CARRIER OF THE LAMBDA 5797 DIB IN THE ISM AND IN THE RED RECTANGLE : A SUPRATHERMALLY ROTATING PAH MOLECULE?

The 5797 Diffuse Interstellar Band has recently been observed in emission at several positions in the Red Rect- angle, with band shape typical of molecular rotational struc- tures of an electronic transition. However, the rotational tem- peratures required to explain the observed band widths depend on the change of the rotational constant, B=B, of the transi- tion but are always signicantly 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 PAHmoleculeorionisthecarrierofthisDIB,weinvestigatethe 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 r 1 . We consider different processes of angular momentum exchange with a free PAH. We show that only an efcient rocket effect (H-atom or H2 molecule ejection) can lead to the suprathermal rotation re- quired by (i) and that its competition with radio emission can explain the power-law behaviour (ii). Using observational con- straints 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 t of the observed bandwidth and its spatialvariation.Finally,wecomputethebandwidthofthetran- sition in the diffuse ISM and in Reflection Nebulae, using the same molecular parameter range. We nd that they are consis- tent with the observed width of the 5797 DIB in the ISM, but that the t with RN observations narrows this range and implies a rotational constant change B=B = 2% and a PAH size of about 40 carbon atoms.