The extinction law for molecular clouds

Context. The large optical and near-IR surveys have made it possible to investigate the properties of dark clouds by means of extinction estimates. There is, however, a need for case studies in more detail in order to investigate the basic assumptions when, say, interpreting reddening in terms of column density. Aims. We determine the extinction curve from the UV to the near-IR for molecular clouds and investigate whether current models can adequately explain this wavelength dependence of the extinction. The aim is also to interpret the extinction in terms of H2 column density. Methods. We applied five different methods, including a new method for simultaneously determining the reddening law and the classification of the background stars. Our method is based on multicolour observations and a grid of model atmospheres. Results. We confirm that the extinction law can be adequately described by a single parameter, RV (the selective to absolute extinction), in accordance with earlier findings. The RV value for B 335 is RV = 4.8. The reddening curve can be accurately reproduced by model calculations. By assuming that all the silicon is bound in silicate grains, we can interpret the reddening in terms of column density, NH = 4.4 (±0.5) × 1021EI − Ks cm-2, corresponding to NH = 2.3 (±0.2) × 1021·AV cm-2, close to that of the diffuse ISM, (1.8 − 2.2) × 1021 cm-2. We show that the density of the B 335 globule outer shells can be modelled as an evolved Ebert-Bonnor gas sphere with ρ ∝ r-2, and estimate the mass of this globule to 2.5M