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

TL;DR

This study models the dust structure of the bipolar proto-planetary nebula M 1--92, revealing significant grain growth in the disk, which suggests disks are crucial for dust grain evolution in such environments.

Contribution

It provides the first detailed radiative transfer modeling of dust size distribution in M 1--92, highlighting the presence of large grains in the disk compared to the lobes.

Findings

Disk contains grains up to 1000 μm in size.

Lobes contain submicron-sized grains (~0.5 μm).

Large grains in disks indicate grain growth processes.

Abstract

We investigate the dust size and dust shell structure of the bipolar proto-planetary nebula M 1--92 by means of radiative transfer modeling. Our models consists of a disk and bipolar lobes that are surrounded by an AGB shell, each component having different dust characteristics. The upper limit of the grain size $a_\mathrm{max}$ in the lobes is estimated to be $0.5 \mu$m from the polarization value in the bipolar lobe. The $a_\mathrm{max}$ value of the disk is constrained with the disk mass (0.2 $M_{\sun}$), which was estimated from 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 mass ratio, a significantly larger dust of $a_\mathrm{max}>100.0 \mu$m, comparing to the dust in the lobe, is expected. We also estimated the disk inner radius, the disk outer radius, and the envelope 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 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 M 1--92. This result suggests that disks play an important role in grain growth.