Polarization Observations: A Pioneering Probe of Grain Growth and Magnetic Fields in Circumstellar Disks

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Planets form in the rotating disk of gas and dust that surround very young stars before nuclear fusion begins in their cores. One of the big mysteries is how that dust grows from small interstellar dust grains to larger and larger objects: from dust to protoplanets. At the moment, there is no clear evidence of dust growth in the disk due to the difficulties in separating dust grain growth from other effects. However, recent discoveries have shown that the dust grains emit polarized light. The polarization of light is sensitive to the dust grain size, so there is a new window into dust properties. The team is observing disks in polarized light to make a comparison of dust grains properties with disk evolution. With this study, they expect to find evidence of the first steps towards planets. In addition, their outreach plans will bring these polarization observations to a larger community, mainly through a STEM workshop, a local girls' astronomy summer camp, and public lectures.The investigators will answer four critical questions: 1) Is dust continuum disk polarization across the (sub)millimeter bands dominated by magnetically aligned grains, radiationally aligned grains, mechanically aligned grains or dust scattering, 2) If magnetically aligned dust grains are not dominant at any wavelength, can we use molecular line polarization to measure the disk magnetic field?; 3) What are the intrinsic disk properties, especially the dust grain size, revealed by the polarization emission?; and 4) How do the disk properties measured by dust polarization observations vary with time? They will create a model toolkit which includes disk polarization from multiple mechanisms under general conditions. The results of this research will be important for understanding observations made with the NSF's Atacama Large Millimeter Array.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.