EDP Sciences
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Volume 428, Number 2, December III 2004
Page(s) 469 - 478
Section Galactic structure, stellar clusters and populations
DOI http://dx.doi.org/10.1051/0004-6361:20034192

A&A 428, 469-478 (2004)
DOI: 10.1051/0004-6361:20034192

The global mass function of M 15

A. Pasquali1, G. De Marchi2, L. Pulone3 and M. S. Brigas1, 4

1  ESO/ST-ECF, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
    e-mail: apasqual@eso.org
2  ESA, Space Telescope Operations Division, 3700 San Martin Drive, Baltimore, MD 21218, USA
    e-mail: gdemarchi@rssd.esa.int
3  INAF, Osservatorio Astronomico di Roma, Via di Frascati 33, 00040 Monte Porzio Catone (RM), Italy
    e-mail: pulone@coma.mporzio.astro.it
4  Osservatorio Astronomico di Cagliari, Strada 54, Poggio dei Pini, 09012 Capoterra, Cagliari, Italy
    e-mail: sbrigas@eso.org

(Received 14 August 2003 / Accepted 21 July 2004 )

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 ~ $7^\prime$ from the cluster centre. The data confirm that the cluster MF has a characteristic mass of ~ 0.3 $M_{\odot}$, as obtained by Paresce & De Marchi (2000) for a stellar field at  $4\farcm6$ from the centre. By combining the present data with those published by other authors for various radial distances (near the centre, at  $20^{\prime\prime}$ and at  $4\farcm6$), 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:  $x \simeq 0.8$ for stars more massive than  0.8 $M_{\odot}$ $x \simeq 0.9$ for  $0.3{-}0.8\,M_\odot$ and  $x \simeq -2.2$ at smaller masses (Salpeter's IMF would have  x=1.35). The best fitting model also suggests that the cluster mass is ~ $5.4 \times 10^5\,M_\odot$ and that the mass-to-light ratio is on average  $M/L_V \simeq 2.1$, with  $M/L_V
\simeq 3.7$ 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.

Key words: globular clusters: general -- globular clusters: individual: M 15

SIMBAD Objects

© ESO 2004