HIPPARCOS AND THE AGE OF THE GALACTIC DISC

We use the Hipparcos colour-magnitude diagram of field stars with Tych o colours to make a new minimum age estimate for the Galactic disc. Th e method is based on fits to the red envelope of subgiants in the Hipp arcos colour-magnitude diagram with synthetic isochrones covering the range of disc metal abundance. The colours and luminosities of the iso chrones as a function of abundance are checked using new techniques in volving 'red-clump' stars in the giant branch region and on the main s equence using G and K dwarfs. We derive a minimum disc age of 8 Gyr, i n good agreement with other methods.