Interstellar absorption in the galactic plane is highly variable from one
direction to another. In this paper colour excesses and distances from a new
open cluster sample are used to investigate the spatial distribution of the
interstellar extinction. An inverse method (Tarantola & Valette, 1982) is used
to construct the extinction map in the galactic plane below ā£bā£<10o. The
Avā(r,l) diagrams are compared with those derived from individual stars
(Arenou et al. 1992, Neckel & Klare 1980). An analytic expression for the
interstellar extinction as a function of galactic longitude and distance in the
solar neighborhood is given. The comparison of the model predictions with
Hipparcos observations in the 4-dimensional space of (V, BāV, Hvā, r)
shows that our extinction model provides a better fit to the data. However, a
new and more detailed extinction model is still lacking.Comment: 16 pages, 16 figures, latex, accepted for pubblication in Astronomy
and Astrophysics Main Journa