THE LOW METALLICITY TAIL OF THE HALO METALLICITY DISTRIBUTION FUNCTION

Ongoing spectroscopy and photometry of stars selected in the HK object ive-prism/interference-filter survey of Beers and colleagues has resul ted in the identification of many hundreds of additional stars in the halo (and possibly the thick disk) of the Galaxy with abundances [Fe/H ] less than or equal to -2.0. A new calibration of the technique for e stimation of metal abundance based on a CaII K index as a function of broadband B - V color is applied to obtain metallicities for stars obs erved with the SSO 2.3m and INT 2.5m telescopes. This new data is comb ined with other samples of extremely metal-deficient stars (Ryan and N orris, 1991a; Beers et al., 1992; Carney et al., 1994) to form a large database of objects of low metallicity. The combined sample is examin ed and compared with expectations derived from a Simple Model of Galac tic chemical evolution. There appears to be a statistically-significan t deficit of stars more metal-weak than [Fe/H] = -3.0. An abundance of [Fe/H] approximate to -4.0 can be taken as the low-metallicity limit for presently-observable stars in the Galaxy.