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 (ISM) 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 International Gamma-Ray Astrophysics Laboratory Galactic emission measurements to constrain the number of PAHs present in the ISM. We find an upper limit of 4.6 x 10(-7) for the PAH abundance (by number, relative to hydrogen).