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

Abstract

Using HST and the WFPC2 we have acquired very deep V- and 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 M₁=10.5(0.2 M_sun) and find that the luminosity function of NGC 2420 turns over at M₁=9.0, and possibly stops altogether by M₁=9.5. The luminosity function of NGC 2477 may flatten at M1 9.5. We compare our open cluster luminosity functions to the solar neighborhood field star luminosity function of Kroupa et al. [MNRAS, 262, 545 (1993)] and the four published HST globular cluster luminosity functions: ω Cen [Elson et al. ApJ, 110, 682 (1995)], 47 Tuc [De Marchi & Paresce, A&A, 304, 211 (1995b)], M15 [De Marchi & Paresce, A&A, 304, 202, (1995a)], and NGC 6397 [Paresce et al., ApJ, 440, 216 (1995)]. We find a smooth relation between the location of the luminosity function turnover and the metallicity for all these low mass star samples which matches the expected M₁ versus [Fe/H] trend for a model star of ≈0.27 M_sun (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.