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We have used the technique of optical spectropolarimetry to study the bipolar proto- planetary nebula M2-9. The lobes of this object contain brightness enhancements which have shown remarkable changes in their E-W positions over the past 40 years (Allen \& Swings 1972). The behavior and origin of these knots are still not fully understood. Van den Bergh (1974) suggested that the lobes of M2-9 are being illuminated by shifting beams of UV radiation that escape between dust clouds moving around a hot central object. Goodrich (1991) predicted that as this beam of UV radiation swept across the face of the nebula, it would leave behind an observable trail of cooling gas.
We present spectropolarimetry of the N2 knot of M2-9. Our slit placement is the same as that of Goodrich so that our spectra sample his proposed recombination tail. Spectroscopic data alone are insufficient to interpret the optical spectra of M2-9 since these spectra are a superposition of reflected and locally emitted radiation that do not accurately describe local conditions. We have isolated the local emission and have derived lines fluxes and physical parameters for the N2 knot and the adjacent nebula. We find that the electron temperature is approximately constant along our nebular cut, which is inconsistent with Goodrich's proposal of recombination tail. We propose that the observed spectra can be explained as follows. At the N2 position the lobe is photoionized by a relatively unobscured ionizing spectrum produced by the central source. At positions away from N2, the ionizing spectrum from the central source is attenuated by dust in the inner regions of the nebula. We have used the photoionization code CLOUDY (Ferland 1993) to predict the line ratios using attenuated ionizing continua and find good agreement with our observations.
This work was supported in part by NSF grant 91-15101.
