Evidence of grain growth in the disk of the bipolar proto-planetary nebula M?1–92

摘要

Aims. We investigate the dust grain size and dust shell structure of the bipolar proto-planetary nebula M1-92. Methods. We performed radiative transfer modeling of the dustshells of M1-92. Our models consists of a disk and bipolar lobesthat are surrounded by an AGB shell, each component having differentdust characteristics. The model parameters were constrained with thepreviously obtained spectral energy distribution, the intensity images,the polarization images based on HST/NICMOS archived data as well asthe previous radio observations in the CO emission line. Results. Our model geometries with the optically thick disk andlobes with a hollow structure reproduce the bright bipolar lobes ofM1-92. The upper limit of the grain size in the lobes is estimated to be m from the polarization value in the bipolar lobe. The value of the disk is constrained with the disk mass (0.2), which was estimated from a previous CO emission line observation. We find a good model with m, which provides an approximated disk mass of 0.15.Even taking into account uncertainties such as the gas-to-dust mass ratio, a significantly larger dust of 100.0~mu$ -->m,comparing to the dust in the lobe, is expected. We also estimated thedisk inner radius, the disk outer radius, the mass-loss rate, and theenvelope mass to be 30(=9AU), 4500AU, kmsyr-1, and 4,respectively, where is the expansion velocity. Conclusions. If the dust existing in the lobes in largeseparations from the central star undergoes little dust processing, thedust sizes preserves the ones in the dust formation. Submicron-sizedgrains are found in many objects besides M1-92, suggesting thatthe size does not depend much on the object properties, such as initialmass of the central star and chemical composition of the stellarsystem. On the other hand, the grain sizes in the disk do. Evidence oflarge grains has been reported in many bipolar PPNs, includingM1-92. This result suggests that disks play an important role ingrain growth. Key words: stars: AGB and post-AGB - planetary nebulae: individual: M1-92 - radiative transfer- polarization