Molecular cloud chemistry and the importance of dielectronic recombination

Abstract

Dielectronic recombination (DR) of singly charged ions is a reaction pathway that is commonly neglected in chemical models of molecular clouds. In this study we include state-of-the-art DR data for He${}^{+}$, C${}^{+}$, N${}^{+}$, O${}^{+}$, Na${}^{+}$, and Mg${}^{+}$ in chemical models used to simulate dense molecular clouds, protostars, and diffuse molecular clouds. We also update the radiative recombination (RR) rate coefficients for H${}^{+}$, He${}^{+}$, C${}^{+}$, N${}^{+}$, O${}^{+}$, Na${}^{+}$, and Mg${}^{+}$ to the current state-of-the-art values. The new RR data has little effect on the models. However, the inclusion of DR results in significant differences in gas-grain models of dense, cold molecular clouds for the evolution of a number of surface and gas-phase species. We find differences of a factor of 2 in the abundance for 74 of the 655 species at times of $10^4$--$10^6$ years in this model when we include DR. Of these 74 species, 16 have at least a factor of 10 difference in abundance. We find the largest differences for species formed on the surface of dust grains. These differences are due primarily to the addition of C${}^{+}$ DR, which increases the neutral C abundance, thereby enhancing the accretion of C onto dust. These results may be important for the warm-up phase of molecular clouds when surface species are desorbed into the gas phase. We also note that no reliable state-of-the-art RR or DR data exist for Si${}^{+}$, P${}^{+}$, S${}^{+}$, Cl${}^{+}$, and Fe${}^{+}$. Modern calculations for these ions are needed to better constrain molecular cloud models.

24 pages, 2 tables, 9 figures. Submitted to ApJ

References in corpus (6)

• Formation of methyl formate and other organic species in the warm-up phase of hot molecular cores
• A model for atomic and molecular interstellar gas: The Meudon PDR code
• Non-thermal desorption from interstellar dust grains via exothermic surface reactions
• Radiative recombination data for modelling dynamic finite-density plasmas
• The effect of uncertainties on chemical models of dark clouds
• Chemistry in Protoplanetary Disks: A Sensitivity Analysis

Cited by in corpus (7)

• A Simple and Accurate Network for Hydrogen and Carbon Chemistry in the ISM
• The initial gas-phase sulfur abundance in the Orion Molecular Cloud from sulfur radio recombination lines
• Dielectronic Recombination of Argon-Like Ions
• HD 62542: Probing the Bare, Dense Core of a Translucent Interstellar Cloud
• Dielectronic recombination data for dynamic finite-density plasmas XV. The silicon isoelectronic sequence
• First detection of [N II] 205 micrometer absorption in interstellar gas
• Influence of galactic arm scale dynamics on the molecular composition of the cold and dense ISM III. Elemental depletion and shortcomings of the current physico-chemical models