Structure of the Mid‐Infrared–emitting Disk around WL 16

WL16 is a unique member of the embedded young stellar population in thenearby rho Ophiuchi cloud core: its extended (900 AU diameter), high surfacebrightness disk is visible only at mid-infrared wavelengths. We presentdiffraction-limited (~ 0.3 arcsec) images, from 7.9 to 24.5 microns, of WL16acquired at the Keck II telescope. We find the central star to be a 250 Lsun, 4Msun, Herbig Ae star, seen through foreground material of the rho Oph cloudcore that provides an extinction of Av=31+/-1 magnitudes. WL16's disk isdetected through all nine observed passbands. We confirm, therefore, that theemitting particles are composed of both polycyclic aromatic hydrocarbons (PAHs)and very small (5--100 Angstroms) graphitic grains (VSGs). We confirm aresolved (1.5" diameter) core component at 7.9 and 8.8 microns, due to emissionfrom positively charged PAHs. An enhancement in the emission at 12.5 microns atthe disk's edges is found for the first time, and signals the presence oflarger (>= 50-80 carbon atoms) PAHs and/or more hydrogenated PAHs than thosefound in the bulk of the disk. We find a disk asymmetry, observed at all ninemid-infrared wavelengths, at projected radii 1"-2.5" (corresponding to 125 AU<= r <= 300 AU) from the central source.Comment: AASTeX preprint2, 16 pages, 8 figures; accepted by the Ap