2001ApJ...548L.217S

Recent observations with the Submillimeter Wave Astronomy Satellite (SWAS) indicate abundances of gaseous H₂O and O₂ in dense molecular clouds that are significantly lower than those found in standard homogeneous chemistry models. We present here results for the thermal and chemical balance of inhomogeneous molecular clouds exposed to ultraviolet radiation in which the abundances of H₂O and O₂ are computed for various density distributions, radiation field strengths, and geometries. It is found that an inhomogeneous density distribution lowers the column densities of H₂O and O₂ compared with the homogeneous case by more than an order of magnitude at the same A_V. O₂ is particularly sensitive to the penetrating ultraviolet radiation, more so than H₂O. The S140 and ρ Ophiuchi clouds are studied as relevant test cases of star-forming and quiescent regions. The SWAS results of S140 can be accommodated naturally in a clumpy model with a mean density of 2x10³ cm^–3 and an enhancement of I_UV=140 compared with the average interstellar radiation field, in agreement with observations of [C I] and ¹³CO of this cloud. Additional radiative transfer computations suggest that this diffuse H₂O component is warm, ∼60-90 K, and can account for the bulk of the 1₁₀-1₀₁ line emission observed by SWAS. The ρ Oph model yields consistent O₂ abundances but too much H₂O, even for [C]/[O]=0.94, if I_UV<10 (respectively <40) for a mean density of 10³ (respectively 10⁴ cm^–3). It is concluded that enhanced photodissociation in clumpy regions can explain the low H₂O and O₂ abundances and emissivities found in the large SWAS beam for extended molecular clouds but that additional freezeout of oxygen onto grains is needed in dense cold cores.