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

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)?

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

Figures 13 and 14 of Ferraro et al 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.

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

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)

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.

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)

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

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

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?

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)

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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