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Astron. Astrophys. 347, 669-675 (1999)

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Suprathermal rotation of PAHs in the ISM *

II. Observational evidence for the rotational broadening of [FORMULA]5797 DIB in reflection nebulae - implication for the carrier size

P. Le Coupanec 1, D. Rouan 1, C. Moutou 2 and A. Léger 3

1 Observatoire de Paris-Meudon, Département Spatial, CNRS F-92195 Meudon Cedex, France
2 Observatoire de Haute-Provence, CNRS, F-04870 Saint-Michel l'Observatoire, France
3 Institut d'Astrophysique Spatiale, CNRS, bat 121, F-91405 Orsay, France

Received 28 July 1998 / Accepted 12 May 1999

Abstract

In a previous paper, we described a model which can explain the [FORMULA] 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 [FORMULA] 40 atoms. 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 [FORMULA] 100 K for any PAH size. The width of the DIB, in such regions, can then be broader than in the classical ISM ([FORMULA] 30 K) 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 [FORMULA] DIB width to those of stars in classical diffuse interstellar medium. These spectra were made for several DIBs such as [FORMULA], [FORMULA] and [FORMULA], with a spectral resolving power of about 60 000. The analysis of the results agrees with our predic tion since the width of the [FORMULA] DIB is broader in a majority of areas with strong UV radiation. Moreover, the broadening is not observed on DIBs [FORMULA] and [FORMULA], 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 [FORMULA] is not due to an instrumental bias. The statistical measurement of the [FORMULA] 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.

Key words: line: profiles – molecular processes – ISM: molecules – ISM: reflection nebulae

* Based on observations made at Observatoire de Haute Provence (CNRS), France

Send offprint requests to: P. Le Coupanec (patricia.lecoupanec@obspm.fr)

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© European Southern Observatory (ESO) 1999

Online publication: June 30, 1999
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