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The interstellar extinction in the solar neighbourhood
I. Statistical approach *
J.-L. Vergely 1,
R. Freire Ferrero 1,
D. Egret 1 and
J. Köppen 1, 2, 3
Received 15 April 1998 / Accepted 29 July 1998
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 Strömgren 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 , 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.
Key words: ISM: clouds ISM: dust, extinction ISM: structure methods: statistical stars: distances solar neighbourhood
Send offprint requests to: J-L. Vergely
Online publication: November 9, 1998