Recent observations have shown that a growing number of the most massive Galactic globular clusters contain multiple populations of stars with different [ Fe/H ] and neutron–capture element abundances .
NGC 6273 has only recently been recognized as a member of this “ iron–complex ” cluster class , and we provide here a chemical and kinematic analysis of > 300 red giant branch ( RGB ) and asymptotic giant branch ( AGB ) member stars using high resolution spectra obtained with the Magellan –M2FS and VLT –FLAMES instruments .
Multiple lines of evidence indicate that NGC 6273 possesses an intrinsic metallicity spread that ranges from about [ Fe/H ] = –2 to –1 dex , and may include at least three populations with different [ Fe/H ] values .
The three populations identified here contain separate first ( Na/Al–poor ) and second ( Na/Al–rich ) generation stars , but a Mg–Al anti–correlation may only be present in stars with [ Fe/H ] \ga –1.65 .
The strong correlation between [ La/Eu ] and [ Fe/H ] suggests that the s–process must have dominated the heavy element enrichment at higher metallicities .
A small group of stars with low [ \alpha /Fe ] is identified and may have been accreted from a former surrounding field star population .
The cluster ’ s large abundance variations are coupled with a complex , extended , and multimodal blue horizontal branch ( HB ) .
The HB morphology and chemical abundances suggest that NGC 6273 may have an origin that is similar to \omega Cen and M 54 .