Stellar Populations in Globular Cluster Cores

1. Nathan Leigh With: Alison Sills and Christian Knigge September 23, 2009 The Lorentz Center, Leiden, the Netherlands Stellar Populations in Globular Cluster Cores

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

4. Figures 13 and 14 of Ferraro et al. (1991): Left: Cumulative radial distributions for the RGB and HB populations in the Galactic globular cluster NGC 6171. Right: Radial variation of the parameter R = NHB/ NRGB.

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)

7. 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.

8. 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)

9. 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

10. 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

11. 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?

13. 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)

14. 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?

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