Astron. Astrophys. 357, 661-668 (2000)

1. Introduction

The bipolar nebula OH 231.8+4.2 (IRAS OH 0739-14, sometimes called the Calabash nebula) is centred on the Mira variable QX Pup and is a source of line and continuum emission at many wavelengths. It displays strong extended optical and IR structure about 40" in length, which is very highly polarized (Scarrott et al. 1990 for the optical; Kobayashi et al. 1978; Heckert & Zeilik 1983; Shure et al. 1995; Kastner & Weintraub 1995 for the IR). At the extremity of the southern bipolar lobe, there are high velocity emission line knots, similar to Herbig-Haro objects (Reipurth 1987). The central variable star has an amplitude of 2 mag., a period of 700 days (Kastner et al. 1992) and a spectral type of M9 III (Cohen 1981; Cohen et al. 1985). The star is not directly visible in the optical on account of a waist of high extinction, but dominates the structure at longer wavelengths (Woodward et al. 1989). Kastner et al. (1992) have resolved the star from the surrounding nebula at K and L and deduce that it is a typical Mira variable. The object was first discovered on account of its OH maser emission (Turner 1971) and mm wave CO mapping has shown a high velocity outflow oriented at 40^o to the plane of the sky with deprojected velocities upto 330 km s^-1 for the southern lobe and 90 km s^-1 for the northern lobe (Alcolea et al. 1996). It is generally considered to be a transition object between the Asymptotic Giant Branch (AGB) and the Planetary Nebula phases in which the high velocity stellar wind is blowing out the AGB dust envelope. The estimated mass of the molecular and dust envelope is estimated to be 0.5 - , consistent with a main sequence mass 3 (Kastner et al. 1998).

Aside from its great astrophysical interest as a short-lived proto-planetary nebula, OH 231.8+4.2 is a useful test source for polarization measurements. From the optical to the L band it possesses high linear polarization (upto 50% in large apertures, Shure et al. 1995) and so is a useful polarization calibrator. It cannot be stated with any certainty that it possesses the most important attribute for a calibrator - that the polarization is non-variable, but the high polarization makes it useful as a calibrator for the polarimetric efficiency and for position angle determination. In addition, since the polarization pattern is centro-symmetric along the bright lobes (although not in the equatorial direction) and extended over a length of about 50", it provides an ideal source to confirm any instrumental depolarization effects. In consequence OH 231.8+4.2 was selected as a test source for the commissioning of the SOFI polarimetric mode. The quality of the instrument and the observations proved to be excellent and yielded the highest resolution near-IR polarization map of this source.

The instrumental set-up and the observations are detailed in Sect. 2 and the polarization results presented in Sect. 3. The spatial polarization structure is discussed in Sect. 4 together with conclusions about the geometry of the elongated dust cloud and the scattering properties of the grains.

© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000
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