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

Abstract

We have mapped the dark cloud L183 in the far-infrared at 100 mu m and 200 mu m 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 10arcmin from the cloud centre is 25 M_sun. We have mapped the cloud in several molecular lines including DCO+(2-1) and H¹³CO+(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 mu m emission where the colour temperature rises above 15 K. The H¹³CO+ emission follows closely the DCO+ distribution but CO isotopes show strong emission even towards the 100 mu m peak. Detailed comparison of the DCO+ and C¹⁸O maps shows sharp variations in the relative intensities of the species. Morphologically the 200 mu m dust emission traces the distribution of dense molecular material as seen e.g. in C¹⁸O lines. A comparison with dust column density shows, however, that C¹⁸O is depleted by a factor of ~ 1.5 in the cloud core. We present results of R- and B-band starcounts. The extinction is much better correlated with the 200 mu m than with the 100 mu m emission. Based on the 200 mu m correlation at low extinction values we deduce a value of ~ 17^m for the visual extinction towards the cloud centre where no background stars are observed anymore. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands and the UK) and with the participation of ISAS and NASA.