1. INTRODUCTION Blue straggler stars (BSSs) are brighter, bluer and more massive than stars occupying the MSTO in clusters. There is mounting evidence to suggest that the preferred BSS formation pathway, whether it is via the coalescence of a primordial binary system or through the collision of two single main-sequence stars, depends largely on the cluster environment. Recently, Piotto et al. (2004) examined the colour-magnitude diagrams of 56 globular clusters (GCs), comparing the frequencies of BSSs to cluster properties like total mass (absolute luminosity) and central density. They found that the BSS frequency varies with the total cluster mass, or more specifically that the more massive clusters have a lower frequency of BSSs, and vice versa. This trend was then extended to the open cluster regime by De Marchi et al. (2006). The aim of this work was to make extracting BSS from CMDs as unambiguous and consistent a process as possible. Then, using these more reliable statistics, explore the link between BSS frequency and environment by looking for correlations with various cluster parameters including velocity dispersion, surface brightness, cluster age, EHB frequency, density, and concentration. Relative Frequencies of Blue Stragglers in Globular Clusters N. Leigh & A. Sills THE DATABASE The colour-magnitude diagrams and photometric databases for 74 galactic globular clusters (GGCs) were taken from Piotto et al. (2002). Of the 74 potential GGCs, only 60 were deemed fit for analysis, with the remaining 14 being discarded due to the poor reliability of the data at fainter magnitudes. The positions of the stars, as well as their magnitudes in the F439W and F555W bands, can be found at the Padova Globular Cluster Group Web pages at http://dipastro.pd.astro.it/globulars. BOUNDARY SELECTION The boundary conditions were ultimately chosen for consistency. The aim was to establish as agreeably as possible a set point in the CMD based on a feature characteristic of every cluster. As such, we chose to take as our starting point the sharpest point in the bend of the MSTO centered on the mass of points that populate it. From there, horizontal, vertical, and sloped lines were defined in the (m 555, m 439 – m 555 )-plane as boundaries to enclose the BSS, HB and EHB populations. RESULTS Both the total cluster mass and the central velocity dispersion seem to be anticorrelated with the frequency of BSS in the cluster core. Moreover, there is a weak correlation between the BSS frequency and the central surface brightness. Amongst the parameters that showed no trend with BSS frequency were the cluster age, central density, concentration, and EHB frequency. REFERENCES De Angeli, F., et al. 2005, AJ, 130, 116 De Marchi, G., et al. 2006, A&A (submitted) Piotto, G., et al. 2002, A&A, 391, 945 Piotto, G., et al. 2004, ApJ, 604, L109 CONCLUSIONS The apparent anticorrelation between total magnitude (or total cluster mass) and core BSS frequency is further evidence of the importance of global cluster dynamics in understanding the nature of these enigmatic stars and points towards the possibility of a preferred formation mechanism operating within the central regions of the cluster. While core blue stragglers seem to form preferentially in the least massive clusters having the lowest velocity dispersions, it remains unknown whether they are created there or are created elsewhere in the cluster and then somehow migrate inwards. In future work, we are developing a semi-analytic model in order to further constrain the connection between BSS formation and cluster dynamics and in so doing explain the observed trends in the data. Image courtesy of NASA F IG. 1. – Colour-magnitude diagram for the globular cluster NGC 5986 with boundaries overlaid. Blue straggler (BSS), horizontal branch (HB), and extended horizontal branch (EHB) stars are labeled in their respective locations. F IG. 2. - Logarithms of the relative BSS frequencies in cluster cores plotted against the total cluster magnitude (or total cluster mass) in the V band. The frequencies were normalized using HB and EHB stars together. F IG. 3. - Logarithms of the relative BSS frequencies in cluster cores plotted against the central velocity dispersion (in km s -1 ). The frequencies were normalized using HB and EHB stars together. F IG. 4. - Logarithms of the relative BSS frequencies in cluster cores plotted against the central surface brightness (in V mag/arcsec 2 ). The frequencies were normalized using HB and EHB stars together. ANALYSIS Once our sample of BS, HB, and EHB stars had been established, we found BSS frequencies inside the cores of each cluster and divided the number of BSS by the number of HB stars (F BSS HB ), EHB stars (F BSS EHB ), as well as HB and EHB stars combined (F BSS HB+EHB ) for normalization. We then looked at possible trends in the data between the resultant BSS frequencies and various cluster parameters including total V magnitude, central density, central velocity dispersion, central surface brightness, and age. We considered only those stars within the cluster core in an attempt to primarily extract collisional BSS. Magnitudes in the V band, distance modulii, extinction values, and central densities can all be found at the Padova Globular Cluster Group Web pages. Cluster parameters including the core radius, concentration, central surface brightness, and central velocity dispersion were taken from the McMaster and Pryor & Meylan (1993) Globular Cluster Catalogue at http://coihue.rutgers.edu/~andresj/gccat.html. Finally, cluster ages can be found in De Angeli et al. (2005). F IG. 5.- Logarithms of the relative BSS frequenciesin cluster cores plotted against age (upper left), concentration (upper right), central density (lower left), and EHB frequency (lower right). ABSTRACT We define a series of selection rules to isolate the blue stragglers from main-sequence turn-off (MSTO) and extended horizontal branch (EHB) stars in the colour-magnitude diagrams (CMDs) of 60 globular clusters taken from HST images of their central cores. Relative frequencies of blue straggler stars are then found using both the HB and EHB for normalization and are subsequently analyzed. We discuss recent results as well as their implications.