Polycyclic Aromatic Hydrocarbons in Protoplanetary Disks around Herbig Ae/Be and T Tauri Stars

Abstract

A distinct set of broad emission features at 3. 3, 6.2, 7.7, 8.6, 11.3, and 12.7 µm, is often detected in protoplanetary disks (PPDs). These features are commonly attributed to polycyclic aromatic hydrocarbons (PAHs). We model these emission features in the infrared spectra of 69 PPDs around 14 T Tauri and 55 Herbig Ae/Be stars in terms of astronomical-PAHs. For each PPD, we derive the size distribution and the charge state of PAHs. We then examine the correlations of the PAH properties (i.e., sizes and ionization fractions) with the stellar properties (e.g., stellar effective temperature, luminosity, and mass). We find that the characteristic size of PAHs shows a tendency of correlating with the stellar effective temperature (T eff ) and interpret this as the preferential photodissociation of small PAHs in systems with higher T eff of which the stellar photons are more energetic. In addition, the PAH size shows a moderate correlation with the red-ward wavelength-shift of the 7.7 µm PAH feature that is commonly observed in disks around cool stars. The ionization fraction of PAHs does not seem to correlate with any stellar parameters. This is because the charging of PAHs depends on not only the stellar properties (e.g., T eff , luminosity) but also the spatial distribution of PAHs in the disks. The mere negative correlation between the PAH size and the stellar age suggests that continuous replenishment of PAHs via the outgassing of cometary bodies and/or the collisional grinding of planetesimals and asteroids is required to maintain the abundance of small PAHs against complete destruction by photodissociation.

CLIcK: a Continuum and Line fItting Kit for circumstellar disks

“…We have also demonstrated that CLIcK can reproduce well the entire Spitzer/IRS water emission spectrum from AA Tau, especially with the power-law temperature profile option, and correctly retrieve the parameters of the water emitting region from a reference spectrum simulated with RADLite. The framework of CLIcK can be easily generalized to solve similar problems in which the dust continuum has to be properly treated, including fitting the emission features from crystalline silicates (e.g., Juhász et al 2010) and polycyclic aromatic hydrocarbons (e.g., Seok & Li 2017) in circumstellar disks.…”

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

“…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).…”

Spitzer’s perspective of polycyclic aromatic hydrocarbons in galaxies

“…Owing to its up to a factor of 100 better sensitivity than ISO/SWS, Spitzer allowed one to extend the mid-IR spectroscopy and imaging into new regimes that ISO could not probe. The Infrared Spectrograph (IRS) on board Spitzer was capable of detecting PAH emission in objects which were too faint for ISO (e.g., the PAH emission at 6-9 µm of protoplanetary disks around T Tauri stars which had previously escaped detection by ISO 46 was unambiguously detected by Spitzer/IRS [47][48][49] ; see Fig. 1e).…”