Diffuse bands in RN

It may be of some interest to assess the question of DIB (Diffuse Interstellar Bands) behaviour in reflection nebulae, even if it has not a direct link with ISO data (optical range). It is often proposed that the species responsible for the IR emission bands may also carry some DIBs (Herbig 1995). In particular, PAH derivatives (ions or radicals) are good candidates for carrying a subsample of DIBs (Léger and d'Hendecourt 1985, Allamandola et al. 1985). A model has been proposed for explaining the DIB profile, based on the suprathermal rotation of a PAH molecule (Rouan et al. 1997). The "rocket effect" uses the ejection of an hydrogen atom by a privileged site of a PAH molecule, and it enables the species to access a high rotational temperature; the absorption bands are thus broadened, especially in the environment of high UV excitation, as in photo-dissociation regions and reflection nebulae. To test the model predictions, we observed the narrow DIB at 5797 Å towards a sample of reflection nebulae, and compared its profile with the "canonical" band of the standard diffuse medium (Le Coupanec et al. 1998). Some results are shown on Fig. . The profile has been corrected each time for the Doppler component of the clouds present towards this line of sight. It has been done by deconvolution of the velocity profile observed in the interstellar Sodium lines. The residual broadening observed for the stars HD34078 and HD216200 is therefore real and may correspond to the rotational broadening predicted by the model of Rouan et al. A more detailed analysis of these observations is given by Le Coupanec et al. (1998).

Other observations of DIBs towards RN (Josafatsson and Snow 1987, Jenniskens et al. 1994) sometimes show a weakening of the band strength compared to the reddening, especially when the incident flux is strong enough for destroying the carriers. For instance, very few DIBs are above the detection level in NGC7023, which is illuminated by a Be star. The carrier disappearing may either be due to the ejection of a small fragment (H, H₂, C₂H₂...) or a photo-ionisation. It conserves the global skeleton of the species, which is itself responsible for the IR emission.

It may be of interest to draw parallels between the IR emission bands and the diffuse band spectrum whenever we are able to distinguish between the contributions of the diffuse clouds and the nebula itself. Such multi-wavelength analysis may indeed constrain the physical nature of the IS species. However, we should not expect a strong correlation between DIB and IR band equivalent widths, even in the case of similar carriers; DIBs are electronic transitions, much more specific than vibrational modes. Other parameters may be related in some way, such as some IR band ratios (12.7/11.3$\mu$m) versus DIB width, to test for the hydrogen ejection mechanisms in strong UV fields.

  

Figure: The profile of the 5797Å DIB in a few lines of sight. At right, interstellar "standards"; at left, reflection nebulae where the band is broadened by 40%, as predicted by the "rocket effect" model (see Le Coupanec et al. 1998). The x-scale is in pixel units (one pixel=0.03Å), and the intensity is normalised. The Doppler profile of the line of sight is deconvolved from the observation of the Sodium lines in the same line of sight. The dotted lines represent the standard interstellar star HD30614 for comparison.