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 (\cite{Paresce2000}) for a stellar field at 4.6 arcmin 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 arcmin), 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/L_V ≃ 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.