Poster Abstracts

Name/Affiliation:  Joanne Hughes (Seattle University)

Title:  NGC 3201: Population Studies from Photometric Metallicities

NGC 3201 is in a rare, retrograde orbit around the Milky Way. This could be interpreted as evidence that it is a captured globular cluster, the remnant of a larger, disrupted system. Since 1998, some spectral samples infer a spread in [Fe/H] of up to 0.4 dex (around an average of [Fe/H]=-1.5 dex), and others found the spread less than 0.12 dex. However, even the studies which do not find a distinguishable spread in [Fe/H] show other chemical signatures of multiple episodes of star formation. High-resolution spectral analysis has been limited to upper-RGB stars because of the S/N required. To explore the chemical composition of unmixed stars, we must observe fainter, less evolved objects, which can only be reached with low-resolution spectra and imaging (as yet). Photometric studies are complicated by a steep extinction gradient towards the cluster, so we both minimized the extinction gradients and used the reddening-free Stromgren index, [m]. We found a method to determine the position-dependent E(B-V)=0.22-0.29 values; this uncertainty level alone would introduce a spread in photometrically estimated [Fe/H]-values of +0.1 dex and -0.3 dex independent of any other factors intrinsic to a star. We reduced the systematic effects to +0.05 and -0.15 dex, which enabled us to show that the spread in [Fe/H] is real but dependent on which RGB stars were sampled. We investigate the relationship of [m] to [Fe/H] and other elements within 1st and 2nd generation stars.