On Graphene in the Interstellar Medium
  The possible detection of C_{24}, a planar graphene, recently reported in
several planetary nebulae by Garciaa-Hernandez et al. (2011, 2012) inspires us
to explore whether and how much graphene could exist in the interstellar medium
(ISM) and how it would reveal its presence through its ultraviolet (UV)
extinction and infrared (IR) emission. In principle, interstellar graphene
could arise from the photochemical processing of polycyclic aromatic
hydrocarbon (PAH) molecules which are abundant in the ISM through a complete
loss of their hydrogen atoms and/or from graphite which is thought to be a
major dust species in the ISM through fragmentation caused by grain-grain
collisional shattering. Both quantum-chemical computations and laboratory
experiments have shown that the exciton-dominated electronic transitions in
graphene cause a strong absorption band near 2755 Angstrom. We calculate the UV
absorption of graphene and place an upper limit of ~5 ppm of C/H (i.e., ~1.9\%
of the total interstellar C) on the interstellar graphene abundance. We also
model the stochastic heating of graphene C_{24} excited by single starlight
photons of the interstellar radiation field in the ISM and calculate its IR
emission spectra. We also derive the abundance of graphene in the ISM to be <5
ppm of C/H by comparing the model emission spectra with that observed in the
ISM.
