The Luminosity Function of ω Centauri

Abstract

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 ~46 and ~7', respectively, away from the cluster center or near the half-light radius of this cluster (rhl 48). 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 MI 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.