Systematic Effects On Photometric Parallaxes For Fgk Dwarfs And Subdwarfs

Hipparcos trigonometric parallaxes were used to establish a preliminary absolute magnitude calibration in terms of effective temperature, metallicity and evolutionary stage indicators for FGK dwarfs and subdwarfs. Photometric determinations of metallicity, needed to establish the calibration, were evaluated by the comparison with known spectroscopic metallicities. The new calibration removes systematic trends present in the calibrations available today and it decreases the rms of the residuals. Key words: Subdwarfs; main-sequence; absolute magnitude; metallicity. 1. INTRODUCTION The methods used to derive the absolute magnitude of FGK dwarfs and subdwarfs stars are based on the definition of a standard relation on a colour-- magnitude diagram, which fits the sequence of stars with standard metallicity (the Hyades metallicity). A term (\DeltaM v ) is then added to account for different metallicities; this is computed from the position of the star in a colour-colour diagram (Wildey et al..