Excited hydrogen and the formation of molecular-hydrogen via associative ionization. 1 : Physical processes and outflows from young stellar objects

Abstract

Rates for the associative ionization reaction H(n=2)+H→H2++e- are calculated. This reaction is found to be a greater contributor to the H2 formation rate than the direct radiative association reaction H(n=2)+H→H2+hν in most regions of astrophysical interest. Chemical models of circumstellar regions are reassessed in the light of this information. We also examine the chemical behaviour of several other excited chemical species. A critical examination reveals that excitation effects are, in general, very important in many astrophysical situations and must be incorporated into the chemistry. H2 has been detected in a variety of circumstellar regions and has a pivotal role in the overall chemistry. The method and efficiency of its formation are therefore of great importance. We test the significance of the associative ionization reaction in several models. These models include a schematic description of the radiative transfer in H I Lyα. The endothermicity (≃ 1.1 eV) of the reaction and the high departures from LTE that are required for the H I (n=2) level to be sufficiently populated restrict its significance to regions of high excitation, such as are found in circumstellar regions. In this paper (I), we investigate the importance of the reaction in winds associated with young stellar objects. In Paper II, the investigation will be extended to include novae, supernovae, planetary nebulae and shocked regions. The results indicate that reactions involving excited atomic states may be very important in a number of circumstellar chemistries. Only exceptionally will reactions involving the higher excited states (n>2) be as significant as those involving H(n =2).