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The temperature and chronology of heavy-element synthesis in low-mass stars

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Abstract

Roughly half of the heavy elements (atomic mass greater than that of iron) are believed to be synthesized in the late evolutionary stages of stars with masses between 0.8 and 8 solar masses. Deep inside the star, nuclei (mainly iron) capture neutrons and progressively build up (through the slow-neutron-capture process1,2, or s-process) heavier elements that are subsequently brought to the stellar surface by convection. Two neutron sources, activated at distinct temperatures, have been proposed: 13C and 22Ne, each releasing one neutron per α-particle (4He) captured1,2,3,4. To explain the measured stellar abundances1,2,3,4,5,6,7, stellar evolution models invoking the 13C neutron source (which operates at temperatures of about one hundred million kelvin) are favoured. Isotopic ratios in primitive meteorites, however, reflecting nucleosynthesis in the previous generations of stars that contributed material to the Solar System, point to higher temperatures (more than three hundred million kelvin), requiring at least a late activation of 22Ne (ref. 1). Here we report a determination of the s-process temperature directly in evolved low-mass giant stars, using zirconium and niobium abundances, independently of stellar evolution models. The derived temperature supports 13C as the s-process neutron source. The radioactive pair 93Zr–93Nb used to estimate the s-process temperature also provides, together with the pair 99Tc–99Ru, chronometric information on the time elapsed since the start of the s-process, which we determine to be one million to three million years.

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Figure 1: The Zr–Nb pair as an s-process thermometer.
Figure 2: KR CMa asymptotic-giant-branch timescale determination.
Figure 3: Asymptotic-giant-branch timescales and infrared excess.

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Acknowledgements

P.N. acknowledges the support of a FRIA (FNRS) fellowship. S.G., L.S. and S.V.E. are FRS-FNRS Research Associates. B.P. is supported in part by the CNRS Programme National de Physique Stellaire. This work has been partly funded by an Action de recherche concertée (ARC) from the Direction générale de l’Enseignement non obligatoire et de la Recherche scientifique, Communauté française de Belgique. It was based on observations obtained using the HERMES spectrograph, which is supported by the Fund for Scientific Research of Flanders (FWO); the Research Council of K.U. Leuven; the Fonds National de la Recherche Scientifique (FRS-FNRS), Belgium; the Royal Observatory of Belgium; the Observatoire de Genève, Switzerland; and the Thüringer Landessternwarte Tautenburg, Germany.

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Authors and Affiliations

Authors

Contributions

S.V.E. and A.J. initiated the project. S.V.E. was supervisor of the PhD thesis of P.N., who computed the stellar atmospheric parameters, the abundances and the grid of S star model atmospheres, with help from B.P. The stellar models were computed by L.S., and the nucleosynthesis predictions were made by S.G. The text was written by S.V.E. and A.J., and edited by the other authors.

Corresponding author

Correspondence to S. Van Eck.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Observed and synthetic spectra around the Tc i 426.227 nm line.

a, Observed spectrum of the intrinsic, Tc-rich star NQ Pup (dots) along with spectral syntheses with log(εTc) = log(N(Tc)/N(H)) + 12.0 = −0.20 (continuous line; see Extended Data Table 3) and no Tc (dashed line). b, Same as a, but for the extrinsic, Tc-poor star V613 Mon; spectral syntheses correspond to log(εTc) = −0.30 (continuous line) and no Tc (dashed line).

Extended Data Figure 2 Predicted versus measured abundances.

We show that Nb and Zr abundances for intrinsic S stars and carbon stars (with only upper limits on the Nb abundance25) are consistent with nucleosynthesis predictions (shaded areas) for a model with a solar initial Nb abundance (±0.15 dex) and different λpm values29 (as labelled). Once all 93Zr has decayed to 93Nb, the models (dotted lines) reproduce the extrinsic S stars. The error bars represent the line-to-line dispersion and the systematic uncertainty from the models. The dashed line is a fit of equation (9) through the two most enriched extrinsic stars.

Extended Data Figure 3 Location of intrinsic S stars in the Hertzsprung–Russell diagram.

We show the location of intrinsic (that is, Tc-rich) S stars (triangles) from our sample in the stellar luminosity/effective temperature diagram21, along with low-mass evolutionary tracks (starting at the zero-age main sequence). The main sequence (MS) is indicated. The black diamond on each evolutionary track indicates where the third dredge-up event starts.

Extended Data Figure 4 Timescale determination for HIP 103476.

a, b, N(Tc)/N(Zr) (a) and [Nb/Zr] (b) predictions from the best-matching model (as judged from panel d, with the same meaning as in Fig. 2). For reasons of clarity we plot only these predictions. c, The deviation between predicted (solid line) and observed (dashed line) abundances in a and b, converted into weight functions fTc,i(t) (blue) and fNb,i(t) (red) (equation (4)). d, The maximum value reached by their product (green) (equation (5)) provides the best age estimate (vertical line). Weight product functions corresponding to less satisfactory models are plotted as grey lines.

Extended Data Table 1 Summary of observations
Extended Data Table 2 Adopted atmospheric parameters for the S and M stars of the present study
Extended Data Table 3 Derived abundances for S, M and C stars
Extended Data Table 4 Sensitivity of Fe, Nb, Zr and Tc abundances to model atmosphere parameter variations (effective temperature, gravity and microturbulence χt)
Extended Data Table 5 Molecular and atomic lines used for computation of the stellar parameters, and atomic lines used for the abundance determinations
Extended Data Table 6 Derived s-process timescales, counted from the first thermal pulse, and masses for the intrinsic S stars of our sample

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Neyskens, P., Van Eck, S., Jorissen, A. et al. The temperature and chronology of heavy-element synthesis in low-mass stars. Nature 517, 174–176 (2015). https://doi.org/10.1038/nature14050

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