Peculiar Massive Globular Clusters in the Milky Way Christian Johnson Smithsonian Astrophysical Observatory

“Typical” Globular Clusters Globular Cluster M13 Asymptotic Giant Branch Horizontal Branch Red Giant Branch B Subgiant Branch Stars exhibit small or negligible age spread Very small (< 12%) metallicity dispersion Simple Stellar Population Main Sequence Turn-off Main Sequence B-I Sandquist+ (2010)

Modern Picture of Globular Clusters Carretta+ (2009) Piotto+ (2015) α-enhanced → rapid enrichment r-process comp.→ rapid enrichment Gratton+ (2004) Johnson+ (2017, in prep.)

Peculiar Massive Clusters ω Cen M 54 NGC 6273 Da Costa (2015) [Fe/H] dispersion not yet measured Mass NGC 6273 NGC 5286 M 22 M 2 A subset of the most massive globular clusters in the Milky Way have significant star-to-star [Fe/H] Variations

Signatures of Peculiar Massive Clusters Johnson+ (2010) Johnson+ (2016) Johnson+ (2015) Carretta+ (2010) NGC 5286 Metal-poor Metal-intermediate Marino+ (2015) Yong+ (2014)

Signatures of Peculiar Massive Clusters Johnson+ (2010) Bellini+ (2009) Han+ (2015) NGC 6273 BHB ω Cen NGC 5286 Marino+ (2015) Distinct stellar groups with different [Fe/H] values

Light Element Variations NGC 6273 M 22 Metal-poor Metal-intermediate Metal-rich Johnson+ (2016) Marino+ (2011) Metal-poor Metal-rich The same light element abundance (anti-)correlations observed in monometallic clusters is also present in each sub-population of all iron-complex clusters Some clusters exhibit abundance pattern changes with [Fe/H]…

Light Element Variations ω Cen NGC 6273 Metal-poor Metal-int. < 65 MK > 65 MK Metal-int. 2 Metal-rich Not seen in other clusters Johnson+ (2016) Johnson+ (2010) Changes in the light element abundance correlations may signify new pollution sources and/or burning temperatures

Alpha Element Variations M 54 Cluster Stars Sagittarius dSph field stars Carretta+ (2010) The M 54 + Sag. dSph system exhibits different [α/Fe]; cluster stars are α-enhanced and field stars have lower [α/Fe]

Alpha Element Variations NGC 6273 ω Cen Pancino+ (2002) Johnson+ (2016) M 2 Yong+ (2014)

Heavy Element Variations Pure s-process ω Cen McWilliam (2011) Marino+ (2015) NGC 6273 Johnson+ (2016)

[Fe/H] dispersion not yet measured Final Thoughts ω Cen M 54 NGC 6273 Da Costa (2015) [Fe/H] dispersion not yet measured Mass NGC 5286 M 22 M 2 The most massive and complex clusters may have been former dwarf galaxy cores and may be identified by their chemical compositions