Whipping IC 63/IC 59

Abstract

Context. IC 63 and IC 59 are a pair of cometary-shaped nebulae in the vicinity of the star γCas (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 [C II] 157 µm and [O I] 63 µm, 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 [C II] 157 µm 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, G 0 , 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 G 0 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 G 0 /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.

Are the carriers of diffuse interstellar bands and extended red emission the same?

“…Finally, the exceptionally high UV radiation field in IC 63 (G 0 150; Andrews et al 2018) allows us to investigate the ratio of two DIBs, λ5797 and λ5780, which has been suggested to be a useful measure of the local interstellar radiation intensity Vos et al 2011). If true, the ratio of EW5797/EW5780 should deviate significantly from that found in the diffuse ISM with typically low interstellar radiation field densities.…”

Evidence of Dust Grain Evolution from Extinction Mapping in the IC 63 Photodissociation Region*

“…Before we discuss these fit results, some remarks should be made about the meaning of these two models and their temperatures. The use of two modified blackbody components does not relate to the existence of a warm and a cool component of the gas in the cloud (Andrews et al 2018). Instead, they each model a different wavelength range in the SED, for which the observed flux is emitted by a different subpopulation of the grains.…”

“…The whole cloud has a projected size of ∼0.5 pc, and the area studied in this work, the tip, is ∼0.1 pc wide. IC 63 is still a dense PDR (1.2 × 10 4 cm −3 ), but it has a relatively low column density (2.3 × 10 20 cm −2 ; Andrews et al 2018). Compared to the Horsehead nebula or the Orion bar, this is about an order of magnitude lower in both number and column density, making IC 63 sufficiently transparent to detect background stars.…”

“…A study using Infrared Space Observatory (ISO) data has characterized the fine structure and H 2 lines in IC 63 (Thi et al 2009), and the distribution of PAHs has been derived from Spitzer data (Fleming et al 2010). Recently, Herschel FIR maps and FIR spectroscopy data were combined with [C II] 157 μm velocity maps from the GREAT instrument on board the SOFIA observatory to revisit IC 63 in greater detail (Andrews et al 2018).…”

“…Here, an edge-on orientation was assumed, meaning that the distance between the tip of IC 63 and γCas is taken to be equal to the projected distance of 1.3 pc. In the work of Andrews et al (2018), a value of G 0 ∼150 is obtained instead, based on measurements of the FIR emission. This implies that IC 63 might be several times further away from γCas than the projected distance, and that the orientation is not truly edge-on.…”