Date of Award
Doctor of Philosophy (PhD)
D. L. Welch
1859 radial velocities (median precision ≃ 1 km s⁻¹) have been measured for 1318 stars in the direction of the Galactic globular cluster NGC 3201. Since the systemic radial velocity for NGC 3201 is 494.2 km s⁻¹, the field and cluster samples separate unambiguously into two distinct samples. Analysis of the radial velocities and APM/CCD photometry for the 879 field stars in our sample has revealed the presence of a probable moving group of ~ 13 stars with radial velocity 75 km s⁻¹ at a distance of 6 - 12 kpc. For the cluster members, we have multiple velocities for 279 stars (known photometric variables excluded) spread over a maximum of six years. Comparison of the observed velocities to simulated datasets containing known numbers of binaries has yielded upper limits to the true binary fraction (for binaries with 0.1 ≤ P ≤ 5 - 10 years and mass ratios in the range 0.1-1.0) of 0.06-0.10 (circular orbits) and 0.15-0.18 (eccentric orbits), consistent with the corresponding incidence among nearby solar-type stars and among a sample of six other Galactic globular clusters with measured binary fractions. NGC 3201 appears to be rotating with an apparent projected rotation amplitude of 1.22±0.25 km s⁻¹. The observed increase in ostensible rotation amplitude with distance from the cluster core may, however, be due to either the projection of the cluster space velocity onto the plane of the sky, or the preferential stripping of stars on prograde orbits near the limiting radius due to the disk-shocking instability identified by Oh and Lin (1992). BV CCD images have been used to derive cluster surface brightness profiles which extend out to ≃ 18. These profiles and the 399 mean radial velocities for nonvariable cluster members have been analysed using both single- and multi-mass King-Michie models and nonparametric techniques. Both methods suggest that the cluster mass-to-light ratio is relatively flat in the range 1.5 - 10 pc: M/Ls ≃ M/Lv = 2.0±0.2 in solar units. The best-fit mass function has a spectral index of x ≃ 0.75 ± 0.25, consistent with recent findings that the form of the mass function depends on the position relative to the potential of the Galaxy.
Côté, Patrick, "The Galactic Globular Cluster NGC 3201: Primordial Binaries and Tidal Dynamics" (1994). Open Access Dissertations and Theses. Paper 3932.