The escape of stars from clusters III. Expansion versus contraction in the evolution of a star cluster

Abstract

view Abstract Citations (23) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The escape of stars from clusters III. Expansion versus contraction in the evolution of a star cluster King, Ivan Abstract . A loose cluster expands as a result of tidal encounters with interstellar clouds, while the ejection of stars causes a dense cluster to contract. The evolution of a cluster of intermediate density can be studied by means of differ- ential equations that follow from the virial theorem and the conservation of energy. The solutions show that the balance between expansive and contractive tendencies is extremely unstable. In practice it is satisfactory to consider a critical radius to divide expanding clusters, to which the pure cloud-encounter theory applies, from contracting clusters, to which the pure ejection theory applies. The critical radius corresponds to the longest possible lifetime for a given number of stars. Present estimates put the critical radius, in the galactic plane, at I 21 pc, with a maximum lifetime of less than I0 years for 100 stars, and 3 or 4 >c I0 years for 1000 stars. A cluster that moves 500 pc above and below the galactic plane has its critical radius doubled and its maximum lifetime tripled. Encounters with field stars also add energy to a cluster, but their effect is always small compared with that of ejection, cloud encounters, or tidal forces. The evolu- tionary picture given here must give the observed distribution of cluster radii a strong bias in favor of values near the critical radius. Publication: The Astronomical Journal Pub Date: December 1958 DOI: 10.1086/107811 Bibcode: 1958AJ.....63..465K full text sources ADS | Related Materials (4) Part 1: 1958AJ.....63..109K Part 2: 1958AJ.....63..114K Part 4: 1959AJ.....64..351K Part 5: 1960AJ.....65..122K