The spatial extent of polycyclic aromatic hydrocarbons emission in the Herbig star HD 179218

¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
e-mail: maotahni@ph1.uni-koeln.de, labadie@ph1.uni-koeln.de
² Department of Astronomy, Faculty of Science, University of Baghdad, Baghdad – Aljadirya, Iraq
³ Laboratoire AIM, CEA/DRF–CNRS–Université Paris Diderot, IRFU/DAS, 91191 Gif-sur-Yvette, France
⁴ Laboratoire Lagrange, CNRS UMR 7293, UNS – Observatoire de la Côte d’Azur, BP 4229, 06304 Nice Cedex 4, France
⁵ W. M. Keck Observatory, 65-1120 Mamalahoa Hwy, Kamuela, HI 96743, USA
⁶ European Space Astronomy Centre (ESAC)/ESA, P.O. Box 78, 28690 Villanueva de la Cañada, Madrid, Spain
⁷ Instituto de Astrofisica de Canarias, C/ Via Lactea s/n, La Laguna, 38200 Tenerife, Spain
⁸ Department of Astronomy, University of Florida, Gainesville, FL 32611, USA
⁹ Institute for Theoretical Physics and Astrophysics, University of Kiel, Leibnizstr. 15, 24118 Kiel, Germany

Received: 27 September 2017
Accepted: 9 November 2017

Abstract

Aim. We investigate, in the mid-infrared, the spatial properties of the polycyclic aromatic hydrocarbons (PAHs) emission in the disk of HD 179218, an intermediate-mass Herbig star at ~300 pc.

Methods. We obtained mid-infrared images in the PAH-1, PAH-2 and Si-6 filters centered at 8.6, 11.3, and 12.5 μm, and N-band low-resolution spectra using CanariCam on the 10-m Gran Telescopio Canarias (GTC). We compared the point spread function (PSF) profiles measured in the PAH filters to the profile derived in the Si-6 filter, where the thermal continuum emission dominates. We performed radiative transfer modeling of the spectral energy distribution (SED) and produced synthetic images in the three filters to investigate different spatial scenarios.

Results. Our data show that the disk emission is spatially resolved in the PAH-1 and PAH-2 filters, while unresolved in the Si-6 filter. Thanks to very good observing conditions, an average full width at half maximum (FWHM) of 0.232′′, 0.280′′ and 0.293′′ is measured in the three filters, respectively. Gaussian disk fitting and quadratic subtraction of the science and calibrator PSFs suggests a lower-limit characteristic angular diameter of the emission of ~100 mas, or ~30 au. The photometric and spectroscopic results are compatible with previous findings. Our radiative transfer (RT) modeling of the continuum suggests that the resolved emission should result from PAH molecules on the disk atmosphere being UV-excited by the central star. Simple geometrical models of the PAH component compared to the underlying continuum point to a PAH emission uniformly extended out to the physical limits of the disk model. Furthermore, our RT best model of the continuum requires a negative exponent of the surface density power-law, in contrast with earlier modeling pointing to a positive exponent.

Conclusions. We have spatially resolved – for the first time to our knowledge – the PAHs emission in the disk of HD 179218 and set constraints on its spatial extent. Based on spatial and spectroscopic considerations as well as on qualitative comparison with IRS 48 and HD 97048, 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_⊙ central star.