Discrepancies between spectroscopy and HST photometry in tagging multiple stellar populations in 22 globular clusters

Multiple populations (MPs) in globular clusters (GCs) are stars distinct by their abundances of light elements. The MPs can be directly separated by measuring abundances of C, N, O, Na, Al, Mg with spectroscopy or indirectly from photometric sequences created by the impact of different chemistry on band passes of particular filters, such as the HST pseudo-colours in the ultraviolet. An attempt to link HST pseudo-colours maps (PCMs) and spectroscopy was done by Marino et al. (2019), using abundances mostly from our FLAMES survey. However, we uncovered that an incomplete census of stars in common was used in their population tagging. We correct the situation by building our own PCMs and matching them with our abundances in 20 GCs, plus two GCs from other sources, doubling the sample with spectroscopic abundances available. We found that the pseudo-colour (magF275W-2*magF336W+magF438W) does not have a monotonic trend with Na abundances, enhanced by proton-capture reactions in MPs. Moreover, on average about 16% of stars with spectroscopic Na abundances show a discrepant tagging of MPs with respect to the HST photometry. Stars with chemistry of second generation (SG) are mistaken for first generation (FG) objects according to HST photometry and vice versa. In general, photometric indices tend to overestimate the fraction of FG stars, in particular in low mass GCs. We offer a simple explanation for these findings. Finally, we publish all our PCMs, with more than 31,800 stars in 22 GCs, with star ID and coordinates, for easy check and reproduction, as it should be in science papers.
Comment: 24 pages, 19 figures, 2 on-line tables plus 23 pages with 21 figures of Appendix; accepted for publication on Astronomy and Astrophysics