SCYON Abstract

Received on November 24 2010

Direct N-body simulations of globular clusters: (I) Palomar 14

AuthorsAkram Hasani Zonoozi (1,2), Andreas H.W. Küpper (2,3), Holger Baumgardt (2,4), Hosein Haghi (1,2), Pavel Kroupa (2), and Michael Hilker (5)
Affiliation(1}) Department of Physics, Institute for Advanced Studies in Basic Sciences (IASBS), P.O. Box 11365-9161, Zanjan, Iran
(2) Argelander Institute für Astronomie (AIfA), Auf dem Hügel 71, 53121 Bonn, Germany
(3) European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
(4) University of Queensland, School of Mathematics and Physics, Brisbane, QLD 4072, Australia
(5) European Southern Observatory, Garching b. München, Germany
Accepted byMonthly Notices of the Royal Astronomical Society
Contacta.hasani@iasbs.ac.ir
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Abstract

We present the first ever direct N-body computations of an old Milky Way globular cluster over its entire life time on a star-by-star basis. Using recent GPU hardware at Bonn University, we have performed a comprehensive set of N-body calculations to model the distant outer halo globular cluster Palomar 14 (Pal 14). Pal 14 is unusual in that its density is about ten times smaller than that in the solar neighborhood. By varying the initial conditions we aim at finding an initial N-body model which reproduces the observational data best in terms of its basic parameters, i.e. half-light radius, mass and velocity dispersion. We furthermore focus on reproducing the mass function slope of Pal 14 which was found to be significantly shallower than in most globular clusters. While some of our models can reproduce Pal 14's basic parameters reasonably well, we find that dynamical mass segregation alone cannot explain the mass function slope of Pal 14 when starting from the canonical Kroupa IMF. In order to seek for an explanation for this discrepancy, we compute additional initial models with varying degrees of primordial mass segregation as well as with a flattened IMF. The necessary degree of primordial mass segregation turns out to be very high, though, such that we prefer the latter hypothesis which we discuss in detail. This modelling has shown that the initial conditions of Pal 14 to reach its current state must have been a half-mass radius of about 20 pc, a mass of about 50000 M(sun), and some mass segregation but in particularly with an already established non-canonical IMF depleted in low-mass stars. Such conditions might be obtained by a violent early gas-expulsion phase from an embedded cluster born with mass segregation. Only at large Galactocentric radii are cluster likely to survive as bound entities the destructive gas-expulsion process we seem to have uncovered for Pal 14.
In addition we compute a model with a 5% primordial binary fraction to test if such a population has an effect on the cluster's evolution. We see no significant effect, though, and moreover find that the binary fraction of Pal 14 stays almost the same and gives the final fraction over its entire life time due to the cluster's extremely low density. Low-density, halo globular clusters might therefore be good targets to test primordial binary fractions of globular clusters.