The global mass function of M 15

Abstract

Data obtained with the NICMOS instrument on board the Hubble Space Telescope (HST) have been used to determine the H-band luminosity function (LF) and mass function (MF) of three stellar fields in the globular cluster M 15, located ∼7′ from the cluster centre. The data confirm that the cluster MF has a characteristic mass of ∼0.3 M⊙, as obtained by Paresce & De Marchi (2000) for a stellar field at 4′.6 from the centre. By combining the present data with those published by other authors for various radial distances (near the centre, at 20″ and at 4′.6), we have studied the radial variation of the LF due to the effects of mass segregation and derived the global mass function (GMF) using the Michie-King approach. The model that simultaneously best fits the LF at various locations, the surface brightness profile and the velocity dispersion profile suggest that the GMF should resemble a segmented power-law with the following indices: x ≃ 0.8 for stars more massive than 0.8 M⊙, x ≃ 0.9 for 0.3-0.8 M⊙ and x ≃ -2.2 at smaller masses (Salpeter's IMF would have x = 1.35). The best fitting model also suggests that the cluster mass is ∼5.4 × 105 M⊙ and that the mass-to-light ratio is on average M/LV ≃ 2.1, with M/L V ≃ 3.7 in the core. A large amount of mass (∼44%) is found in the cluster core in the form of stellar heavy remnants, which may be sufficient to explain the mass segregation in M 15 without invoking the presence of an intermediate-mass black hole.