The Luminosity Function of ω Centauri

Deep Hubble Space Telescope ( HST ) Wide Field Planetary Camera 2 observations of the stellar population in the globular cluster ω Cen (NGC 5139) have been used to measure the luminosity function of main-sequence stars at the low-mass end. Two fields have been investigated, located ∼4 6 and ∼7', respectively, away from the cluster center or near the half-light radius of this cluster ( r hl ≃ 4 8). The color-magnitude diagrams derived from these data show the cluster main sequence extending to the detection limit at I ≃ 24. Information on both color and magnitude is used to build the luminosity functions of main-sequence stars in these fields, and the two independent determinations are found to agree very well with each other within the observational uncertainty. Both functions show a peak in the stellar distribution around M I ≃ 9, followed by a drop at fainter magnitudes well before photometric incompleteness becomes significant, as is typical of other globular clusters observed with the HST . This result is at variance with previous claims that the luminosity function of ω Cen stays flat at low masses but is in excellent agreement with recent Near Infrared Camera and Multi-Object Spectrograph observations.