The spatial extent of Polycyclic Aromatic Hydrocarbons emission in the Herbig star HD 179218

Abstract

We investigate in the mid-IR the spatial properties of the PAHs emission in the disk of HD179218. We obtained mid-IR images in the PAH1, PAH2 and Si6 filters at 8.6, 11.3 and 12.5 mu, and N band low-resolution spectra using CanariCam on the GTC. We compared the PSFs measured in the PAH filters to the PSF derived in the Si6 filter, where the thermal continuum dominates. We performed radiative transfer modelling of the spectral energy distribution and produced synthetic images in the three filters to investigate different spatial scenarios. Our data show that the disk emission is spatially resolved in the PAHs filters, while unresolved in the Si6 filter. An average FHWM of 0.232", 0.280" and 0.293" is measured in the three filters. Gaussian disk fitting and quadratic subtraction of the science and calibrator suggest a lower-limit characteristic angular diameter of the emission of circa 100 mas (circa 40 au). The photometric and spectroscopic results are compatible with previous findings. Our radiative transfer (RT) modelling of the continuum suggests that the resolved emission results from PAH molecules on the disk atmosphere being UV-excited by the central star. Geometrical models of the PAH component compared to the underlying continuum point at a PAH emission uniformly extended out to the physical limits of the disk's model. Also, our RT best model of the continuum requires a negative exponent of the surface density power-law, in contrast to earlier modelling pointing at a positive exponent. Based on spatial and spectroscopic considerations as well as on qualitative comparison with IRS48 and HD97048, we favor a scenario in which PAHs extend out to large radii across the flared disk surface and are at the same time predominantly in an ionized charge state due to the strong UV radiation field of the 180 L_sun central star.

Accepted for publication in A&A; 12 pages, 9 figures, 7 tables

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