1999A&A...347..669L

In a previous paper, we described a model which can explain the λ5797 diffuse interstellar band (DIB) profile as seen in absorption in the diffuse interstellar medium and in emission in the Red Rectangle (RR), as a rotational envelope of electronic transitions where the molecular carrier is a free PAH of size≃40atoms. One of the strongest predictions is the behaviour of the rotational temperature of PAH in the case of regions rich in UV such as Reflection Nebulae: it must be suprathermal with respect to the gas temperature but clamped to ≃100K for any PAH size. The width of the DIB, in such regions, can then be broader than in the classical ISM (T_rot≃30K) if the values of the molecular transition are favorable. In order to test this prediction, we have obtained high resolution spectra of 27 reddened early type stars, mostly in reflection nebulae, in order to compare their λ5797 DIB width to those of stars in classical diffuse interstellar medium. These spectra were made for several DIBs such as λ 5797, λ6379 and λ6613, with a spectral resolving power of about 60000. The analysis of the results agrees with our predic tion since the width of the λ5797 DIB is broader in a majority of areas with strong UV radiation. Moreover, the broadening is not observed on DIBs λ6379 and λ6613, indicating that the molecular parameters of the electronic transitions at the origin of the different DIBs are pretty variable from one DIB to another and confirming that the measured broadening on λ5797 is not due to an instrumental bias. The statistical measurement of the λ5797 width in this medium permits the derivation of new constraints on the size of the carrier of this DIB, a molecule that should have 30 to 45 carbon atoms if, as we propose, it is indeed a PAH.