PROBING THE STRUCTURE OF MOLECULAR CLOUD CORES - OBSERVATIONS AND MODELING OF C-I AND (CO)-O-18 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 clump s identified by previous high-resolution HCO+ and dust continuum data do not stand out. Using H-2 column densities estimated from dust conti nuum measurements, we find that the CO abundance may be reduced by fac tors 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 o f collapsing clouds in which molecules accrete on to dust grains. Alte rnative interpretations, retaining normal abundances. and relying on s ubtle optical depth and beam filling effects, are considered, but show n 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 exac t contribution from C atoms deeper into the core as their emission can not be separated from that arising at the surface. Non-LTE radiative t ransfer modelling of the (CO)-O-18 emission towards the two Class O so urces HH24MMS and HH25MMS confirms a widespread reduction of the CO ab undance by a factor of greater than 10 within a radius of 0.3 pc and n ot just close to the clumps. In HH24MMS, the abundance is required to rise again towards the centre of the model clouds in accordance with t he rise in temperature near to the central embedded object where CO is desorbed from grains. Application of the same radiative transfer mode l to the C I emission provides little constraint on the carbon abundan ce profile, although fits can be obtained for reasonable forms. The de pletion of CO in the core, coupled with the lack of an infrared cluste r, suggests that HH24-26 may be in the process of forming its first ge neration of stars.