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

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3. Imagery results

Figs. 4 to 8 allow intercomparisons of the spatial distribution of several infrared and visible emissions over Sh 152, namely those arising from the 3.3 and 6.2 µm UIRBs, the 12 µm continuum, the H[FORMULA] line and the ERE. The main results shown by these figures can be summarized as follows.

[FIGURE] Fig. 4. 6.2 µm (colors) and 3.3 µm (solid contours) emission bands in Sh 152

[FIGURE] Fig. 5. 6.2 µm emission band (colors) and H[FORMULA] emission (solid contours) in Sh 152

[FIGURE] Fig. 6. ERE distribution (colors) and 6.2 µm emission (solid contours) in Sh 152

[FIGURE] Fig. 7. ERE distribution (colors) and H[FORMULA] emission (solid contours) in Sh 152

[FIGURE] Fig. 8. ERE distribution (colors) and 12 µm continuum emission (solid contours) in Sh 152

The 3.3 and 6.2 µm emission bands have the same spatial distribution over the entire nebula, suggesting that their carriers could be the same. The maxima of the infrared emissions arise from regions located outside the ionized region (traced by H[FORMULA] emission) but the ERE contours are found to closely coincide with those of the ionized region and thus significantly differ from the UIRBs distribution. This anti-correlated spatial distribution of ERE and UIRBs is very similar to the results of Kerr et al. (1999) for the Red Rectangle nebula. Also, the 12 µm continuum emission extends over the area where the ERE intensity reaches its maximum. This coincidence is in favour of grains as carriers of the ERE (i) because the 12 µm emission is thought to be the short wavelength part of a strong thermal emission from grains (see, for example, IR spectra of ultra compact galactic HII regions presented by Roelfsema et al. 1998) and (ii) because such grains can exist in Sh 152 at the distance from the exciting star where the observed coincidence occurs. In effect, this area is located about 0.2 pc from the star (assuming a distance of 3.5 kpc for Sh 152, Heydari-Malayeri & Testor 1981). According to Lamy & Perrin (1997), the temperature of a dust particule located between 10-3 pc and 0.2 pc from an O9.5V star would be lower than 200 K for a silicate grain and 400K for a carbonaceous grain.

Fig. 9 presents the four continuum images of Sh 152 at 6.911, 8.222, 10.52 and 12.00 µm taken with ISOCAM. In these images, the flux is normalized to the maximum observed in the LW6 filter, centered at 7.7 µm. At the location of the ERE maximum, the infrared images show flux values increasing with wavelength: this is in agreement with thermal emission from grains. It should be noted that the ionizing potential of OIII and SIV are about the same ([FORMULA] 35eV) and that, since the [OIII][FORMULA]5007Å emission is extremely weak (Fig. 4b of Heydari-Malayeri & Testor 1981) at the location of the maximum of the 10.52 µm emission, the ISOCAM 10.52 µm image is unlikely to be contaminated by the [SIV] 10.521 µm emission line.

[FIGURE] Fig. 9. Continuum emission in Sh 152 observed with the ISOCAM CVF at 6.911, 8.222, 10.52 and 12.00 µm

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001