Grain growth in protoplanetary disks is the critical first step in the formation of planetesimals and ultimately planets. This process is not well constrained, however, because grain sizes are…

This is a grant award from the Shared Services Center to the Rector & Visitors of the University of Virginia (UVA) for a research project titled "Probing Disk Magnetic Fields and Grain Growth through (Sub)Millimeter Polarization." The $331,958.50 grant, awarded on July 27, 2018, supports UVA's research to better understand the process of grain growth in protoplanetary disks, which is a critical first step in planet formation. The research aims to develop polarization as a reliable tool for studying disk magnetic fields and grain growth. The project will account for polarization from both direct emission by aligned grains and dust scattering, and will consider optical depth effects and non-axisymmetric disk features. This grant does not have a set-aside designation, indicating open competition. UVA, a prominent public research university, has a history of conducting advanced scientific research for federal agencies across various domains.

Grain growth in protoplanetary disks is the critical first step in the formation of planetesimals and ultimately planets. This process is not well constrained, however, because grain sizes are difficult to measure directly. A potential observational breakthrough came from the recent detection of spatially-resolved polarized disk emission at (sub)millimeter wavelengths. The conventional interpretation is that the polarization is produced by magnetically aligned dust grains. If true, IT would provide the long-sought-after evidence that young star disks indeed have magnetic fields, which are widely believed to play a key role in determining the structure and evolution of the disks, which, in turn, strongly affect the formation and migration of planets. However, the PI's team has recently shown that the polarization observed in one of the best studied disks to date (HL TAU) is more consistent with the pattern produced by scattering off large dust grains instead. If this is true in general, IT would complicate the use of polarization to probe the disk magnetic field. On the other hand, IT would open up an exciting new window on grain growth, the poorly constrained first step toward planet formation. In order to develop the (sub)millimeter polarization into a reliable tool for probing the disk magnetic field and grain growth, we propose a comprehensive program that simultaneously accounts for the polarizations from both direct emission by aligned grains and dust scattering. We will go beyond our preliminary work by including optical depth effects and non-axisymmetric features. The former is required for direct comparison with observations at millimeter and especially submillimeter wavelengths, where disks are often optically thick. The latter is necessary for interpreting the polarization patterns produced by important disk features, such as spiral arms and dust traps, that are intrinsically non-axisymmetric. The optical depth effects will be determined by iteratively solving the radiative transfer equation for polarized light. The non-axisymmetric features will be treated through monte-carlo simulations. Our proposed program is timely in view of alma, which is starting to revolutionize disk polarization observations. The PI and collaborators are deeply involved with polarization observations. The proposed program will provide the necessary theoretical tools for interpreting the observations and maximizing the scientific return of the hard-won data. Because of the strong interplay between polarizations from direct emission and scattering, neither disk magnetic field nor grain growth can be ascertained individually without detailed modeling efforts such as ours that account for both polarization mechanisms simultaneously. By enabling the detection of magnetic fields and grain growth in young star disks, our proposed program directly supports one of the stated science goals of NASA's astrophysics division: ``understand how individual STARS form and how those processes ultimately affect the formation of planetary systems.'' the tools that we plan to develop for polarization at (sub)millimeter will be useful for interpreting NASA's sofia/hawc+ polarization observations at far infrared as well.