The Mid-Infrared Polarization of the Herbig Ae Star WL 16: An Interstellar Origin?

TL;DR

This study uses high-resolution mid-infrared polarimetric imaging and spectroscopy to analyze the polarization of WL 16, revealing that the observed polarization is mainly due to interstellar dust aligned with magnetic fields, and providing insights into the dust properties and magnetic environment in the Ophiuchus region.

Contribution

The paper presents the first high-resolution mid-IR polarimetric images of WL 16, demonstrating that its polarization arises from interstellar dust aligned with magnetic fields, and constrains dust grain polarization efficiency in the molecular cloud.

Findings

Polarization vectors are aligned with the magnetic field orientation.

Mid-IR polarization is mainly due to dichroic absorption by aligned dust grains.

PAH emission features show anti-correlation between brightness and ionization fraction.

Abstract

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.