Near-infrared polarimetric imaging of the bipolar nebula OH 231.8+4.2: The death of a beta Pic-like system

Abstract

The source OH 231.8+4.2 consists of a Mira variable star embedded in a highly collimated bipolar nebula, and thus appears to be representative of a transitional stage between red giant and planetary nebula. We obtained polarimetric near-infrared images of this source to ascertain its density structure. The patterns of polarization revealed in J and K images are characteristic of scattering of light from the central star by dust distributed in bipolar lobes. The signature of a dense circumstellar disk, previously hypothesized to explain the obscuration of the central star at optical and near-infrared wavelengths, is evident in the polarization maps and in a map of J-K color. Combined with the polarization information, the measured color gradients across the bipolar lobes suggest that the lobes are evacuated bubbles. We use the images to derive absorbing and scattering column densities of dust grains as functions of position within the nebula. The results for the total absorbing and scattering dust masses are similar, at 10 <SUP>-2</SUP> Solar mass. This similarity lends further weight to a model in which the bulk of the dust mass of the nebula lies along the lobe walls. Various lines of evidence suggest that only a small fraction of the total nebula mass resides in the circumstellar disk. This finding would appear to set OH 231.8+4.2 apart from other classical evolved bipolar nebulae, in which disks may dominate the circumstellar masses. We consider the possibility that the disk in OH 231.8+4.2 originated as a result of the influence of a binary companion to the Mira variable. Alternatively, we propose that the OH 231.8+4.2 nebula may be the end point of stellar evolution for an early A star that was surrounded throughout its main-sequence lifetime by a particulate disk. Such a disk might have resembled the dusty disk around beta Pic and, like that structure, would have had to be continually replenished in order to survive until the central star's ascent of the red giant branch. This model naturally explains this confinement of bipolar planetary nebulae to low galactic latitudes.