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Quantifying the universality of the stellar initial mass function in old star clusters

Authors

  • Nathan Leigh,

    Corresponding author
    1. Department of Physics and Astronomy, McMaster University, 1280 Main Street West, Hamilton, ON L8S 4M1, Canada
    2. Space Science Department, European Space Agency, Keplerlaan 1, 2200 AG Noordwijk, the Netherlands
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  • Stefan Umbreit,

    Corresponding author
    1. Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Northwestern University, Evanston, IL 60208, USA
    2. Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA
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  • Alison Sills,

    Corresponding author
    1. Department of Physics and Astronomy, McMaster University, 1280 Main Street West, Hamilton, ON L8S 4M1, Canada
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  • Christian Knigge,

    Corresponding author
    1. School of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ
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  • Guido de Marchi,

    Corresponding author
    1. Space Science Department, European Space Agency, Keplerlaan 1, 2200 AG Noordwijk, the Netherlands
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  • Evert Glebbeek,

    Corresponding author
    1. Department of Physics and Astronomy, McMaster University, 1280 Main Street West, Hamilton, ON L8S 4M1, Canada
    2. Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, the Netherlands
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  • Ata Sarajedini

    Corresponding author
    1. Department of Astronomy, University of Florida, Gainesville, FL 32611, USA
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E-mail: leighn@mcmaster.ca (NL); s-umbreit@northwestern.edu (SU); asills@mcmaster.ca (ASi); christian@astro.soton.ac.uk (CK); gdemarchi@esa.rssd.int (GD); e.glebbeek@astro.ru.nl (EG); ata@astro.ufl.edu (ASa)

ABSTRACT

We present a new technique to quantify cluster-to-cluster variations in the observed present-day stellar mass functions of a large sample of star clusters. Our method quantifies these differences as a function of both the stellar mass and the total cluster mass, and offers the advantage that it is insensitive to the precise functional form of the mass function. We applied our technique to data taken from the Advanced Camera for Surveys (ACS) Survey for Globular Clusters, from which we obtained completeness-corrected stellar mass functions in the mass range 0.25–0.75 M for a sample of 27 clusters. The results of our observational analysis were then compared to Monte Carlo simulations for globular cluster evolution spanning a range of initial mass functions, total number of stars, concentrations and virial radii.

We show that the present-day mass functions of the clusters in our sample can be reproduced by assuming a universal initial mass function for all clusters, and that the cluster-to-cluster differences are consistent with what is expected from two-body relaxation. A more complete exploration of the initial cluster conditions will be needed in future studies to better constrain the precise functional form of the initial mass function. This study is a first step towards using our technique to constrain the dynamical histories of a large sample of old Galactic star clusters and, by extension, star formation in the early Universe.

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