Low temperature spectroscopy: From ground to space

Abstract

Low temperature molecular spectroscopy represents an essential tool in laboratory astrophysics. This point is illustrated, here, through the case of the diffuse interstellar bands (DIBs), the longest unsolved enigma in astrophysical spectroscopy. Identifying the carriers responsible for these bands is a key element for a correct understanding of the energetic mechanisms governing the origin and evolution of the interstellar medium (ISM). The multidisciplinary approach adopted to address the DIB issue is described through its three components: astronomical observations, laboratory simulations, and theoretical modeling. A special emphasis is given to the experimental results obtained using matrix isolation spectroscopy (MIS). The potential molecular carriers for the DIBs are briefly discussed and a detailed study of the UV-to-NIR absorption spectra of neutral and ionized polycyclic aromatic hydrocarbons (PAHs) -for which the most focused effort to obtain astrophysically relevant laboratory data has been made to date- is presented. It is shown that, in the case of PAHs, small neutral molecules (≤equal to C-atoms) absorb only in the UV and cannot contribute to the known DIBs although they may contribute to the interstellar extinction curve. A search for their spectral signatures should allow one to set significant limits for the abundances of specific PAHs in the diffuse ISM. When ionized, small PAHs also absorb in the visible and NIR close to the position of well-known DIBs. It is found that the absorption spectra of compact PAH ions are always clearly dominated by one single band. As a result, the observed distribution of the DIBs between strong and weak features can be explained, in the context of the PAH proposal, in terms of the distribution of the band carriers between compact and non — compact PAH ions. Based on these promisingresults it is argued that PAH ions constitute good candidates to explain (some of) the DIBs.