THE ABSOLUTE MAGNITUDE OF THE EARLY-TYPE MK STANDARDS FROM HIPPARCOS PARALLAXES

We analyse the standards of the MK system with the help of Hipparcos p arallaxes, using only stars for which the error of the absolute magnit ude is less than or equal to 0.3 mag. We find that the main sequence i s a wide band and that, although in general giants and dwarfs have dif ferent absolute magnitudes, the separation between luminosity classes V and III is not clear. Furthermore, there are a number of exceptions to the strict relation between luminosity class and absolute magnitude . We analyse similarly the system of standards defined by Garrison & G ray (1994) separating low and high rotational velocity standards. We f ind similar effects as in the original MK system. We propose a revisio n of the MK standards, to eliminate the most deviant cases.