SIMBAD references

We present a comparison of the predictions of stellar models with the luminosity of the lower main sequence (5.5 < M_V< 7.3) using K dwarfs in the Hipparcos catalogue. The parallaxes of our comparison stars are known to better than 15 per cent and metallicities have been recently determined from photometry. A major advantage of our comparison is that distances in our sample are known with good accuracy, while tests that involve open and globular clusters are constrained by potentially inaccurate distances. We show that the luminosity of the lower main sequence is a simple function of metallicity, deriving the relation ΔM_V= 0.045 77 - 0.843 75 x[Fe/H], where ΔM_Vis the luminosity difference relative to a fiducial, solar metallicity isochrone. We compare the data with a range of isochrones from the literature. None of the isochrone sets fits the data at all metallicities, although some models do clearly better than others. In particular, metal-rich isochrones seem to be difficult to construct.

The relationship between luminosity, colour and metallicity for K dwarfs is found to be very tight. We are thus able to derive metallicities for K dwarfs based on their position in the Hipparcos colour-magnitude diagram with accuracies better than 0.1 dex. The metallicity-luminosity relation for K dwarfs leads to a new distance indicator with a wide range of possible applications.