Far-infrared and molecular line observations of Lynds 183 - Studies of cold gas and dust

We have mapped the dark cloud L183 in the far-infrared at 100 mum and 200 mum with the ISOPHOT photometer aboard the ISO satellite. The observations make it possible for the first time to study the distribution and properties of the large dust grains in L183 without confusion from smaller grains. The observations show clear colour temperature variations which are likely to be caused by changes in the emission properties of the dust particles. In the cloud core the far-infrared colour temperature drops below 12 K. The data allow a new determination of the cloud mass and the mass distribution based on dust emission. The estimated mass within a radius of 10' from the cloud centre is 25 M.. We have mapped the cloud in several molecular lines including DCO+(2-1) and (HCO)-C-13+(1-0). These species are believed to be tracers of cold and dense molecular material and we detect a strong anticorrelation between the DCO+ emission and the dust colour temperatures. In particular, the DCO+(2-1) emission is not detected towards the maximum of the 100 mum emission where the colour temperature rises above 15 K. The (HCO+)-C-13 emission follows closely the DCO+ distribution but CO isotopes show strong emission even towards the 100 mum peak. Detailed comparison of the DCO+ and (CO)-O-18 maps shows sharp variations in the relative intensities of the species. Morphologically the 200 mum dust emission traces the distribution of dense molecular material as seen e.g. in (CO)-O-18 lines. A comparison with dust column density shows, however, that (CO)-O-18 is depleted by a factor of similar to1.5 in the cloud core. We present results of R- and B-band starcounts. The extinction is much better correlated with the 200 mum than with the 100 mum emission. Based on the 200 mum correlation at low extinction values we deduce a value of similar to17(m) for the visual extinction towards the cloud centre where no background stars are observed anymore.