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Photograph of Ross Church

Ross Church

Senior lecturer

Photograph of Ross Church

Close stellar encounters at the Galactic Centre - I. The effect on the observed stellar populations


  • Alessandra Mastrobuono-Battisti
  • Ross P. Church
  • Melvyn B. Davies

Summary, in English

We model the effects of collisions and close encounters on the stellar populations observed in the Milky Way nuclear
stellar cluster (NSC). Our analysis is based on N -body simulations in which the NSC forms by accretion of massive
stellar clusters around a supermassive black hole. We attach stellar populations to our N -body particles and follow the
evolution of their stars, and the rate of collisions and close encounters. The most common encounters are collisions
between pairs of main-sequence stars, which lead to mergers: destructive collisions between main-sequence stars
and compact objects are rare. We find that the effects of collisions on the stellar populations are small for three
reasons. First, our models possess a core which limits the maximum stellar density. Secondly, the velocity dispersion
in the NSC is similar to the surface escape velocities of the stars, which minimises the collision rate. Finally, whilst
collisions between main-sequence stars destroy bright giants by accelerating their evolution, they also create them
by accelerating the evolution of lower-mass stars. These two effects approximately cancel out. We also investigate
whether the G2 cloud could be a fuzzball: a compact stellar core which has accreted a tenuous envelope in a close
encounter with a red giant. We conclude that fuzzballs with cores below 2 M have thermal times-scales too short
to reproduce G2. A fuzzball with a black-hole core could reproduce the surface properties of G2 but the production
rate of such objects in our model is low.


  • Lund Observatory
  • eSSENCE: The e-Science Collaboration
  • Mathematics (Faculty of Sciences)

Publishing year







Monthly Notices of the Royal Astronomical Society





Document type

Journal article


Oxford University Press


  • Astronomy, Astrophysics and Cosmology


  • methods: numerical
  • stars: evolution
  • Galaxy: evolution
  • Galaxy: nucleus
  • Galaxy: stellar content
  • Astrophysics - Astrophysics of Galaxies




  • ISSN: 1365-2966