Protostellar Disks:
I am interested in learning how protostellar disks evolve to form planets and solar systems. The formation of planets starts when the protostar is less than ~1 million years old; indeed, HL Tau has
recently received much attention due to the ALMA discovery of the youngest known planetary system.
My main focus is determining what (sub)millimeter dust continuum polarization signatures tell us about grain growth and planet formation in protostellar disks. I found the first resolved detection of polarization in a Class I/II protostar (see Figure below, published in Nature).
Usually, polarization at these wavelengths are expected to show the magnetic field morphology. However, the polarization morphology is not that which is expected from grains aligned with the magnetic field.
Figure from Stephens et al. (2014). Polarimetric map (with polarization vectors rotated by 90º to show the 'inferred' magnetic field orientation) of HL Tau with the blue color scale showing the 1.3 mm polarized flux. Contours show the 1.3 mm continuum flux. Vectors are shown in blue for 3σ > P > 2σ and red for P > 3σ. The dashed line
shows the major axis of the disk.
In Stephens et al. (2017) I followed up with multi-wavelength observations with ALMA, as seen in the figure below. The morphology is mostly circular at 3.1 mm, mostly uniform at 870 μm, and a mix between the morphologies at 1.3 mm.
We interpret the 3.1 mm polarization observations to be emission from dust grains aligned perpendicular to the radiation anisotropy, and we interpret the 870 μm observations to be from Rayleigh scattering from grains in the disk. 1.3 mm is a mix between these two morphologies.
Now that we have a good idea what causes polarization in disks, we can use such observations to constrain the size of dust grains in disks.
Once we complete many more observations, we can constrain how grains grow in disks as these disks form planets and step toward new solar systems.
Figure from Stephens et al. (2017). Multi-wavelength polarimetric maps (vectors NOT rotated) of HL Tau at 3.1 mm, 1.3 mm, and 870 μm. Contours show the continuum flux at each wavelength. Magnetic field vectors are shown in red for P > 3σ.