Variation in the near-infrared surface brightness distribution of the bipolar nebula OH 231.8 + 4.2

Near-infrared images of the bipolar nebula OH 231.8 + 4.2 obtained over a 3 yr period suggest that its surface brightness distribution varies with time. We present a model in which the variations are explained by the differences in travel times for photons scattered off dust grains at various positions along the polar axis of the nebula. This formulation allows us to determine the brightness fluctuations and heliocentric distance (d) of the embedded source and the inclination of the nebula (i). To explain the observed variations, the central star must possess a 2.2 μm amplitude of ∼2 mag - larger than determined previously from aperture photometry and consistent with the classification of the central star as a Mira variable, rather than as a supergiant. We estimate d ∼ 1300 pc, and i ≳ 35° with the south lobe further away. These results are consistent with those in the recent literature, although the value of d is sensitive to the adopted phases of the observations. Comparison with published 2.2 μm photometry suggests that the nebula is brighter at maximum and shows a larger variation in integrated surface brightness than measured ∼10 yr ago; there is evidence that over the same interval the period of the central star increased or the phase of variation shifted. Overall, these results demonstrate the potential utility of long-term monitoring, via near-IR imaging, of reflection nebulae around variable stars.