Polycyclic aromatic hydrocarbon ionization as a tracer of gas flows through protoplanetary disk gaps

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: maaskant@strw.leidenuniv.nl
² Anton Pannekoek Astronomical Institute, University of Amsterdam, PO Box 94249, 1090 GE Amsterdam, The Netherlands
³ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands

Received: 27 November 2013
Accepted: 27 January 2014

Abstract

Context. Planet-forming disks of gas and dust around young stars contain polycyclic aromatic hydrocarbons (PAHs).

Aims. We aim to characterize how the charge state of PAHs can be used as a probe of flows of gas through protoplanetary gaps. In this context, our goal is to understand the PAH spectra of four transitional disks. In addition, we want to explain the observed correlation between PAH ionization (traced by the I_6.2/I_11.3 feature ratio) and the disk mass (traced by the 1.3 mm luminosity).

Methods. We implement a model to calculate the charge state of PAHs in the Monte Carlo radiative transfer code MCMax. The emission spectra and ionization balance are calculated in the parameter space set by the properties of the star and the disk.

Results. A benchmark modeling grid is presented that shows how PAH ionization and luminosity behave as a function of star and disk properties. The PAH ionization is most sensitive to ultraviolet (UV) radiation and the electron density. In optically thick disks, where the UV field is low and the electron density is high, PAHs are predominantly neutral. Ionized PAHs trace low-density optically thin disk regions where the UV field is high and the electron density is low. Such regions are characteristic of gas flows through the gaps of transitional disks. We demonstrate that fitting the PAH spectra of four transitional disks requires a contribution of ionized PAHs in “gas flows” through the gap.

Conclusions. The PAH spectra of transitional disks can be understood as superpositions of neutral and ionized PAHs. For HD 97048, neutral PAHs in the optically thick disk dominate the spectrum. In the cases of HD 169142, HD 135344 B and Oph IRS 48, small amounts of ionized PAHs located in the “gas flows” through the gap are strong contributors to the total PAH luminosity. The observed trend in the sample of Herbig stars between the disk mass and PAH ionization may imply that lower-mass disks have larger gaps. Ionized PAHs in gas flows through these gaps contribute strongly to their spectra.