The abundance and emission of H2O and O-2 in clumpy molecular clouds

Recent observations with the Submillimeter Wave Astronomy Satellite (SWAS) indicate abundances of gaseous H2O and O-2 in dense molecular clouds that a re significantly lower than those found in standard homogeneous chemistry m odels. We present here results for the thermal and chemical balance of inho mogeneous molecular clouds exposed to ultraviolet radiation in which the ab undances of H2O and O-2 are computed for various density distributions, rad iation field strengths, and geometries. It is found that an inhomogeneous d ensity distribution lowers the column densities of H2O and O-2 compared wit h the homogeneous case by more than an order of magnitude at the same A(nu) . O-2 is particularly sensitive to the penetrating ultraviolet radiation, m ore so than H2O. The S140 and rho Ophiuchi clouds are studied as relevant t est cases of star-forming and quiescent regions. The SWAS results of S140 c an be accommodated naturally in a clumpy model with a mean density of 2 x 1 0 cm(-3) and an enhancement of I-UV = 140 compared with the average interst ellar radiation field, in agreement with observations of [C I] and (CO)-C-1 3 of this cloud. Additional radiative transfer computations suggest that th is diffuse H2O component is warm, similar to 60- 90 K, and can account for the bulk of the 1(10) -1(01) line emission observed by SWAS. The rho Oph mo del yields consistent O-2 abundances but too much H2O, even for, [C]/[O] = 0.94, if I-UV < 10 (respectively <40) for a mean density of 10(3) (respecti vely 10(4) cm(-3)). It is concluded that enhanced photodissociation in clum py regions can explain the low H2O and O-2 abundances and emissivities foun d in the large SWAS beam for extended molecular clouds but that additional freezeout of oxygen onto grains is needed in dense cold cores.