SIMBAD references

Techniques used to remove atmospheric extinction cannot be used for interstellar extinction, because objects can be observed at only a single extinction, which varies from object to object. The interstellar extinction is thus unknowable without the use of prior information about the object. In this paper we show how to apply such knowledge in the case of normal stars. Using standard passbands, an accepted average optical interstellar extinction law, and Kurucz flux tables, we derive useful colour-dependent interstellar extinction relations for synthetic versions of two widely used sets of broad band photometric passbands, Johnson-Cousins UBVRI and Washington CMT₁T₂. For the Washington system, we define a new internal reddening system. For bands blueward of 500 nm in either photometric system the reddenings for main sequence stars and evolved stars are quite different, and even can be non-linear as a function of temperature colour. These corrections are important for the correct characterisation of the intrinsic reddening properties of interstellar dust, for fitting isochrones accurately, for determining stellar metallicities, for obtaining correct standard reddenings for reddened galactic clusters, and indeed for the determination of basic stellar parameters for single stars such as T_eff and M_bol. To make the reddenings to different objects comparable, we standardise the reddening on that of a solar metallicity main sequence star of T_eff=17000 K (B3-4 V). The functional dependence of the extinction curves on abundance is not as important in the Johnson-Cousins as in the Washington system. We compute new theoretical reddening vectors in both systems, and evaluate the size of the nonlinearity with extinction (Forbes effect).