Probing the structure of molecular cloud cores:: observations and modelling of C I and C18O in HH24-26

We describe observations of the (CO)-O-18 J = 2 --> 1, 3 --> 2 and C I P-3(1) --> P-3(0) lines towards the HH24-26 molecular cloud core. The (CO)-O-18 traces the north-south molecular ridge, but the dense clumps identified by previous high-resolution HCO+ and dust continuum data do not stand out. Using H-2 column densities estimated from dust continuum measurements, we find that the CO abundance may be reduced by factors of at least 10 towards three positions (two of which are Class O protostars). Depending on the assumptions employed, the reduction may be as high as similar to 50 towards the clump positions. The magnitude of the reduced abundances is in good agreement with chemical models of collapsing clouds in which molecules accrete on to dust grains. Alternative interpretations, retaining normal abundances. and relying on subtle optical depth and beam filling effects, are considered, but shown to be less likely. The contrast in C I line intensity is low across the source. The greater part of the emission probably arises from the outer surface of the cloud, but it is impossible to determine the exact contribution from C atoms deeper into the core as their emission cannot be separated from that arising at the surface. Non-LTE radiative transfer modelling of the (CO)-O-18 emission towards the two Class O sources HH24MMS and HH25MMS confirms a widespread reduction of the CO abundance by a factor of greater than 10 within a radius of 0.3 pc and not just close to the clumps. In HH24MMS, the abundance is required to rise again towards the centre of the model clouds in accordance with the rise in temperature near to the central embedded object where CO is desorbed from grains. Application of the same radiative transfer model to the C I emission provides little constraint on the carbon abundance profile, although fits can be obtained for reasonable forms. The depletion of CO in the core, coupled with the lack of an infrared cluster, suggests that HH24-26 may be in the process of forming its first generation of stars.