2018A&A...619A.170A

Context. IC 63 and IC 59 are a pair of cometary-shaped nebulae in the vicinity of the star γ Cas (also known as Tsih, "the Whip"). Both nebulae have very different optical appearances, despite the fact that both objects lie at similar projected distances from the star: IC 63 shows bright rims and filaments, while IC 59 looks more homogeneous and faint.
Aims. We aim to perform a general description of the two nebulae from an observational standpoint in order to study the physical conditions at the UV-illuminated surfaces of these objects.
Methods. We used the available data on both nebulae taken with Spitzer and Herschel to study the infrared emission at the tip of both clouds, and derive the intensity of the UV radiation field. Using the photodetector array camera and spectrometer (PACS) in line spectroscopy mode, we obtained the intensity of the cooling lines [CII] 157µm and [OI] 63µm, and we used these to estimate the density in these regions. Excitation diagrams of molecular hydrogen were obtained to derive the gas temperature. We also used [CII] 157µm velocity maps of IC 59 taken with German REceiver for Astronomy at Terahertz frequencies (GREAT) on board SOFIA to explore the gas dynamics in this nebula.
Results. We find that the IR emission from polycyclic aromatic hydrocarbons (PAHs) is very similar at the tip of both nebulae. Even though it varies in intensity between the two, the derived PAH band ratios are remarkably similar. These ratios are similar to those found in the more shielded regions of other nebulae such as NGC 7023 and NGC 2023. Regarding the physical conditions, we get that while in IC 63 the intensity of the UV field, G₀, is a factor of approximately ten higher than in IC 59, the density n at the tip of IC 59 is lower than in IC 63 by a similar factor. For both objects we derive G₀ values significantly lower than what previous works have so far assumed. Comparison with other reflection nebulae photo-dissociation regions (PDRs) and known correlations support our claim that both IC 63 and IC 59 are low-UV irradiated environments.
Conclusions. We conclude that the tips of IC 63 and IC 59 are about three and five times farther away from the star than their respective projected distances. The similarity of the mid-infrared emission between the two nebulae is consistent not only with both objects being overdensities within the same region around γCas, but it is also consistent with the similar G₀/n 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 IC 59, we find evidence of photo-evaporation due to the incident radiation from γCas.