Electron‐Ion Recombination on Grains and Polycyclic Aromatic Hydrocarbons

With the high-resolution spectroscopy now available in the optical andsatellite UV, it is possible to determine the neutral/ionized column densityratios for several different elements in a single cloud. Assuming ionizationequilibrium for each element, one can make several independent determinationsof the electron density. For the clouds for which such an analysis has beencarried out, these different estimates disagree by large factors, suggestingthat some process (or processes) besides photoionization and radiativerecombination might play an important role in the ionization balance. Onecandidate process is collisions of ions with dust grains. Making use of recent work quantifying the abundances of polycyclic aromatichydrocarbon molecules and other grains in the interstellar medium, as well asrecent models for grain charging, we estimate the grain-assisted ionrecombination rates for several astrophysically important elements. We findthat these rates are comparable to the rates for radiative recombination forconditions typical of the cold neutral medium. Including grain-assisted ionrecombination in the ionization equilibrium analysis leads to increasedconsistency in the various electron density estimates for the gas along theline of sight to 23 Orionis. However, not all of the discrepancies can beeliminated in this way; we speculate on some other processes that might play arole. We also note that grain-assisted recombination of H+ and He+ leads tosignificantly lower electron fractions than usually assumed for the coldneutral medium.Comment: LaTeX(12 pages, 8 figures, uses emulateapj5.sty, apjfonts.sty); submitted to ApJ; corrected typo