Stellar Populations in Globular Cluster Cores

1
Stellar Populations in Globular Cluster Cores

• Nathan Leigh
• With Alison Sills and Christian Knigge
• September 23, 2009
• The Lorentz Center, Leiden, the Netherlands

2
Introduction

• What are the observational signatures of stellar
  mergers?
• ? blue stragglers?
• ? abundance anomalies (e.g. Ferraro et al. 2006)?
• ? rapid rotation (e.g. Glebbeek, Pols Hurley
  2008)?
• ? dynamical evolution of clusters (e.g Portegies
  Zwart et al. 2004)?
• ? evolved merger products (e.g. Sills, Karakas
  Lattanzio 2009)?

3
Introduction

• Stellar populations in GCs are typically studied
  on a cluster-by-cluster basis (e.g. Sandquist
  Hess 2008)
• Relative sizes CMD morphologies are used to
  constrain rate of stellar evolution and degree of
  He enrichment (e.g. Ferraro et al, 1991 Romano
  et al. 2007)
• LFs and SB profiles are used to learn about
  dynamical evolution of clusters, universality of
  stellar MF, etc. (e.g. de Marchi Pulone 2007)
• Very few trends found to account for
  cluster-to-cluster discrepancies reported in
  these studies

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5
Our Approach

• Apply a cluster-independent selection criterion
  to the colour-magnitude diagrams of 56 globular
  clusters taken from Piotto et al.s (2002) HST
  database
• This provides the number of RGB, MSTO and HB
  stars in the core of each cluster
• The size of each stellar population is compared
  to the core mass

6
Motivation

• All things being equal, the number of stars in
  the cluster core belonging to each stellar
  population should scale linearly with the core
  mass
• However, all things are not equal
• ? the rate of two-body relaxation increases
  with decreasing cluster mass (e.g. Spitzer 1987)
• ? the collision rate increases with increasing
  cluster mass (e.g. Davies, Piotto De Angeli
  2004)
• ? the core binary fraction could depend on the
  core mass (e.g. Sollima 2008 Knigge, Leigh
  Sills 2009)
• ? globular clusters may not be simple stellar
  populations (e.g. Anderson et al. 2009)

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Figure 1 of Leigh, Sills Knigge (2009)
Colour-magnitude diagram for NGC 362 in the
(F439W-F555W)-F555W plane. Boundaries enclosing
the selected RGB, HB and MSTO populations are
shown.
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NMSTO (1.02 0.01)log Ncore/103 (2.66
0.01) NRGB (0.89 0.03)log Ncore/103 (2.04
0.02) NHB (0.91 0.10)log Ncore/103 (1.58
0.05)
Figure 2 of Leigh, Sills Knigge (2009)
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Implications

• The number of RGB stars in GC cores does not
  direct trace the total stellar population in
  those cores
• The number of RGB stars scales sub-linearly with
  core mass as the 3-? level
• The ratio NRGB/NMSTO suggests a surplus of RGB
  stars in the least massive cores

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Stellar Evolution

• No reason to expect the rate of stellar evolution
  to depend on the cluster mass
• Many of the most massive GCs are thought to be
  enriched in helium (e.g. Anderson et al. 2009)
• This could depress the slope of the RGB sample,
  however it suggests a deficiency of RGB stars in
  the most massive GCs

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The Suspects

• Single star dynamics?
• - two-body relaxation?
• - increased cross-section for collision?
• Binary effects?
• - Roche lobe overflow in binaries?
• Evolved blue stragglers?
• - contamination from merger products?

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NBS (0.47 0.06) log Ncore/103 (1.22
0.02) NRGB (0.89 0.03)log Ncore/103 (2.04
0.02) NRGB-BS (0.94 0.04) log Ncore/103
(1.97 0.02)
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Summary

• Compared NRGB, NMSTO NHB to Mcore in 56 GCs
• Applicable to studies of both cluster and stellar
  evolution
• NRGB scales sub-linearly with Mcore at the 3-?
  level
• Contamination of RGB sample from evolved merger
  products?

15
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