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

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.

Tiny grains shining bright in the gaps of Herbig Ae transitional discs

“…Our models have a similar powerlaw density structure to those used in other works (e.g. Bruderer et al 2014; Maaskant et al 2014, Oph IRS 48, both, respectively), as seen in Figure 6. Alternative density structures (e.g.…”

“…ous modelling of infrared spectra of both Oph IRS 48 and HD 169142 have indicated the likely presence of PAHs within the gas gap regions (regions depleted in CO; Geers et al 2007b;Maaskant et al 2014;Seok & Li 2017). Oph IRS 48 is located in the ρ Ophiuchus star forming region, as catalogued by Elias (1978) and confirmed by Wilking et al (1989).…”

“…The discs of both Oph IRS 48 and HD 169142 are known to contain PAHs (e.g. Geers et al 2007b;Maaskant et al 2014;Seok & Li 2016, Oph IRS 48, both, HD 169142, respectively). The reasoning behind the choice of dust is explained further in Section 6.1.…”

A Multi-instrument and Multi-wavelength High Angular Resolution Study of MWC 614: Quantum Heated Particles Inside the Disk Cavity*

“…Maaskant et al (2014) argued that the level of PAH ionization, measured by the ratio between the PAH line at 6.2 μm and the one at 11.3 μm, can indicate the presence of a gap in the disk. Using the spectra by Seok & Li (2017), we computed the PAH ionization ratio I 6.2 /I 11.3 (which gives the ratio of the equivalent width of the 6.2 and 11.3 μm lines) and found a value of around 3.…”

“…Another feature displayed by gapped disks is a high ionization degree of polycyclic aromatic hydrocarbons (PAHs) (Maaskant et al (2014). PAHs, and more generally all quantum heated particles (QHPs), can be quantum heated when directly exposed to the ultraviolet (UV) flux from the central star (Purcell 1976;Draine & Li 2001).…”