e.g. Morris et al., 1987; Sánchez Contreras et al., 1997). This source is classified as O-rich, due to the presence of H2O, OH, and SiO maser emission (Genzel & Downes, 1977; Bowers & Morris, 1984; Jewell et al., 1991) and the large variety of oxygen bearing molecules. In general, many carbon containing molecules are found in OH 231.8+4.2, suggesting that an active chemistry takes place in this nebula.

The pronounced axial symmetry, the large velocities reached by the gas, and the presence of shocks, are characteristics of the OH 231.8+4.2 nebula that are also frequently observed in post-AGB objects, i.e. objects that have left the Asymptotic Giant Branch (AGB) and are evolving toward the Planetary Nebula (PN) stage. For that reason, the mentioned properties of OH 231.8+4.2 have been interpreted in the overall PNe shaping scenario, i.e. they are thought to be the result of the impact of a recent, highly collimated wind on the old, slowly expanding envelope that was ejected by the star during its AGB evolution (Reipurth, 1987; Sanchez Contreras et al., 1997; Sanchez Contreras et al., 2000; Cohen et al., 1985). We note, however, that the structure and kinematics as well as the presence of shocks in the nebula, are absolutely unexpected for its late-type central star, a Mira variable. Thus, the contradiction between the apparent latter evolutionary stage of the envelope (post-AGB) and that of the central star (on the AGB) poses a problem to determine the evolutionary status of OH 231.8+4.2.

Throughout this paper the nebula distance, luminosity and inclination (with respect to the plane of the sky) have been assumed to be, respectively, [FORMULA] 1500 pc, [FORMULA] 104 [FORMULA], and[FORMULA] 36o (with the north lobe pointing toward the observer). These values are relatively well known, the methods used to their determination being described in Bowers & Morris (1984), Reipurth (1987), Woodward et al. (1989), Kastner et al. (1992), and Shure et al. (1995).

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000