Comparison of two different extinction laws with Hipparcos observations

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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 |b| < 10ᵒ. The Aᵥ (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, B-V, Hᵥ, r) shows that our extinction model provides a better fit to the data. However, a new and more detailed extinction model is still lacking.