The interstellar extinction in the solar neighbourhood - I. Statistical approach

Assuming some specific distribution of the interstellar clouds, we interpre t the projected stellar extinction in terms of mean extinction by cloud as well as cloud density on the line of sight. Individual stellar extinctions are derived from Stromgren photometry and from accurate distances based on Hipparcos parallaxes. Extinction profiles as a function of galactic latitude allow us to determin e scale heights of 35 and 55 pc, characterizing the cloud density perpendic ular to the galactic plane. Assuming that clouds are distributed according to a Poisson law, we find th at observations are best fitted with a model consisting of at least three d ifferent kinds of clouds: weak extinction clouds with a characteristic exti nction E(b - y) = 0.012, medium extinction clouds of about 0.05, and dark c louds with extinction larger than 0.1. Angular and distance correlation functions provide additional information a bout cloud sizes. We present also a theoretical support for the approach we follow, and we co mplete the analysis by numerical simulations in order to determine ranges o f possible values for the main parameters of our model.