Whipping IC63/IC59

The mid-IR spectra of photodissociation regions (PDRs) are dominated by the well-known emission features at 3.3, 6.2, 7.7, 11.3, and 12.7 μm, generally attributed to polycyclic aromatic hydrocarbon molecules (PAHs). PAHs drive much of the physics and the chemistry in these PDRs, e.g. by heating the gas and as a catalyst in the formation of molecular hydrogen on their surfaces. Thus, PAHs and PDRs are intimately connected, and a complete knowledge of PDRs requires a good understanding of the properties of the PAH population and vice-versa, a complete knowledge of the PAH population requires a good understanding of the local physical conditions. IC63 and IC59 are a pair of cometary-shaped PDRs in the vicinity of the star γ Cas (also known as Tsih, "the Whip"). We use available data on both nebulae taken with Spitzer, Herschel and SOFIA to study the infrared emission at the tip of both clouds, and derive the intensity of the UV radiation field, the density and the gas temperature. We find that the PAH emission is very similar at the tip of both nebulae. Their PAH ratios are similar to those found in the more shielded regions of other nebulae. While in IC63 the intensity of the UV field, G0, is a factor of ~10 higher than in IC59, the density n at the tip of IC59 is lower than in IC63 by a similar factor. We derive G0 values significantly lower than reported in previous works. Comparison with other PDRs and known correlations support our claim that both IC63 and IC59 are low-UV irradiated environments. We conclude that the tips of IC63 and IC59 are about 3 and 5 times farther away from the star than their respective projected distances. The similarity of the MIR emission between the two nebulae is consistent not only with both objects being overdensities within the same region around γ Cas, but it is also consistent with the similar G0/n ratio and ionization parameters, which altogether rule the evolution of the hydrogenation and ionization level of the emitting population of PAHs. Finally, regarding the kinematics of the material in IC59, we find evidence of photo-evaporation due to the incident radiation from γ Cas.