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

Abstract

Assuming some specific distribution of the interstellar clouds, we interpret 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 Stroemgren photometry and from accurate distances based on Hipparcos parallaxes. Extinction profiles as a function of galactic latitude allow us to determine scale heights of 35 and 55 pc, characterizing the cloud density perpendicular to the galactic plane. Assuming that clouds are distributed according to a Poisson law, we find that observations are best fitted with a model consisting of at least three different kinds of clouds: weak extinction clouds with a characteristic extinction E(b-y)=0.012, medium extinction clouds of about 0.05, and dark clouds with extinction larger than 0.1. Angular and distance correlation functions provide additional information about cloud sizes. We present also a theoretical support for the approach we follow, and we complete the analysis by numerical simulations in order to determine ranges of possible values for the main parameters of our model. Using observations from the ESA Hipparcos space astrometry mission.