Astron. Astrophys. 327, 689-698 (1997)

1. Introduction

OH 231.8+4.2 (also known as OH0739-14, the Calabash Nebula, and the Rotten Egg Nebula; hereafter OH 231.8) is one of the best studied protoplanetary nebulae (PPNe). Its relative proximity and large angular extent are important advantages for the analysis of the complex dynamics often present in this kind of objects. In the visible, OH 231.8 shows a clear axial symmetry with two lobes at both sides of the central star, two collimated streamers and an obscuring ridge between the lobes (Reipurth 1987, Kastner et al . 1992, and references therein). The axis of the nebula is oriented almost in the north-south direction; the length of the nebula along this axis is about . The size in the perpendicular direction is in most of the nebula, except for a somewhat larger size in the southern extension. The inclination of the axis with respect to the plane of the sky is quite well known, i 40 , with the south lobe further away (see Shure et al. 1995, Kastner et al. 1992). The southern lobe is about twice more extended than the northern one, being ended by a remarkable bow-shaped feature. Herbig-Haro objects are observed at the front of the lobes, which are probably related to shocks caused by wind interaction, a phenomenon thought to often take place in the post-AGB phase (Cohen et al. 1985). Optical spectroscopy indicates that the spectral type of the hidden central star is M9 III (Cohen, 1981). However, the blue excess detected in its spectrum could suggest the presence of a hotter companion.

The distance to OH 231.8, particularly well known for a PPN, is of the order of 1300/tan(i) pc, i.e. 1500 pc for i = 40 , as derived from the comparison of phase lag and angular extent measurements in both OH maser emission and IR continuum (Bowers and Morris 1984, Kastner et al. 1992). Kastner et al. estimates that errors in this distance should not exceed 400 pc. For this distance, the total extent of the nebula is 10¹⁸ cm and the total luminosity is 10⁴ .

OH 231.8 is also a remarkable molecular line emitter. The ¹² CO line profiles are found to be wide, between about -80 and +250 km s^-1 LSR, with an intense central component 40 km s^-1 wide, as well as weaker wings (Alcolea et al. 1996). These authors conclude that the total mass probed by CO is 0.5-1 ; probably it is the dominant component of the nebular material and was ejected during the post-AGB phase. A compact central clump that shows a low expansion velocity ( 20 km s^-1), contains 0.4 . This part of the circumstellar envelope does not seem to be affected by wind interaction during the evolution from the AGB phase toward planetary nebula. The red and blue wings arise respectively from the south and north parts of the nebula. It is remarkable that the strong velocity gradient in the axial direction does not practically vary along the whole nebula, 6 km s^-1 per arcsec, leading to a flow (deprojected) velocity as high as 330 km s^-1. The present high velocities are probably due to shock interaction with the fast, bipolar post-AGB ejections. The molecular mass that axially flows at more than 40 km s^-1 is very large, at least 0.1 , following Alcolea et al.

OH 231.8 is also a peculiar OH maser emitter. The OH lines are less wide than those of CO, occupying between -15 and +80 km s^-1 LSR. The OH source has a extent of in the axial direction and in the perpendicular one, being centered on the star position (Morris et al. 1982). OH 231.8 also presents strong H₂ O and SiO maser emission. Such maser emission is only observed in oxygen-rich objects, so we conclude that OH 231.8 is very probably O-rich. However, this source is unusual in its chemistry because of the relatively strong emission from carbon-bearing molecules like HCN, HNC and CS, normally indicators of a carbon-rich chemistry. The strong emission of sulfur-containing molecules like H₂ S, SO₂ and SO is also remarkable (Ukita and Morris 1983, Omont et al. 1993). Up to date, the molecular abundances calculated for OH 231.8 are only average values that do not account for the possible differences between the various velocity components of the nebula, and must mainly correspond to the central clump. The abundance and excitation state of the diffuser regions rapidly flowing along the axis was up to now practically unknown. The purpose of this paper is to improve our knowledge on the mass distribution and the chemical composition of OH 231.8, particularly in the axial gas flow.

© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998
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