PAHs in circumstellar disks around Herbig Ae/Be stars

Abstract

We investigate the presence and properties of PAHs on the surface of circumstellar disks around Herbig Ae/Be stars by comparing the predictions of disk models with observations. We present results of a radiation transfer code for disks heated by the central star, in hydrostatic equilibrium in the vertical direction (flared disks). The dust is a mixture of large grains in thermal equilibrium, transiently heated small grains and PAHs. Special attention is given to the influence of the stellar, disk and PAHs properties on the strength of the PAH emission lines and their spatial distribution. The models predict an infrared SED showing PAH features at 3.3, 6.2, 7.7, and 11.3 \um clearly visible above the continuum, and with some of them very strong. The PAH emission, spatially extended, comes mostly from the outer disk region ($R\sim$100 AU) while the continuum emission at similar wavelengths, mostly due to warm large grains, is confined to the innermost disk regions ($R\sim$few AU). We compare the model results to infrared observations from ISO and ground-based telescopes of some thirty Herbig Ae/Be stars. Most of the observed PAH features in objects with spectral type later than B9 are well described by our disk models and we discuss in some detail the PAH characteristics one can derive from the existing data. Objects with strong radiation field (generally earlier than about B9) have the 3.3 $\mu$m feature (often the only one observed) much weaker than predicted, and we discuss possible explanations (dissipation of the disk, photoevaporation or modification of the PAH properties).