Abundance of strontium in the Galactic globular cluster 47 Tuc

TL;DR

This study addresses whether second-population stars in the globular cluster 47 Tuc show enhanced Sr, a first-peak s-process element, relative to 1P stars, testing the idea that nucleosynthetic polluters also contributed to s-process material. Using archival UVES spectra, the authors perform 1D LTE Na and Sr abundances with NLTE corrections for Na, while carefully handling CN blending and a V blend near the Sr I 650.3991 nm line; they also evaluate 3D convection effects with CO5BOLD and 3D–1D LTE corrections. The analysis yields a mean Sr enhancement of ⟨[Sr/Fe]⟩ = 0.18 ± 0.08 and a weak, not statistically robust, Sr–Na correlation, alongside a Zr–Na pattern from previous work, suggesting that polluters enriched 2P stars with light elements and may have produced some s-process material. The Sr/Zr ratio hints at either AGB stars (1–4 M⊙) or fast-rotating massive stars (10–20 M⊙, vrot ≈ 200–300 km s−1) as potential polluters, though larger samples and additional s-process element measurements (e.g., Y, Rb, Ce, Ba) are needed to discriminate among scenarios and refine nucleosynthesis models in globular clusters.

Abstract

Aims. We have determined Sr abundance in a sample of 31 red giant branch stars located in the Galactic globular cluster 47 Tuc with the aim to identify potential differences in the Sr abundance between first population (1P, Na-poor) and second population (2P, Na-rich) stars. Methods. We derived the Na and Sr abundances from the archival spectra obtained with the UVES spectrograph. To do this, we used 1D ATLAS9 model atmospheres and a 1D local thermodynamic equilibrium spectral synthesis method. Particular attention was paid to assessing the potential impact of CN line blending on the obtained Sr abundances. Furthermore, we evaluated the potential influence of convection on the Sr line formation by using 3D hydrodynamical model atmospheres computed with the CO5BOLD code. Results. Our results suggest a weak correlation between the abundances of Sr and Na. Together with a similar correlation between the abundances of Zr and Na determined in our previous study, our analysis of Sr suggests that polluters that have enriched 2P stars with light elements may have produced some s-process elements as well. The mean Sr abundance determined in 31 red giant branch stars of 47~Tuc is $\langle {\rm [Sr/Fe]} \rangle = 0.18\pm0.08$ (the error denotes the standard deviation due to the star-to-star abundance scatter). This value is within the range of the Sr abundance variation that is observed in Galactic field stars of similar metallicity. The mean [Sr/Zr] abundance ratio in our sample stars suggests that the two s-process elements could have been synthesized by either low-mass asymptotic giant branch stars ($M=1-4 {\rm M}_{\odot}$) or massive ($M=10-20 {\rm M}_{\odot}$) fast-rotating ($v_{\rm rot}=200-300$ km/s) stars.

Figures (19)

• Figure 1: Color-magnitude diagram of 47 Tuc with the target RGB stars marked as red circles (photometry from BS09).
• Figure 2: Effective temperatures determined using the Gaia DR3 photometry vs. those obtained using photometry from BS09. The mean difference between the two sets of effective temperatures and the scatter around this value (measured as the standard deviation, $\sigma$) are shown in the top left corner of the figure.
• Figure 3: Difference between the microturbulence velocities determined from the Fe i lines in the UVES and GIRAFFE spectra. The mean difference between the two sets of values and their scatter around the mean (measured as standard deviation, $\sigma$) are shown in the bottom panel of the figure.
• Figure 4: Abundances in the target RGB stars vs the effective temperature and microturbulence velocity in individual stars. The linear fits to the data are shown as red lines.
• Figure 5: Typical observed (dots) and best-fit synthetic LTE profiles of the Na i 615.4225 and 616.0747 nm lines (solid red lines) in the UVES spectra of two RGB stars in 47 Tuc: Na-poor (5968, 1P, top row) and Na-rich (27678, 2P, bottom row). The identification numbers of each star, their atmospheric parameters, and the determined Na abundances are provided in the leftmost panels of each row.