Hunting Axion Dark Matter with Protoplanetary Disk Polarimetry
  We find that the polarimetric observations of protoplanetary disks are useful
to search for ultra-light axion dark matter. Axion dark matter predicts the
rotation of the linear polarization plane of propagating light, and
protoplanetary disks are ideal targets to observe it. We show that a recent
observation puts the tightest constraint on the axion-photon coupling constant
for axion mass $m\lesssim10^{-21}$eV.
