Dust sedimentation in protoplanetary disks with polycyclic aromatic hydrocarbons

Abstract

Context. Dust sedimentation is known to affect the infrared spectra and images of disks. In particular the far-infrared emission may be reduced by strong sedimentation. However, dust grains of different sizes sediment to different depths in the disk. Spectral features of one species may thus be enhanced, while those of other species may be suppressed. Aims. Polycyclic aromatic hydrocarbons (PAHs) are among the smallest "dust grains". We investigate how the presence of PAHs in protoplanetary disks affects the disk's spectral energy distribution (SED) and feature strengths when the thermal (large) grains are allowed to sediment. Methods. We calculate the vertical distribution of dust grains for both the PAH "dust" and the thermal dust grains.We include vertical settling and vertical mixing via turbulence. The results are inserted into a Monte-Carlo radiative transfer code to compute the SEDs. Results. For high turbulence the sedimentation barely affects the spectrum. For low turbulence, however, the PAHs still stay well-mixed in the disk's surface layer, while the 0.1 μm size grains sediment deep into the disk. This strongly enhances the PAH features relative to the continuum (by factors of 2 to 10), while the far-infrared flux is reduced. This predicts that sources with weak far-infrared flux have stronger PAH features, which is – at least among Herbig Ae stars – opposite to what is observed, suggesting that sedimentation is not the only factor responsible for the weak mid- to far-infrared excess in some disks. We speculate that coagulation might be a solution, reducing both the mid- to far-infrared flux and the PAH features.

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