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Astron. Astrophys. 331, 742-748 (1998)

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1. Introduction

A surprisingly high level of mid-IR emission at 12 and 25 µm was detected by IRAS (see Boulanger et al. 1985) from many of the high latitude cirrus clouds which were most prominently seen in the IRAS 100 µm band (Low et al. 1984). This emission constitutes in many cases 20 - 30% of the total IR radiation energy of the cloud and cannot be explained as equilibrium emission by the classical dust grains. Puget et al. (1985) have proposed that the IRAS 12 µm emission of cirrus clouds is largely due to the unidentified infrared (UIR) bands. There has still been no unambiguous identification of the specific carriers of the UIR bands, the strongest of which are located at 3.3, 6.2, 7.7, 8.6, 11.3, and 12.7 µm. However, there is good evidence for the carriers being some form of carbonaceous material excited by UV photons: either large free polycyclic aromatic hydrocarbon molecules (PAHs) (Léger & Puget 1984) or grains consisting of hydrogenated amorphous carbon (HACs) (Duley & Williams 1981), quenched carbonaceous composites (QCCs) (Sakata et al. 1984), or coal (Papoular et al. 1989).

The PAH model has been supported by observations of the diffuse galactic disk emission: the photometric detection of the 3.3 µm and 6.2 µm UIR bands using the AROME balloon-borne experiment (Giard et al. 1988; Ristorcelli et al. 1994), and the ISOPHOT and IRTS spectrophotometry of the 6.2 - 11.3 µm and 3.3 - 11.3 µm UIR bands in lines of sight towards the inner Galaxy (Mattila et al. 1996, Tanaka et al. 1996, Onaka et al. 1996). Further support has been provided also by COBE/DIRBE broad band photometry of intermediate and high latitude cirrus in the 3.5 - 25 µm wavelength region (Bernard et al. 1994, 1996; Dwek et al. 1997).

The UIR emission bands have been observed in a number of bright galactic objects, such as planetary nebulae (PN), HII regions, and reflection nebulae (RN) around early type stars (Gillett et al. 1973), as well as in some external galaxies. These objects are characterized by UV radiation densities several orders of magnitude above the average interstellar radiation field (ISRF) representative of the solar neighbourhood. No spectrally resolved observations have been available so far on the near- or mid-IR emission of an individual isolated cirrus cloud.

The present investigation was initiated to answer the following questions:
(1) Are the UIR emission bands at 3.3, 6.2, 7.7, 8.6, 11.3, and 12.7 µm present also in the diffuse emission of local cirrus clouds where the UV ISRF has a typical solar neighbourhood value?
(2) Can the IRAS band 1 (12 µm) cirrus emission be explained as the integrated emission of the UIR bands?
(3) What are the relative intensities of the UIR bands in comparison with the bright galactic sources, the PNs, RNs, and the HII regions?

Using ISOPHOT-P1 between 3.3 and 16 µm we observed the cirrus/translucent cloud G 300.2-16.8 in the Chamaeleon dark cloud complex at a distance of [FORMULA] 140 - 200 pc from the sun (Franco 1991). There is recent indication that the cloud might actually be at half the distance if associated to a nearby star for which a Hipparcos determined distance had just become available. G 300.2-16.8 is characterized by a large IRAS 12/100µm ratio of 0.14 (Laureijs et al. 1989). The signal in the different IRAS bands peaks at different positions within the cloud, which indicates local dust composition changes. The relatively high galactic latitude ([FORMULA]) helps to avoid confusion with unrelated structures along the line of sight. Since the cirrus signal is only a few per cent of the zodiacal emission at the mid-IR wavelengths the high ecliptic latitude of G 300.2-16.8 ([FORMULA]) is an important factor in reducing noise and drift problems.

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© European Southern Observatory (ESO) 1998

Online publication: February 16, 1998
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