Carrier of the Emission λ5797 DIB in the Red Rectangle: Suprathermally Rotating PAH?

Abstract

Assuming that free PAHs are the carriers of the λ5797 DIB observed in emission in the Red Rectangle, we investigate the physics of the rotation of such a molecule in order to interpret: i) a band structure indicating a very high rotational temperature (450 to 150 K, much higher than the gas temperature) and ii) a decrease of this temperature with distance from the star r ⁻¹. We consider several processes that exchange angular momentum with a free PAH, and we conclude that only an efficient rocket effect (H-atom ejection) leads 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 Red Rectangle, we build a self-consistent model that leads to a good fit of the observed rotational temperature variation. It implies rather small PAH size (10–30 carbons).