Gas-Grain Modeling of Interstellar O _\textbf2

Molecular oxygen (O _2 ) is essential to human beings on the earth. Although elemental oxygen is rather abundant, O _2 is rare in the interstellar medium. It was only detected in two galactic and one extra-galactic region. The inconsistency between observations and theoretical studies is a big challenge for astrochemical models. Here we report a two-phase modeling research of molecular oxygen, using the Nautilus gas-grain code. We apply the isothermal cold dense models in the interstellar medium with two typical sets of initial elemental abundances, as well as the warm-up models with various physical conditions. Under cold dense conditions, we find that the timescales for gas-phase CO, O _2 and H _2 O to reach peak values are dependent on the hydrogen density and are shortened when hydrogen density increases. In warm-up models, O _2 abundances are in good agreement with observations at temperatures rising after 10 ^5 yr. In both isothermal and warm-up models, the steady-state O _2 fractional abundance is independent of the hydrogen density, as long as the temperature is high enough ( > 30 K), at which O _2 is prevented from significant depleting onto grain surface. In addition, low density is preferable for the formation of O _2 , whether molecular oxygen is under cold conditions or in warm regions.