Dynamical studies of globular clusters based on photoelectric radial velocities of individual stars and on the observed mass function. II - M13

Details Dynamical studies of globular clusters based on photoelectric radial velocities of individual stars and on the observed mass function. II - M13 Dynamical studies of globular clusters based on photoelectric radial velocities of individual stars and on the observed mass function. II - M13

May 1987

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987aj.....93.1114l&link_type=abstract

Astronomical Journal (ISSN 0004-6256), vol. 93, May 1987, p. 1114-1136. Research supported by the John D. and Catherine T. MacAr

Astronomy and Astrophysics

Astronomy

111

Globular Clusters, Radial Velocity, Stellar Mass, Stellar Motions, Curve Fitting, Mass To Light Ratios, Photoelectron Spectroscopy, Stellar Luminosity, White Dwarf Stars

Scientific paper

We have measured radial velocities for 147 stars in the globular cluster M13 with typical errors of about 1 km s-1. Six of these are probable new velocity variables. The one-dimensional central velocity dispersion in the giants is about 7 km s-1 , and the cluster is rotating with a maximum rotational velocity of about 5 km s-1. We have constructed truncated Maxwellian models of M13, using the observations of Lupton and Gunn to constrain the mass function in the region 0.5-0.8 Msun. In order to reconcile the mass-to-light ratio derived from the dynamics with that of the assumed mass spectrum, we had to assume that half the mass of the cluster was not observed. This mass cannot be present as heavy remnants, and the expected mass in white dwarfs is too low by a factor of about 20. If we assume that this mass takes the form of low-mass stars, the global mass function has a maximum at about 0.65 Msun, a minimum at about 0.5 Msun ,and then a very sharp rise towards lower masses. We see no evidence for any truncation of the mass function. There is no evidence for any heavy remnants not accounted for by the adopted mass function. We find that isotropic models of the velocity dispersion do not fit our velocity measurements, and that the transition to anisotropy occurs at 3-5 core radii. We use the proper-motion data of Cudworth and Monet in conjunction with our models to derive a distance of 6.5 kpc to the cluster, with formal errors of + 9%, - 6%.

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