Probing the Connection between PAHs and Hydrogen (H, H₂) in the Laboratory and in the Interstellar Medium

Summary

Polycyclic aromatic hydrocarbons (PAHs) could play an important role in interstellar chemistry. In particular, it is important to evaluate their possible contribution to the formation of H₂. To address this question, recent laboratory results and new observations are presented. Although still preliminary, these first results are very encouraging. First, the photodissociation of PAHs isolated in ion traps and exposed to UV light involves the loss of pairs of hydrogen atoms which are likely to form H₂ molecules. Second, the PAH and H₂ emission observed in the photodissociation region associated with the young stellar object S106-IR was found to coincide at some positions. This suggests a coupling between the interstellar PAH and H₂ populations. More results are expected in the near future.

Introduction

The Infrared Space Observatory (ISO) has recently showed the ubiquity of the emission features at 3.3, 6.2, 7.7, 8.6, 11.3 and 12.7 µm in the interstellar medium (First ISO Results 1996, Boulanger 1999). Amongst the carriers for these features, polycyclic aromatic hydrocarbons (PAHs) are the best candidates as far as an excitation mechanism and a reasonable spectral agreement are concerned. A lot of infrared spectroscopy has been performed in the laboratory since the initial proposal by Léger & Puget (1984) and Allamandola et al. (1985) to find the laboratory species whose spectrum match the interstellar spectrum (Szczepanski & Vala 1993, Joblin et al. 1995, Cook et al. 1998, Allamandola & Hudgins 1999, Hudgins & Allamandola 1999, etc…).