2010A&A...510A..30M

We investigate the dust grain size and dust shell structure of the bipolar proto-planetary nebula M1-92. We performed radiative transfer modeling of the dust shells of M1-92. Our models consists of a disk and bipolar lobes that are surrounded by an AGB shell, each component having different dust characteristics. The model parameters were constrained with the previously obtained spectral energy distribution, the intensity images, the polarization images based on HST/NICMOS archived data as well as the previous radio observations in the CO emission line. Our model geometries with the optically thick disk and lobes with a hollow structure reproduce the bright bipolar lobes of M1-92. The upper limit of the grain size a_max_in the lobes is estimated to be 0.5µm from the polarization value in the bipolar lobe. The a_max_value of the disk is constrained with the disk mass (0.2M_☉), which was estimated from a previous CO emission line observation. We find a good model with a_max₌1000.0µm, which provides an approximated disk mass of 0.15M_☉. Even taking into account uncertainties such as the gas-to-dust mass ratio, a significantly larger dust of a_max>100.0µm, comparing to the dust in the lobe, is expected. We also estimated the disk inner radius, the disk outer radius, the mass-loss rate, and the envelope mass to be 30R_*(=9AU), 4500AU, 7.5x10^–6[v_exp km/s_]M☉_/yr, and 4M_☉, respectively, where v_exp is the expansion velocity. If the dust existing in the lobes in large separations from the central star undergoes little dust processing, the dust sizes preserves the ones in the dust formation. Submicron-sized grains are found in many objects besides M∼1-92, suggesting that the size does not depend much on the object properties, such as initial mass of the central star and chemical composition of the stellar system. On the other hand, the grain sizes in the disk do. Evidence of large grains has been reported in many bipolar PPNs, including M1-92. This result suggests that disks play an important role in grain growth.