A SEARCH FOR MOLECULAR-OXYGEN IN COLD DARK CLOUDS

A search for molecular oxygen has been carried out towards TMC2, L134N and B335. We have obtained 1 x sigma upper limits to the O2/CO abunda nce ratio of approximately 0.15, 0.19 and 0.29 in TMC2, L134N, and B33 5 respectively. These are the first limits to the O2 abundance obtaine d towards dark clouds, and they are similar to those obtained previous ly towards warm clouds. Chemical equilibrium models predict a value of 0.5 for the O2/CO ratio toward dark clouds if we assume the cosmic va lue for the C/O abundance ratio (C/O approximately 0.4). This value is larger than the limits obtained towards L134N and TMC2 by a factor gr eater-than-or-equal-to 2. A C/O ratio different from the cosmic value is proposed to explain the low O2 fractional abundance. In fact, chemi cal equilibrium models give an O2 abundance consistent with our limits and in agreement with previous estimated molecular abundances towards dark clouds if we assume C/O approximately 0.7. Other possible explan ations for the low O2 fractional abundances are also discussed.