IC 59 has a lower surface brightness than IC 63, which may be the result of both lower column densities as well as a larger physical distance from the illuminating star (Blouin et al. 1997; Karr, Noriega-Crespo & Martin 2005) compared to IC 63. Because of their rare illumination geometry and proximity to Earth, these two nebulae have been studied intensively at wavelengths ranging from the Lyman limit of hydrogen to the hydrogen line and continuum near 21 cm as ideal test cases for PDR models (Perrin & Sivan 1992; Jansen, van Dishoeck & Black 1994; Blouin et al. 1997; Luhman et al. 1997; Hurwitz 1998; Habart et al. 2004; France et al. 2005; Karr et al. 2005; Thi et al. 2009; Fleming et al. 2010; Miao et al. 2010). As a result of these studies, the physical conditions in these nebulae are exceptionally well constrained. The distances of the nebulae from the illuminating star are about 1 pc, assuming that they are located near the plane of the sky occupied by γ Cas (Friedmann et al. 1996). Thus, the incident radiation field at locations within the two nebulae is essentially unidirectional, with the contribution from γ Cas about 650 times (Habart et al. 2004) more intense than the average ISRF in the solar vicinity at the front PDR in IC 63 facing γ Cas. This leads to well-defined PDRs that are viewed essentially edge-on. This is the case particularly in IC 63. Thus, it is possible to observe radiation from different emission processes requiring photons of different energies for their excitation, such as recombination lines from ionized hydrogen and sulphur, fluorescence from photoexcited molecular hydrogen, emissions from ionized and neutral polycyclic aromatic hydrocarbons (PAHs), and dust-scattered continuum at different depths behind the front of the PDR. In this regard, IC 63 is a more ideal geometry for the morphological study of ERE than NGC 7023, where the illuminating star, HD 200775, is centrally embedded and where projection effects make it more difficult to spatially separate different emission processes (Witt et al. 2006; Berné et al. 2008). One of the prime objectives of this study is to determine the spatial distribution of the ERE in IC 59 and IC 63 in relation to other emissions with known excitation requirements. For this analysis, we take advantage of the wavelength dependence of the opacity of the ISM consisting of atomic and molecular gas and interstellar dust in the UV–optical spectral range (Ryter 1996). An added benefit of the special geometry of IC 59 and IC 63 with respect to γ Cas is that scattering by dust occurring here at large scattering angles near 90° is rather inefficient, which makes it easier to separate other UV/optical emission components from the scattered light, including the ERE. The subsequent chapters of this paper are organized as follows: In Section 2, we present new optical observations of IC 59 and IC 63 and discuss their reductions and absolute intensity calibrations. Additional archival observations obtained from the WISE, Spitzer and Herschel Space Observatories will be introduced as well. In Section 3, we present the analysis of the ERE morphology in the nebulae based on digital image subtraction and division. We will estimate the ERE intensity using the colour-difference technique and determine the wavelength region of ERE excitation. A discussion of the implications for the ERE emission process and specific ERE carrier models will follow in Section 4, with conclusions to be presented in Section 5. Data tables with the results of measurements of the relative surface brightnesses of IC59 and IC63 are presented in the appendices.
Citation: Friedmann et al. 1996
Citation function: Uses