Positron Annihilation on Polycyclic Aromatic Hydrocarbon molecules in
  the Interstellar Medium
  We examine the annihilation of positrons on polycyclic aromatic hydrocarbon
(PAH) molecules in interstellar medium conditions. We estimate the annihilation
rates of positrons on PAHs by a semi-empirical approach. We show that PAHs can
play a significant role in the overall galactic positron annihilation picture
and use the annihilation rates and INTEGRAL galactic emission measurements to
constrain the amount of PAHs present in the ISM. We find an upper limit of 4.6
x 10^-7 for the PAH abundance.
