H i-to-H₂ Transitions in Dust-free Interstellar Gas

We present numerical computations and analysis of atomic-to-molecular (H i-to-H2) transitions in cool (?100 K), low-metallicity, dust-free (primordial) gas which molecule formation occurs via cosmic-ray-driven negative ion chemistry removal is by a combination far-UV photodissociation cosmic-ray ionization dissociation. For any temperature, the behavior depends on ratio Lyman–Werner (LW) band FUV intensity to density, ILW/n, rate ?/n. sets H i-to-H2 abundance profiles for wide range ?/n ILW/n gas. determine conditions H2 absorption-line self-shielding optically thick clouds enables transition from atomic molecular form ionization-driven chemistry. also examine effects energy losses density points. unit Galactic interstellar field (ILW = 1) with LW flux 2.07 × 107 photons cm?2 s?1 uniform ? 10?16 s?1, an at total hydrogen column 4 1021 cm?2, within 3 yr, volume n 106 cm?3 100 K. these parameters, limit reached dust-to-gas , may be overall metallicities relative solar values.