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Astron. Astrophys. 324, 661-673 (1997)


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Carrier of the 5797 DIB in the ISM and in the Red Rectangle: a suprathermally rotating PAH molecule?

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

1 Observatoire de Paris-Meudon, Département Spatial, CNRS F-92195 Meudon Cedex, France (e-mail: rouan@obspm.fr, lecoupanec@obspm.fr)
2 Institut d'Astrophysique Spatiale, CNRS, bat 121, F-91405 Orsay, France (e-mail:leger@iaslab.ias.fr)

Received 10 July 1996 / Accepted 20 November 1996

Abstract

The [FORMULA] Diffuse Interstellar Band has recently been observed in emission at several positions in the Red Rectangle, with band shape typical of molecular rotational structures of an electronic transition. However, the rotational temperatures required to explain the observed band widths depend on the change of the rotational constant, [FORMULA], of the transition but are always significantly higher than those expected for the hydrogen gas. We address the question of whether such a suprathermal rotation can be understood. Assuming that a free PAH molecule or ion is the carrier of this DIB, we investigate the physics of the rotation of such a molecule in order to interpret the observational characteristics of this emission in the RR: (i) a band structure indicating a rotational temperature larger than 450K at 6 arcsec from the central star; (ii) a decrease of this temperature with distance from the star as [FORMULA]. We consider different processes of angular momentum exchange with a free PAH. We show that only an efficient rocket effect (H-atom or [FORMULA] molecule ejection) can lead to the suprathermal rotation required by (i) and that its competition with radio emission can explain the power-law behaviour (ii). Using observational constraints for the gas and radiation densities in the RR, we build a model and show that there is a range of molecular parameters that leads to both a good fit of the observed bandwidth and its spatial variation. Finally, we compute the bandwidth of the transition in the diffuse ISM and in Reflection Nebulae, using the same molecular parameter range. We find that they are consistent with the observed width of the [FORMULA] DIB in the ISM, but that the fit with RN observations narrows this range and implies a rotational constant change [FORMULA] and a PAH size of about 40 carbon atoms.

Key words: ISM: Red rectangle = HD 44179 – ISM: molecules – reflection nebulae – molecular processes

Send offprint requests to: D. Rouan

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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