The mid-infrared polarization of the Herbig Ae star WL 16: an interstellar origin?

We present high-resolution (0.4 arcsec) 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 disc of ∼ 900 au in diameter, making it one of the most extended Herbig Ae/Be discs as seen in the mid-IR. The star is behind, or embedded in, the ρ Ophiuchus molecular cloud, and obscured by 28 mag 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|$^\circ$|(east from north) and a polarization fraction of ∼ 2.0 per cent. 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 towards 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, p10.3/A10.3, for the dust grains in the ρ Ophiuchus molecular cloud to be ≃1.0 per cent mag−1. WL 16 has polycyclic aromatic hydrocarbon (PAH) emission features detected at 8.6, 11.2, 12.0, and 12.7 μm by our spectroscopic data, and we find an anticorrelation between the PAH surface brightness and the PAH ionization fraction between the north-west and south-west sides of the disc.