The mass function of ω Centauri down to 0.15 M_solar

Abstract

By means of deep FORS1/VLT and ACS/Hubble Space Telescope observations of a wide area in the stellar system ω Cen we measured the luminosity function of main-sequence stars down to R = 22.6 and I_F814W = 24.5. The luminosity functions obtained have been converted into mass functions and compared with analytical initial mass functions (IMFs) available in the literature. The mass function obtained, reaching M ~ 0.15M_solar, can be well reproduced by a broken power law with indices α = -2.3 for M > 0.5M_solar and α = -0.8 for M < 0.5M_solar. Since the stellar populations of ω Cen have been proved to be actually unaffected by dynamical evolution processes, the mass function measured in this stellar system should represent the best approximation of the IMF of a star cluster. The comparison with the MF measured in other Galactic globular clusters suggests that possible primordial differences in the slope of the low-mass end of their MF could exist.

Based on observations collected at the European Southern Observatory within the observing program 74.D-0369(B). Also based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.

E-mail: antonio.sollima@bo.astro.it