Astronomy and Astrophysics – Astronomy
Scientific paper
Oct 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apjs..100..347d&link_type=abstract
Astrophysical Journal Supplement v.100, p.347
Astronomy and Astrophysics
Astronomy
30
Celestial Mechanics, Stellar Dynamics, Galaxy: Globular Clusters: Individual Ngc Number: Ngc 6397
Scientific paper
I present a detailed examination of Fokker-Planck models for the globular cluster NGC 6397. I show that these models provide a good match to observations of the surface density profile, mass functions at three radii, and the velocity dispersion profile. The constraint of requiring the best matches to the mass functions and surface density profiles to occur simultaneously defines a surface in an initial parameter space consisting of the cluster concentration, mass, and limiting radius. I discuss various techniques for locating this surface and the dependence of the quality of the matches on the position of the model on the surface, the initial mass function, and the retention rate of neutron stars. The quality of the matches is usually strongly related to the age of the models, but one initial mass function was found for which the quality of the matches is independent of age. The models give an estimate for the current cluster mass of 6.6±0.5 x l0 Msun. Variations in the model matches with different initial mass functions suggest that more than 1400 Msun is in neutron stars and that less than one-third of the mass is in stars less massive than 0.2 Msun. This mass gives a V-band mass-to-light ratio of 1.2 in solar units. The models and data provide weaker estimates of the structural parameters but suggest that the core radius is less than ll" (0.3 pc) and the tidal radius is 10±2' (17±4 pc). In turn, by assuming a flat rotation curve for the Galaxy, the mass and tidal radius suggest that the latter was set at a distance of 2.5 kpc from the Galactic center.
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