A reliable transformation of HIPPARCOS H_p magnitudes into Johnson V and B magnitudes

Abstract

A comparison of accurate UBV magnitudes, derived from numerous observations at Hvar and SkalnatePleso, and of the mean Hipparcos \hp magnitudes for a number of constant stars showed a very good mutual correspondence of these two data sets. Simple transformation formul\ae\ are presented which allow calculating Johnson V and B magnitudes from the \hp magnitude and known B-V and U-B colours. For constant stars with well-known values of both colours the accuracy of the transformation is clearly better than 0\m01. At the same time, the transformation is not critically sensitive to the exact values of B-V and U-B. It is applicable over a wide range of colours (B-V between -0\m25 and 2\m0) and works well also for reddened stars. However, since it was defined for stars brighter than about 8\m0 and for reddenings smaller than about 1\m0, its application outside these limits should be made with some caution and further tested. Since the B-V and U-B colours are known for the majority of brighter stars and since there are many classes of variable stars which do change colours only very mildly during their light changes (like the majority of Be stars) or for which the instantaneous colours can be predicted or estimated from existing optical observations, the transformations presented here may turn out to be very useful for many researchers who need to combine Hipparcos and optical photometry into one homogeneous data set.