Polyynyl-Substituted PAH Molecules and DIB Carriers

Polycyclic aromatic hydrocarbon (PAH) molecules have been long considered promising candidates for the carriers of the diffuse interstellar bands (DIBs). The PAH-DIB hypothesis, however, raises two major issues. First, the number of interstellar PAH species is potentially orders of magnitude larger than the number of DIBs. Second, the absorption spectrum of a PAH is in general dominated by bands found at UV wavelengths while, conversely, DIBs are absent from the UV wavelength domain and arise at visible and near IR wavelengths. These issues do not necessarily weaken the PAH-DIB hypothesis and can actually allow us to refine it. In that context, we analyze the UV/vis absorption spectra of PAH molecules isolated in Ne matrices and propose that polyynyl-substituted PAHs, or similar species, are valid candidates for the carriers of the DIBs. Finally, a possible lifecycle for DIB-carrying PAHs is presented.