Polyacenes and diffuse interstellar bands
  The identification of the carriers of the diffuse interstellar bands (DIBs)
remains to be established, with the exception of five bands attributed to C60+,
although it is generally agreed that DIB carriers should be large carbon-based
molecules (with ~10-100 atoms) in the gas phase, such as polycyclic aromatic
hydrocarbons (PAHs), long carbon chains or fullerenes. More specific possible
carriers among PAHs are investigated, namely elongated molecules, which could
explain a correlation between the DIB wavelength and the apparent UV resilience
of their carriers. We address the case of polyacenes, C4N+2-H2N+4, with N~10-18
fused rectilinear aligned hexagons. Polyacenes are attractive DIB carrier
candidates because their high symmetry and large linear size allow them to form
regular series of bands in the visible range with strengths larger than most
other PAHs, as confirmed by recent laboratory results up to undecacene
(C46H26). Those with very strong bands in the DIB spectral domain are just at
the limit of stability against UV photodissociation. They are part of the
prominent PAH family of interstellar carbon compounds, meaning that only ~10-5
of the total PAH abundance is enough to account for a medium-strength DIB.
After summarizing the current knowledge about the properties of polyacenes and
recent laboratory results, the likelihood that they might meet the criteria for
being carriers of some DIBs is addressed by reviewing the following properties:
wavelength and strength of their series of visible bands; interstellar
stability and abundances, charge state and hydrogenation; and DIB rotation
profiles. No definite inconsistency has been identified that precludes
polyacenes from being the carriers of some DIBs with medium or weak strength,
including the so-called C2 DIBs. But additional experimental data about long
acenes and their visible bands are needed to make robust conclusions
