Warm dust surface chemistry. H2 and HD formation
  Molecular hydrogen (H2) is the main constituent of the gas in the
planet-forming disks that surround many PMS stars. H2 can be incorporated in
the atmosphere of the giant planets. HD has been detected in a few disks and
can be considered the most reliable tracer of H2. We wish to form H2 and HD
efficiently for the varied conditions encountered in protoplanetary disks: the
densities vary from 1E4 to 1E16 cm^-3; the dust temperatures range from 5 to
1500 K, the gas temperatures go from 5 to a few 1000 Kelvin, and the
ultraviolet field can be 1E7 stronger than the standard interstellar field. We
implemented a comprehensive model of H2 and HD formation on cold and warm grain
surfaces and via hydrogenated PAHs in the physico-chemical code ProDiMo. The H2
and HD formation can proceed via the Langmuir-Hinshelwood and Eley-Ridel
mechanisms for physisorbed or chemisorbed H (D) atoms. H2 and HD also form by H
(D) abstraction from hydrogenated neutral and ionised PAHs and via gas phase
reactions. H2 and HD are formed efficiently on dust grain surfaces from 10 to
700 K. All the deuterium is converted into HD in UV shielded regions as soon as
H2 is formed by gas-phase D abstraction reactions. The detailed model compares
well with standard analytical prescriptions for H2 (HD) formation. At low
temperatures, H2 is formed from the encounter of two physisorbed atoms. HD
molecules form on the grain surfaces and in the gas-phase. At temperatures
greater than 20 K, the meeting between a weakly bound H- (or D-) atom or a
gas-phase H (D) atom and a chemisorbed atom is the most efficient H2 formation
route. H2 formation through hydrogenated PAHs alone is efficient above 80 K.
The contribution of hydrogenated PAHs to the overall H2 and HD formation is
relatively low if chemisorption on silicate is taken into account and if a
small hydrogen abstraction cross-section is used.
