Spatially resolving polycyclic aromatic hydrocarbons in Herbig Ae disks with VISIR-NEAR at the VLT

Abstract

Context. The emission from polycyclic aromatic hydrocarbons (PAHs) arises from the uppermost layers of protoplanetary disks, higher than the optical/near-infrared scattered light and similar to the emission from the highly thick 12CO millimeter lines. The PAH intensity profiles trace the gas distribution and can constrain the penetration depth of UV radiation. Aims. We aim to constrain the spatial intensity profiles of the four strongest PAH emission features in the telluric N-band spectral region. Thereby, we seek to constrain the dependence of PAH properties on the (radial) location in the disk, such as charge state, the interrelation with the presence and dynamics of small silicate grains, and the correlation of PAH emission with gas or dust. Methods. We used the long-slit spectroscopy mode of the VISIR-NEAR experiment to perform diffraction-limited observations of eight nearby Herbig Ae protoplanetary disks. We extracted spectra for various locations along the slit with a spectral resolution of R ≈ 300 and performed a compositional fit at each spatial location using spectral templates of silicates and the four PAH bands. This yields the intensity versus location profiles of each species. Results. We obtained spatially resolved intensity profiles of the PAH emission features in the N band for five objects (AB Aurigae, HD 97048, HD 100546, HD 163296, and HD 169142). We observe two kinds of PAH emission geometry in our sample: centrally peaked (HD 97048) and ring-like (AB Aurigae, HD 100546, HD 163296, and potentially HD 169142). Comparing the spatial PAH emission profiles with near-infrared scattered light images, we find a strong correlation in the disk substructure but a difference in radial intensity decay rate. The PAH emission shows a less steep decline with distance from the star. Finally, we find a correlation between the presence of (sub)micron-sized silicate grains and the depletion of PAH emission within the inner regions of the disks. Conclusions. In this work we find the following: (1) PAH emission traces the extent of Herbig Ae disks to a considerable radial distance. (2) The correlation between the presence of silicate emission within the inner regions of disks and the depletion of PAH emission can result from dust-mixing and PAH coagulation mechanisms and competition over UV photons. (3) For all objects in our sample, PAHs undergo stochastic heating across the entire spatial extent of the disk and are not saturated. (4) The difference in radial intensity decay rates between the PAHs and scattered-light profiles may be attributed to shadowing and dust-settling effects, which impact the scattering grains more so than the PAHs.