Modeling the Possible Abundance of H− Anions in Dense Molecular Clouds

More than 300 different molecules have been observed in interstellar environments by now. Molecular cations were detected in the interstellar medium as early as the 1930s, while the currently known eight molecular anions have only been observed since 2006. The hydrogen atomic anion is not one of them, although it has long been surmised that H− has played an important role in the formation of H2 in the early universe. Here we present realistic estimates of the relative abundance of this smallest anion in the interstellar medium via modeling its kinetic evolution in typical dense molecular clouds. By progressively reducing the number of reactions linked to the creation and destruction of H−, we show that only a small sub-set of reactive processes is significantly contributing to the final H− abundances in the two types of dense molecular clouds that we have considered. The fractional abundances for H− are found to stabilize between 2 and 8 × 10−13. Since this amount turns out to be not far from the observed abundances of the recently detected molecular anions, the present modeling suggests that the H− ion could play a significant role in the chemical formation of the observed interstellar molecular negative ions.