HST luminosity functions of the globular clusters M10, M22, and M55. A comparison with other clusters

Abstract

From a combination of deep Hubble Space Telescope V and I images with groundbased images in the same bands, we have obtained color-magnitude diagrams of M10, M22, and M55, extending from just above the hydrogen burning limit to the tip of the red giant branch, down to the white dwarf cooling sequence. We have used the color-magnitude arrays to extract main sequence luminosity functions (LFs) from the turnoff to m ~ 0.13m_sun. The LFs of M10 is significantly steeper than that for the other two clusters. The difference cannot be due to a difference in metallicity. A comparison with the LFs from Piotto et al. (1997), shows a large spread in the LF slopes. This spread is also present in the local mass functions (MFs) obtained from the observed LFs using different theoretical mass-luminosity relations. The dispersion in the MF slopes remains also after removing the mass segregation effects by using multimass King-Michie models. The globular cluster MF slopes are also flatter than the MF slope of the field stars and of the Galactic clusters in the same mass interval. We interpret the MF slope dispersion and the MF flatness as an evidence of dynamical evolution which makes the present day globular cluster stellar MFs different from the initial MFs. The slopes of the present day MFs exclude that the low mass star can be dynamically relevant for the Galactic globular clusters. Based on HST observations retrieved from the ESO ST-ECF Archive, and on observations made at the European Southern Observatory, La Silla, Chile, and at the JKT telescope at La Palma, Islas Canarias.