* Astronomy

Title: The Mid-Infrared Polarisation of the Herbig Ae Star WL 16: An Interstellar Origin?
Author: Han Zhang, Charles M. Telesco, Eric Pantin, Dan Li, Christopher M. Wright, Naibí Mariñas, Peter Barnes, Aigen Li, Christopher Packham

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 ~ 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 polarisation of WL 16, mainly arises from aligned elongated dust grains present along the line of sight, suggesting a uniform morphology of polarisation vectors with an orientation of 33° (East from North) and a polarisation fraction of ~ 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 polarisation toward WL 16 is produced by the dichroic absorption of magnetically aligned foreground dust grains by a uniform magnetic field. Using polarisations of WL 16 and Elias 29, a nearby polarisation standard star, we constrain the polarisation efficiency, {p10.3/A10.3}, for the dust grains in the rho Ophiuchus molecular cloud to be ~ 1.0% mag^-1. WL 16 has polycyclic aromatic hydrocarbon (PAH) emission features detected at 8.6, 11.2, 12.0, and 12.7 m by our spectroscopic data, and we find an anti-correlation between the PAH surface brightness and the PAH ionisation fraction between the NW and SW sides of the disk.

Read more (569kb, PDF)