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.

💡 Research Summary

The paper investigates how polycyclic aromatic hydrocarbon (PAH) molecules influence the annihilation of positrons in the interstellar medium (ISM) and evaluates whether PAHs could account for a significant fraction of the Galactic 511 keV gamma‑ray emission observed by INTEGRAL. The authors adopt a semi‑empirical methodology that combines laboratory measurements of positron‑PAH reaction cross‑sections with astrophysical modeling of the ISM environment.

First, they compile experimental data for a range of PAH species (benzene, naphthalene, pyrene, anthracene, etc.) and extract the dependence of the capture cross‑section on molecular size, electron affinity, and the kinetic energy of the incoming positron. By assuming a Maxwell‑Boltzmann distribution for positron velocities at a typical ISM temperature of ~100 K, they calculate temperature‑dependent rate coefficients k(T) for each PAH. The resulting rates indicate that, per molecule, PAHs are 10–30 % more efficient at annihilating positrons than free electrons under the same conditions.

Second, the authors construct a Galactic PAH abundance model. They assume a power‑law mass distribution for PAHs and define a dimensionless abundance parameter X, representing the number of PAH molecules per hydrogen atom. By varying X from 10⁻⁸ to 10⁻⁵, they compute the expected 511 keV line intensity contributed by PAH‑mediated annihilation across the Galactic bulge and disk.

Third, they compare these model predictions with the 511 keV flux measured by the SPI spectrometer on INTEGRAL, which shows a total Galactic flux of roughly 10⁻³ ph cm⁻² s⁻¹, with a pronounced bulge component. A χ² minimization between model and data yields an upper limit for the PAH abundance of X ≤ 4.6 × 10⁻⁷. This limit is lower than the typical PAH abundances inferred from infrared observations (10⁻⁶–10⁻⁵), implying that PAHs cannot dominate the Galactic positron annihilation budget, though they may contribute a non‑negligible fraction.

The paper’s strengths lie in its interdisciplinary approach: it bridges laboratory atomic physics with large‑scale astrophysical observations, providing a concrete quantitative framework for assessing PAH contributions. However, several uncertainties remain. The experimental cross‑sections are available for only a limited set of PAH molecules, and extrapolations to larger, more complex PAHs introduce systematic errors. The ISM is not homogeneous; temperature, density, and ionisation fraction vary dramatically, potentially altering the effective rate coefficients. Moreover, the positron energy distribution in the Galaxy is likely broader than the simple thermal assumption used here.

In conclusion, the study demonstrates that PAH molecules can act as efficient positron traps and that, under realistic Galactic conditions, they could account for up to a few tens of percent of the observed annihilation signal if present at the upper limit of their abundance. The derived upper bound of 4.6 × 10⁻⁷ for the PAH‑to‑hydrogen ratio provides a new constraint for models of interstellar chemistry and for interpretations of the 511 keV line. Future work should aim at expanding the laboratory database of positron‑PAH cross‑sections, incorporating more sophisticated ISM models, and possibly using spatially resolved gamma‑ray data to isolate regions where PAH‑mediated annihilation might be most significant.