Barium isotopic ratios in metal-poor stars: calibrating the method with globular clusters

TL;DR

The paper addresses how to disentangle s- and r-process contributions to Ba in metal-poor stars by using Ba isotopic ratios as a diagnostic. It develops a calibration of microturbulence specifically for Ba lines by analyzing subordinate and resonance Ba features in a homogeneous globular-cluster sample (NGC 6752) and cross-checks with 3D models. The authors derive Ba abundances and isotopic fractions, finding that turn-off stars appear s-process dominated while some giants show r-process signatures, implying diversity within the cluster and raising questions about multiple populations and membership. The method provides a practical framework for isotopic diagnostics in metal-poor stars and sets the stage for applying these calibrations to larger samples and additional clusters.

Abstract

Identifying the nucleosynthesis processes behind heavy-element enrichment in stellar atmospheres is challenging. It typically relies on comparing observed abundance-to-iron ratios with theoretical predictions relative to the Sun, but this method is prone to uncertainty due to limitations of classical 1D hydrostatic models. One promising but still underexplored approach is to measure the isotopic composition of stellar atmospheres by focusing on elements that have both slow (s)-process and rapid (r)-process contributions. While the study of total elemental abundances offers a simplified view, isotopic ratios are directly linked to the underlying nucleosynthesis processes. Our aim is to provide a reliable method for quantifying the contributions of the s- and r-processes to barium in stellar atmospheres. This is achieved by determining barium isotopic ratios using 1D atmospheric models in combination with a carefully calibrated microturbulence, based on the comparison between subordinate and resonance Ba lines. In this initial study, we use member stars of the globular cluster NGC 6752 to calibrate the microturbulence ($v_{mic}$) value for both subordinate and resonance barium lines across different stellar evolutionary stages. This allows us to provide a reliable estimate of $v_{mic}$ that can be applied to accurately determine barium abundances and isotopic ratios in stars ranging from the main sequence to the upper red giant branch. The $v_{mic}$ scale adapted for barium subordinate lines is consistent with that derived from 3D model atmospheres, and thus the $T_{\mathrm{eff}}$-log $g$ dependent relations of the later can be used safely. The $v_{mic}$ for the resonance line at $λ$4934 Angstrom -- for the determination of the isotopic ratio -- is higher, and depends on the equivalent width (EW). We provide calibrated relations between $v_{mic}$ and EW for measuring isotopic ratios.

Figures (20)

• Figure 1: Theoretical curves of growth for the two Ba resonance lines. Equivalent widths (EW) were computed in spectra synthesised by MARCS 1D atmosphere models adopting the atmospheric parameters $T_{\mathrm{eff}}$ = 5540 K, log g = 2.45, [Fe/H] = $-2.00$ dex, and $v_{mic} = 1.5$ km s$^{-1}$. The barium abundance in the horizontal axis is neglected on purpose, as the scale depends on the abundance itself and the stellar parameters. The location of our stars is indicated by the red circles, where they are designated according to their evolutionary state as follows: turn-off (TO), subgiant (SG), base of the red giant branch (bRGB), over the clump RGB (ocRGB), and upper RGB (uRGB).
• Figure 2: Synthetic profiles of the line at $\lambda$4934 Å. The synthesis is made with the code Turbospectrum gerber2023 and MARCS models; see details in Sect. \ref{['sec:isotopes']}. The atmospheric parameters for the synthesis are similar to those of the star GES J19102677-6003089 (TO-1) in Table \ref{['tab:atmo_param']}. The spectrum has R = 493000 and infinite S/N. The solid line (r-process), dashed line (s-process), and blue shade (i-process) profiles are synthesised with the isotopic ratios in Table \ref{['tab:ratios']}.
• Figure 3: Colour-magnitude diagram of NGC 6752. Grey and red symbols represent stars in the cluster field and the sample analysed in the present work, respectively. The photometry for both samples was taken from Gaia e dr3. An isochrone of 12 Gyr is overplotted as a reference.
• Figure 4: Kiel diagram of the program stars. Red circles display $T_{\mathrm{eff}}$ and log g$^{NLTE}$. Gray circles display log g$^{LTE}$. Quantities are listed in Table \ref{['tab:atmo_param']}.
• Figure 5: LTE barium abundance as function of $v_{mic}$ for the subordinate lines 5853, 6141, and 6496 Å. Each trend is related to one star and is colour-coded according to log g. Shades enclose the most probable ranges of A(Ba) of the TO-1 and TO-2 stars; see main text. Dashed gray and red lines indicate the mean A(Ba) in the shades for each star, respectively.