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

摘要

We investigate the dust size and dust shell structure of the bipolarproto-planetary nebula M 1--92 by means of radiative transfer modeling. Ourmodels consists of a disk and bipolar lobes that are surrounded by an AGBshell, each component having different dust characteristics. The upper limit ofthe grain size $a_\mathrm{max}$ in the lobes is estimated to be $0.5 \mu$m fromthe polarization value in the bipolar lobe. The $a_\mathrm{max}$ value of thedisk is constrained with the disk mass (0.2 $M_{\sun}$), which was estimatedfrom a previous CO emission line observation. We find a good model with$a_\mathrm{max}=1000.0 \mu$m, which provides an approximated disk mass of 0.15$M_{\sun}$. Even taking into account uncertainties such as the gas-to-dust massratio, a significantly larger dust of $a_\mathrm{max}>100.0 \mu$m, comparing tothe dust in the lobe, is expected. We also estimated the disk inner radius, the disk outer radius, and theenvelope mass to be 30 $R_\star$(=9 AU), 4500 AU, and 4 $M_{\sun}$,respectively, where $v_\mathrm{exp}$ is the expansion velocity. If the dustexisting in the lobes in large separations from the central star undergoeslittle dust processing, the dust sizes preserves the ones in the dustformation. 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 asinitial mass of the central star and chemical composition of the stellarsystem. On the other hand, the grain sizes in the disk do. Evidence of largegrains has been reported in many bipolar PPNs, including M 1--92. This resultsuggests that disks play an important role in grain growth.