  We present high-resolution (0".4) mid-infrared (mid-IR) polarimetric images
and spectra of WL 16, a Herbig Ae star at a distance of 125 pc. WL 16 is
surrounded by a protoplanetary disk of $\sim$ 900 AU in diameter, making it one
of the most extended Herbig Ae/Be disks as seen in the mid-IR. The star is
behind, or embedded in, the $\rho$ Ophiuchus molecular cloud, and obscured by
28 magnitudes of extinction at optical wavelengths by the foreground cloud.
Mid-IR polarization of WL 16, mainly arises from aligned elongated dust grains
present along the line of sight, suggesting a uniform morphology of
polarization vectors with an orientation of 33\degr (East from North) and a
polarization fraction of $\sim$ 2.0\%. This orientation is consistent with
previous polarimetric surveys in the optical and near-IR bands to probe
large-scale magnetic fields in the Ophiuchus star formation region, indicating
that the observed mid-IR polarization toward WL 16 is produced by the dichroic
absorption of magnetically aligned foreground dust grains by a uniform magnetic
field. Using polarizations of WL 16 and Elias 29, a nearby polarization
standard star, we constrain the polarization efficiency,
\textit{$p_{10.3}/A_{10.3}$}, for the dust grains in the $\rho$ Ophiuchus
molecular cloud to be $\sim$ 1.0\% mag$^{-1}$. WL 16 has polycyclic aromatic
hydrocarbon (PAH) emission features detected at 8.6, 11.2, 12.0, and 12.7
$\mu$m by our spectroscopic data, and we find an anti-correlation between the
PAH surface brightness and the PAH ionization fraction between the NW and SW
sides of the disk.

Earth and Planetary Astrophysics;Solar and Stellar Astrophysics