Polarization from Aligned Dust Grains in the β Pic Debris Disk

We present 870 $\mu$m ALMA polarization observations of thermal dust emission from the iconic, edge-on debris disk $\beta$ Pic. While spatially resolved map does not exhibit detectable polarized emission, we detect at $\sim$3$\sigma$ level when averaging across entire disk. The corresponding fraction is $P_\textrm{frac}$ = $0.51 \pm 0.19$%. position angle $\chi$ aligned with minor axis disk, as expected models grains via radiative alignment torques (RAT) respect to a toroidal magnetic field ($B$-RAT) or anisotropy in radiation ($k$-RAT). When outer versus inner thirds find that arises primarily SW third. perform synthetic assuming grain both $k$-RAT and $B$-RAT. Both produce fractions close our observed value averaged average likely mechanism producing A comparison timescales relevant also yields same conclusion. For realistic aspect ratios (i.e., $s > 1.1$), imply low grain-alignment efficiencies.