PAHs in protoplanetary disks: emission and X-ray destruction

TL;DR

This study models PAH emission in protoplanetary disks, examining how photon energy, hydrogenation, and X-ray effects influence PAH survival and emission, with implications for observations of T Tauri and Herbig Ae stars.

Contribution

It introduces a detailed 3D Monte Carlo radiative transfer model for PAH emission, incorporating stochastic heating, X-ray destruction, and disk halo effects, advancing understanding of PAH observability.

Findings

PAH band ratios depend on photon energy and hydrogenation, not ionization.

X-ray luminosity influences PAH destruction, explaining detection differences.

Halos affect PAH emission visibility at high inclinations.

Abstract

We study the PAH emission from protoplanetary disks. First, we discuss the dependence of the PAH band ratios on the hardness of the absorbed photons and the temperature of the stars. We show that the photon energy together with a varying degree of the PAH hydrogenation accounts for most of the observed PAH band ratios without the need to change the ionization degree of the molecules. We present an accurate treatment of stochastic heated grains in a vectorized three dimensional Monte Carlo dust radiative transfer code. The program is verified against results using ray tracing techniques. Disk models are presented for T Tauri and Herbig Ae stars. Particular attention is given to the photo-dissociation of the molecules. We consider beside PAH destruction also the survival of the molecules by vertical mixing within the disk. By applying typical X-ray luminosities the model accounts for the low PAH detection probability observed in T Tauri and the high PAH detection statistics found in Herbig Ae disks. Spherical halos above the disks are considered. We show that halos reduce the observed PAH band-to-continuum ratios when observed at high inclination. Finally, mid-IR images of disks around Herbig Ae disks are presented. We show that they are easier to resolve when PAH emission dominate.