Some Structural Properties of Galactic and Globular Clusters. I.

Abstract

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Some Structural Properties of Galactic and Globular Clusters. I. Michie, Richard W. ; Kinman, T. D. Abstract 67, 582,1962), theoretical models have been constructed using the Boltzmann equation with encounters. The velocity space distribution is not restricted to complete isotropy everywhere, and the potential is obtained from Poisson's equation. A further property is that some models include an approximation to the effects of the tidal force by requiring the escape velocity to go to zero at finite distance from the center. In dimensionless variables, the model is specified by one parameter C for any given strength of the tidal force. When C is zero, the velocity distribution is isotropic, while large positive C produces the elongated orbits. The calculations indicate the tidal force determines the structure of the outermost parts of the system, but it is the anisotropy of velocities which determine the structure of the inner regions. Models have been obtained for 12 galactic clusters using star counts to faint magnitudes. The model parameter has a wide range, but generally C 10-2.5 which implies quite elongated orbits. Models for 35 globular clusters have been obtained from the Kron and Mayall data (Astron. J. 65, 581,1960) and for seven of these, models were also obtained by using surface density data. For this overlap of seven, the two methods agree very well. Again, the model parameter for the globular clusters is generally large, implying elongated orbits. Among the galactic clusters, there is no evidence of any correlation of the model parameter with the strength of the tidal force. The tidal force is not of importance in determining the major part of the system, but completely determines the outermost regions. The intrinsic structural properties of the young clusters then reflect conditions in the gas cloud out of which the system formed. The highly elongated orbits in some systems must be due to a more rapid rate of collapse of the original cloud of gas, the rate being determined in part by the turbulent pressure and also possibly by the strength of the magnetic field. Publication: The Astronomical Journal Pub Date: 1963 DOI: 10.1086/109045 Bibcode: 1963AJ.....68Q.286M full text sources ADS | Related Materials (1) Part 2: 1963AJ.....68R.282K