Spatial distribution of unidentified bands and extended red emission in compact galactic HII regions

S. Darbon ¹, A. Zavagno ², C. Savine ¹, V. Ducci ², J.-M. Perrin ¹, & J.-P. Sivan ¹

¹ Observatoire de Haute Provance du CNRS, 04870 Saint Michel l'Observatoire, France

² Observatoire de Marseille, 2 place Leverrier, 13248 Marseille Cedex 04, France

Extended red emission (ERE) is a continuous emission band observed in the red part (600-800 nm) of the spectrum of various astrophysical objects, commonly attributed to hydrogenated amorphous carbon grains. A series of emission bands in the 3-16  range, the so-called UIRBs, is also observed in dusty environments and commonly attributed to polycyclic aromatic molecules and/or carbonaceous materials. However, recent ISO results show that the identification of these band carriers is still a matter of debate. New observations are needed to improve our knowledge of the carriers' properties. In particular, the existence (or absence) of a link between the two types of emission (ERE or UIRB) would constrain the carriers' identification. In this context, we have begun an observational program of imaging of five compact galactic HII regions. Images in the 500-850 nm range with ISOCAM (see Zavagno and Ducci, this conference) through filters centered on the UIRBs.

We present the first results of this study obtained ofr the compact HII region Sharpless 152. This region clearly exhibits the two types of emission: ERE and UIRB. Their spatial distributions are found to be significantly different. The ERE emission coincides with the emission of the ionized gas (revealed by and H image), where the UIRBs are found at the interface between the ionized and neutral medium. This indicates that the ERE carriers may better survive in harsh environments than the UIRBs ones. These observational results are in agreement with the hypothesis of cabonaceous carriers of the ERE: hydroganated environment and UV irradiation favour luminescence of low-rank carbonaceous grains.

ISOGAL images (see Omont et al. this conference) of this region at 6.2 and 15  are also presented (see also Copet et al., this conference). These images reveal the large-scale structure of the dust distribution and the presence of a deeply embedded source barely detected at 800 .