Probing the role of polycyclic aromatic hydrocarbons in the photoelectric heating within photodissociation regions

Aims: We observationally investigate the relation between the photoelectric heating efficiency in photodissociation regions (PDRs) and the charge of polycyclic aromatic hydrocarbons (PAHs), which are considered to play a key role in photoelectric heating.
Methods: Using PACS onboard Herschel, we observed six PDRs spanning a wide range of far-ultraviolet radiation fields (G₀ = 100-10⁵). To measure the photoelectric heating efficiency, we obtained the intensities of the main cooling lines in these PDRs, i.e., the [O i] 63 μm, 145 μm, and [C ii] 158 μm, as well as the far-infrared (FIR) continuum intensity. We used Spitzer/IRS spectroscopic mapping observations to investigate the mid-infrared (MIR; 5.5-14 μm) PAH features in the same regions. We decomposed the MIR PAH emission into that of neutral (PAH⁰) and positively ionized (PAH⁺) species to derive the fraction of the positively charged PAHs in each region, and compare it to the photoelectric heating efficiency.
Results: The heating efficiency traced by ([O i] 63 μm + [O i] 145 μm + [C ii] 158 μm)/TIR, where TIR is the total infrared flux, ranges between 0.1% and 0.9% in different sources, and the fraction of PAH⁺ relative to (PAH⁰+ PAH⁺) spans from 0 (+11)% to 87 (±10)%. All positions with a high PAH⁺ fraction show a low heating efficiency, and all positions with a high heating efficiency have a low PAH⁺ fraction, supporting the scenario in which a positive grain charge results in a decreased heating efficiency. Theoretical estimates of the photoelectric heating efficiency show a stronger dependence on the charging parameter γ = G₀T^1/2/n_e than the observed efficiency reported in this study, and the discrepancy is significant at low γ. The photoelectric heating efficiency on PAHs, traced by ([O i] 63 μm + [O i] 145 μm + [C ii] 158 μm)/(PAH-band emission + [O i] 63 μm + [O i] 145 μm + [C ii] 158 μm), shows a much better match between the observations and the theoretical estimates.
Conclusions: The good agreement of the photoelectric heating efficiency on PAHs with a theoretical model indicates the dominant contribution of PAHs to the photoelectric heating. This study demonstrates the fundamental role that PAHs have in photoelectric heating. More studies of their charging behavior are crucial to understand the thermal balance of the interstellar medium.

Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA.