Molecular oxygen: A new astrophysical maser?

New excitation calculations for molecular oxygen, which predict maser emission for plausible interstellar conditions, are reported. Collisional rate coefficients for the collision system O2-He were calculated for several kinetic energies in the range 20-500 K. Excitation calculations have been performed for a typical dark cloud and for the core and envelope parts of a giant molecular cloud. It is found that for hot ((Tsub kin) approximately greater than 200 K)), dense (nH2 approximately equals 105/cu cm) interstellar clouds the NJ = NN to NN-1 transitions (1 less than N approximately less than 23) of O2 may become significantly inverted, provided that the O2 abundance (per velocity gradient) is high, X(O2)/(dv/dr) greater than 10-4/km/sec/pc. All these transitions occur at frequencies between 54 and 63 GHz. The physical conditions needed for the masers to exist are similar to, but less severe than, what is required for the 22 GHz water maser line.