Our team, which includes astronomers from the ANU's Research School of Astronomy and Astrophysics and Monash University, has combined HST photometric data with high resolution UVES@VLT spectra collected under the ESO program 0101.D-0113. Thanks to this valuable dataset we have inferred the chemical abundances for many elements and radial velocities of stars distributed along the extended 1G of the GC NGC3201 (Marino et al. 2019).
In Figure 2 below, the abundance ratios [La/Y] and [Ba/Y] have been taken as indicative of the heavy to light n-capture element abundance ratio ([hs/ls]), which is sensitive to the neutron exposure and neutron density. Clearly, the stars with more extreme lower values of ΔF275W,F814W have higher [hs/ls] and lower Fe abundances. The higher [hs/ls] of two stars, that are also enriched in n-capture elements, might suggest direct mass transfer between the star and previous low-mass, low-metallicity AGB stars (e.g. Karakas & Lattanzio 2014). According to the variations in radial velocities, the stars with lowest ΔF275W,F814W are consistent with being binaries. Thus, our results suggest that a combination of two phenomena may account for the 1G spread in this GC: (1) a tiny variation in metals, of the order of 0.10 dex; and (2) binarity, which alleviates the size of chemical inhomogeneity required to account for the photometric spread of 1G stars.