Self-consistent modelling of star cluster evolution: Initial conditions and the origin of the stellar population II halo of the Milky Way

• Applicant: Professor Dr. Pavel Kroupa
• Subject Area: Astrophysics and Astronomy ; African, American and Oceania Studies
• Term: from 2013 to 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 234732163

• Final Report Year: 2018

Final Report Abstract

This work constrains the initial densities of globular clusters, their mass segregation and the initial binary fraction within them. This results in globular clusters depopulating their stars, which become members of the Population II halo, with a preference for low-mass stars, whereby this mass-dependent loss of stars is enhanced over the pure two-body relaxation evaporation. The work of Seungkyung Oh discovered that clusters with an initial mass near 3000 M have a maximum ejection fraction of their massive stellar content independently of whether they are born binary rich and independent of their initial density. But clusters with a higher density have a higher ejection fraction than clusters with a lower density, and a high initial binary fraction also increases the ejection rates of massive stars. The agreement of the observed stellar mass functions in M31 clusters with the models indicates that we may have achieved an excellent grasp of the physics of these systems. Future directions would be to study explicitly an early population of globular clusters which quickly loose stars due to gas expulsion, although the most recent paper (Brinkmann et al. 2017) suggests that gas expulsion does not lead to unbinding a significant fraction of stars from clusters with initial masses larger than about 106 M . The young globular clusters will have ejected a large fraction of their massive stars which would have exploded outside of their clusters, a process of chemical enrichment of the Population II halo needing further study.

Publications

• 2013. The state of globular clusters at birth: emergence from the gas-embedded phase. Monthly Notices of the Royal Astronomical Society 436, 3399-3412
  Leigh, N., Giersz, M., Webb, J. J., Hypki, A., De Marchi, G., Kroupa, P., Sills, A.
  
• 2015. Dependency of Dynamical Ejections of O Stars on the Masses of Very Young Star Clusters. The Astrophysical Journal 805, 92
  Oh, S., Kroupa, P., Pflamm-Altenburg, J.
  
• 2015. Possible smoking-gun evidence for initial mass segregation in re-virialized post-gas expulsion globular clusters. Monthly Notices of the Royal Astronomical Society 454, 3872-3885
  Haghi, H., Zonoozi, A. H., Kroupa, P., Banerjee, S., Baumgardt, H.
  
• 2015. The state of globular clusters at birth - II. Primordial binaries. Monthly Notices of the Royal Astronomical Society 446, 226-239
  Leigh, N. W. C., Giersz, M., Marks, M., Webb, J. J., Hypki, A., Heinke, C. O., Kroupa, P., Sills, A.
  
• 2016. A Possible Solution for the M/L-[Fe/H] Relation of Globular Clusters in M3. I. A Metallicity- and Density-dependent Top-heavy IMF. The Astrophysical Journal 826, 89
  Zonoozi, A. H., Haghi, H., Kroupa, P.
  
• 2016. Dynamical ejections of massive stars from young star clusters under diverse initial conditions. Astronomy and Astrophysics 590, A107
  Oh, S., Kroupa, P.
  
• 2017. Direct N-body simulations of globular clusters - III. Palomar 4 on an eccentric orbit. Monthly Notices of the Royal Astronomical Society 467, 758-767
  Zonoozi, A. H., Haghi, H., Kroupa, P., Küpper, A. H. W., Baumgardt, H.
  
• 2017. The bound fraction of young star clusters. Astronomy and Astrophysics 600, A49
  Brinkmann, N., Banerjee, S., Motwani, B., Kroupa, P.