MIDINFRARED IMAGING OF WL-16 - THE SPATIAL-DISTRIBUTION OF THE HYDROCARBON EMISSION FEATURES

We present arcsec-resolution images at 8.2, 10.0 and 11.3 mu m of the unusual young object WL 16 in Ophiuchus, which has an extended envelop e of fluorescing hydrocarbon molecules. To the limit of achieved sensi tivity, the faint 10.0-mu m continuum has a surface-brightness distrib ution that is not distinguishable from those at 8.2 and 11.3 mu m, whe re the luminosity is known to be dominated by the polycyclic aromatic hydrocarbon (PAH) emission features. We conclude that the 10-mu m cont inuum either arises from non-equilibrium heating of small dust grains that are well mixed with the hydrocarbons or is quasi-continuous emiss ion from the PAH particles themselves, rather than thermal equilibrium emission from macroscopic dust grains, and that there is no significa nt silicate absorption variation across the source. The extended hydro carbon emission may trace a flattened, equatorial distribution of circ umstellar material or arise in bipolar lobes. The former case is sligh tly favoured, based on currently available data, and would imply that WL 16 is a relatively evolved Herbig Ae star, the equatorial plane of which has been almost cleared of normal dust, leaving only fluorescing hydrocarbons and larger coagulated particles as a possibly transient fossil of the original circumstellar disc.