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

2. “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)

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

4. 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

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

6. 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

7. 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]…

8. 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

9. 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]

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

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

12. [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