Suprathermal rotation of PAHs in the ISM
I. Is the width of the UIBs affected?
P. Le Coupanec 1,
D. Rouan 1 and
A. Léger 2
Received 11 December 1997 / Accepted 12 June 1998
We have shown in a previous paper that the broadening of the 5797 DIB seen in emission in the Red Rectangle, can be explained by a rotational envelope indicating a high rotational temperature, and that, indeed, a molecular carrier such as a PAH, could reach an angular velocity much higher than expected from thermal equilibrium. Here, we apply our physical model of the rotation of PAHs in different conditions met in the ISM: circumstellar envelope, cirrus, dense cloud edges and reflection nebulae. We show that the prediction of a largely suprathermal rotational temperature is extended to all cases: except for the special case of the reflection nebulae, where the temperature is suprathermal but clamped to only 100K for any PAH size, we find that rotational temperatures within the range 100 - 105 K could be current in all other situations as long as the PAH size is between 100 and 600 atoms. In a second step, we explore the possible consequence of a fast rotation of a PAH on the width of the Unidentified Infrared Bands, prompted by our prediction that suprathermal rotation could be current and the recent ISO results showing that the UIBs intensity ratios and band widths are similar whatever the environmental parameters are. The evaluation of the rotation-induced broadening is performed, and we show that it is, in any cases, lower than 6 cm-1, i.e. much too low compared to the observed width of the UIB. We conclude that the observed universality of UIB 's width cannot be explained by a rotational structure, even if very suprathermal rotation is predicted.
Key words: ISM: molecules molecular processes infrared: ISM: lines and bands
Send offprint requests to: P. Le Coupanec
© European Southern Observatory (ESO) 1998
Online publication: September 8, 1998