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Astron. Astrophys. 364, 723-731 (2000)

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1. Introduction

The Extended Red Emission (ERE) is a continuous emission band generally observed in the red part (600-800 nm) of the spectrum of various dusty astrophysical objects: reflection nebulae (Schmidt et al. 1980; Witt & Boroson 1990), planetary nebulae (Furton & Witt 1992), galactic and extragalactic HII regions (Perrin & Sivan 1992; Sivan & Perrin 1993; Darbon et al. 1998), high-latitude galactic cirrus clouds (Szomoru & Guhathakurta 1998), the halo of the galaxy M82 (Perrin et al. 1995) and the diffuse galactic medium (Gordon et al. 1998). It has been commonly explained by luminescence from Quenched Carbonaceous Composites (f-QCC) (Sakata et al. 1992), Hydrogenated Amorphous Carbon (HAC) grains (see for example Furton & Witt 1993; Seahra & Duley 1999), silicon nanocrystals (Witt et al. 1998; Ledoux et al. 1998) or from isolated molecules such as free Polycyclic Aromatic Hydrocarbon (PAH) molecules (see for example d'Hendecourt et al. 1986; Léger et al. 1988). Emission bands in the 3-16 µm range, the so-called Unidentified InfraRed Bands (UIRBs), are also observed in dusty environments (e.g. Sellgren 1981) and commonly attributed to PAH molecules (see for example Puget & Léger 1989; Allamandola et al. 1989) and/or carbonaceous materials (see for example Papoular et al. 1989). The existence (or absence) of a spatial correlation between UIRBs and ERE might be useful to put constraints on the nature of the carriers.

Compact HII regions, which are bright and dusty objects, seemed to us well suited for this kind of study. We therefore carried out a program of imagery and spectrophotometry of compact HII regions in order to detect and map ERE and UIRBs. This paper reports on the results obtained for Sh 152, a compact HII region, previously known as an intense near-IR source (Frogel & Persson 1972). Among the three main stars present in this object (see Fig. 2 in Heydari-Malayeri & Testor 1981), only the component S 152.1, of spectral type O9V (Hunter & Massey 1990), plays a role in the ionization and excitation of the nebula. An ionization front is clearly visible at the south and the east of S 152.1 and is closely associated with dust (Heydari-Malayeri & Testor 1981; Cox, Deharveng & Caplan 1987).

Sect. 2 describes the observations and data reduction. Sect. 3 presents the spatial correlation between visible and IR emissions. Sect. 4 shows the main spectrophotometric results we have interpreted in terms of dust scattering and luminescence. Discussion and conclusions are given in Sect. 5.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001
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