The Metallicity Dependence of the Stellar Luminosity and Initial Mass Functions: HST Observations of Open and Globular Clusters

Using HST and the WFPC2 we have acquired very deep Vand I-band photometry of stars in NGC 2420 and NGC 2477 to study cluster luminosity functions at approximately solar metallicity. We have determined these cluster luminosity functions down to MI = 10.5 (0.2 M⊙) and find that the luminosity function of NGC 2420 turns over at MI ≈ 9.0, and possibly stops altogether by MI ≈ 9.5. The luminosity function of NGC 2477 may flatten at MI ≥ 9.5. We compare our open cluster luminosity functions to the solar neighborhood field star luminosity function of Kroupa, Tout & Gilmore (1993) and the four published HST globular cluster luminosity functions: ω Cen (Elson et al. 1995), 47 Tuc (De Marchi & Paresce 1995b), M 15 (De Marchi & Paresce 1995a), and NGC 6397 (Paresce, De Marchi & Romaniello 1995). We find a smooth relation between the location of the luminosity function turn-over and the metallicity for all these low mass star samples which matches the expected MI versus [Fe/H] trend for a model star of ≈ 0.27 M⊙ (Saumon 1995; Alexander et al. 1996). We interpret this smooth and systematic behavior in the cluster luminosity functions as strong evidence in favor of an invariant initial mass function and a metallicity-dependent mass-luminosity relation. Subject headings: stars: low-mass, luminosity function, mass function – open clusters and associations: individual (NGC 2420, NGC 2477) Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc, under NASA contract NAS 5-26555. Institute of Astronomy, Madingley Road, Cambridge CB3 0HA address: WIYN Telescope, NOAO, PO Box 26732, Tucson, AZ 85726-6732, USA Royal Greenwich Observatory, Madingley Road, Cambridge CB3 0EZ