Abundance of zirconium in the globular cluster 47 Tucanae: a possible Zr–Na correlation?

Abstract

We determined abundances of Na and Zr in the atmospheres of 237 RGB stars in Galactic globular cluster (GGC) 47 Tuc (NGC 104), with a primary objective of investigating possible differences between the abundances of Zr in the first generation (1P) and second generation (2P) stars. For the abundance analysis, we used archival UVES/GIRAFFE spectra obtained during three different observing programmes. Abundances were determined from two Na I and three Zr I lines, using 1D hydrostatic ATLAS9 model atmospheres. The target stars for the abundance analysis were limited to those with 4200 ≤ Teff ≤ 4800 K. This is the largest sample of GGC stars in which Na and Zr abundances have been studied so far. While our mean [Na/Fe] and [Zr/Fe] ratios agree well with those determined in the earlier studies, we find a weak but statistically significant correlation in the [Zr/Fe] – [Na/Fe] plane. A comparison of the mean [Zr/Fe] abundance ratios in the 1P and 2P stars suggests a small but statistically significant Zr over-abundance in the 2P stars, ∆[Zr/Fe]2P−1P ≈ +0.06 dex. Also, our analysis shows that stars enriched in both Zr and Na are more centrally concentrated. However, we find no correlation between their distance from the cluster centre and their full spatial velocity, as indicated by the velocity dispersions at different mean values of [Zr/Fe] and [Na/Fe]. While there may be some influence of CN line blends on the determined Zr abundances, it seems very unlikely that the detected Zr–Na correlation, for the slightly higher Zr abundances in the 2P stars, would be caused by the CN blending alone. The obtained results indicate that, in 47 Tuc, some amount of Zr should have been synthesised by the same polluters that enriched 2P stars with the light elements. While sizeable amounts of Zr may be synthesised by both AGB stars (M ~ 1.5–5 M⊙) and massive rotating stars (M ~ 12–25 M⊙, υrot > 150 km s−1), our data alone do not allow us to distinguish which of the two scenarios, or whether or not a combination of both, could have operated in this GGC.

Abundance of barium in the atmospheres of red giants in the Galactic globular cluster NGC 104 (47 Tuc)

“…Some other studies have shown tentative signs for a possible correlation between the abundances of light and s-process elements in other Type I GGCs, such as signatures for Sr-Na and Y-Na relations in M 4 (Spite et al 2016;Villanova & Geisler 2011; but see also D'Orazi et al 2013). In addition, our recent analysis of Zr abundance in 237 RGB stars in 47 Tuc suggests the existence of weak but statistically significant Na-Zr correlation and 2P-1P Zr abundance difference of ∆[Zr/Fe] 2P−1P ≈ 0.06 (Kolomiecas et al 2021). The existence of such correlations would indicate that the light and s-process elements should have been produced in the same polluters that have enriched the 2P stars in some elements and depleted in others.…”

“…We also note that no Ba-Na correlation was detected by D' Orazi et al (2010) in their analysis of 110 RGB stars in 47 Tuc, which indirectly supports our assertion that the [Ba/Fe] − [Na/Fe] anti-correlation detected in our study is, in fact, spurious. This said, it would be nevertheless interesting to see whether the Ba-Na -or any correlation between any other s-process and light elements -could be seen in other GGCs, especially given the tentative detection of Zr-Na correlation in the RGB stars of 47 Tuc that is suggested in the analysis of Kolomiecas et al (2021).…”

“…Individual spectra were retrieved for the analysis from the ESO Advanced Data Products Archive 1 . Our stellar sample overlaps with that utilised by Kolomiecas et al (2021) in their study of Zr in 47 Tuc (228 stars in common), but it also includes 33 additional targets that were not analysed in Kolomiecas et al (2021).…”

“…Atmospheric parameters for the majority of targets (228 stars) were determined by Kolomiecas et al (2021). For the remaining 33 stars, we used the same procedure as utilised by the latter authors.…”

“…Because the stellar sample used in the present study overlaps with the one investigated by Kolomiecas et al (2021), abundances of Fe and Na in targets common to the two studies (228 stars) were taken from Kolomiecas et al (2021). In addition, we determined Fe and Na abundances in 33 RGB stars and Ba abundances in 261 stars.…”