UV photodestruction of CH bonds and the evolution of the 3.4 μm feature carrier

I. The case of aliphatic and aromatic molecular species

¹ Raymond and Beverly Sackler Laboratory for Astrophysics at Leiden Observatory, 2300 RA Leiden, The Netherlands
² Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy

Corresponding author: G. M. Muñoz Caro, munoz@strw.leidenuniv.nl

Received: 26 June 2000
Accepted: 28 November 2000

Abstract

Experiments simulating the processing of various hydrocarbon species under diffuse and dense cloud conditions by UV irradiation were performed. The results indicate that such molecules will be efficiently dehydrogenated in interstellar space. It is argued that the presence of hydrogen in the aliphatic grain material in diffuse clouds results from an equilibrium between dehydrogenation by UV processing and re-hydrogenation by the impinging atomic gas. In dense clouds, the presence of the ice layer will prevent the re-hydrogenation process, causing the carbonaceous grain material to be gradually de-hydrogenated if UV photons are able to penetrate into the dense medium. The implications of this study for the evolution of the carbonaceous component of dust in the interstellar medium are discussed.