Structure of the Mid-Infrared Emitting Disk Around WL16

Abstract

WL16 is a unique member of the embedded young stellar population in the nearby rho Ophiuchi cloud core: its extended (900 AU diameter), high surface brightness disk is visible only at mid-infrared wavelengths. We present diffraction-limited (~ 0.3 arcsec) images, from 7.9 to 24.5 microns, of WL16 acquired at the Keck II telescope. We find the central star to be a 250 Lsun, 4 Msun, Herbig Ae star, seen through foreground material of the rho Oph cloud core that provides an extinction of Av=31+/-1 magnitudes. WL16's disk is detected through all nine observed passbands. We confirm, therefore, that the emitting particles are composed of both polycyclic aromatic hydrocarbons (PAHs) and very small (5--100 Angstroms) graphitic grains (VSGs). We confirm a resolved (1.5" diameter) core component at 7.9 and 8.8 microns, due to emission from positively charged PAHs. An enhancement in the emission at 12.5 microns at the disk's edges is found for the first time, and signals the presence of larger (>= 50-80 carbon atoms) PAHs and/or more hydrogenated PAHs than those found in the bulk of the disk. We find a disk asymmetry, observed at all nine mid-infrared wavelengths, at projected radii 1"-2.5" (corresponding to 125 AU <= r <= 300 AU) from the central source.