| Popis: |
Recent observations with the Submillimeter Wave Astronomy Satellite (SWAS) indicate abundances of gaseous H2O and O-2 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 O-2 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 O-2 compared with 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, more so than H2O. The S140 and rho 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 2 x 10 cm(-3) and an enhancement of I-UV = 140 compared with the average interstellar radiation field, in agreement with observations of [C I] and (CO)-C-13 of this cloud. Additional radiative transfer computations suggest that this 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 model yields consistent O-2 abundances but too much H2O, even for, [C]/[O] = 0.94, if I-UV |
