Astronomy and Astrophysics – Astronomy
Scientific paper
Jun 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006aas...208.1001d&link_type=abstract
American Astronomical Society Meeting 208, #10.01; Bulletin of the American Astronomical Society, Vol. 38, p.88
Astronomy and Astrophysics
Astronomy
Scientific paper
We present a deep investigation of the main sequence (MS) of the globular cluster M12 (NGC 6218) in the V and R bands. The observations cover an area of 3.4´ x 3.4´ around the cluster centre and probe the stellar population out to the cluster's half-mass radius (rh≈2.2´). The colour-magnitude diagram (CMD) reveals a narrow and well defined MS extending down to the 5σ detection limit at V≈25, or about 6 magnitudes below the MS turn-off, corresponding to stars of 0.25 Msun. The luminosity function (LF) obtained with these data shows a marked radial gradient, in that the ratio of lower- and higher-mass stars increases monotonically with radius. The mass function (MF) measured at the half mass radius, and as such representative of the cluster's global properties, is surprisingly flat. Over the range 0.4 - 0.8 Msun, the number of stars per unit mass follows a power-law distribution of the type dN/dm ∝ m0, where, for comparison, Salpeter's IMF would be dN/dm ∝ m-2.35. We show that such a flat MF does not represent the cluster's IMF but is the result of severe tidal stripping of the stars from the cluster due to its interaction with the Galaxy's gravitational field. These results cannot be reconciled with the predictions of recent theoretical models that imply a relatively insignificant loss of stars from M12 as measured by its expected very long time to disruption. They are more consistent with the orbital parameters based on the Hipparcos reference system that imply a much higher degree of interaction of this cluster with the Galaxy than assumed by those models. If the orbit of a cluster is known, the slope of its MF is useful in discriminating between the various models of the Galactic potential.
de Marchi Guido
Paresce Francesco
Pulone Luigi
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