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



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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  1. Käppeler, F. The origin of the heavy elements: the s process. Prog. Part. Nucl. Phys. 43, 419–483 (1999)

    Article  ADS  Google Scholar 

  2. Käppeler, F., Gallino, R., Bisterzo, S. & Aoki, W. The s process: nuclear physics, stellar models, and observations. Rev. Mod. Phys. 83, 157–193 (2011)

    Article  ADS  Google Scholar 

  3. Straniero, O. et al. Radiative C-13 burning in asymptotic giant branch stars and s-processing. Astrophys. J. 440, L85–L87 (1995)

    Article  ADS  CAS  Google Scholar 

  4. Goriely, S. & Mowlavi, N. Neutron-capture nucleosynthesis in AGB stars. Astron. Astrophys. 362, 599–614 (2000)

    ADS  CAS  Google Scholar 

  5. Van Eck, S., Goriely, S., Jorissen, A. & Plez, B. Discovery of three lead-rich stars. Nature 412, 793–795 (2001)

    Article  ADS  CAS  Google Scholar 

  6. Cristallo, S. et al. Evolution, nucleosynthesis, and yields of low-mass asymptotic giant branch stars at different metallicities. II. The FRUITY Database. Astrophys. J. Suppl. Ser. 197, 17 (2011)

    Article  ADS  Google Scholar 

  7. Trippella, O., Busso, M., Maiorca, E., Käppeler, F. & Palmerini, S. s-processing in AGB stars revisited. I. Does the main component constrain the neutron source in the 13C pocket? Astrophys. J. 787, 41 (2014)

    Article  ADS  Google Scholar 

  8. Raskin, G. et al. HERMES: a high-resolution fibre-fed spectrograph for the Mercator telescope. Astron. Astrophys. 526, A69 (2011)

    Article  Google Scholar 

  9. Neyskens, P. Exploring S Stars. PhD thesis, Univ. Libre de Bruxelles. (2014)

  10. Gustafsson, B. et al. A grid of MARCS model atmospheres for late-type stars. I. Methods and general properties. Astron. Astrophys. 486, 951–970 (2008)

    Article  ADS  CAS  Google Scholar 

  11. Van Eck, S. et al. A grid of S stars MARCS model atmospheres. J. Phys. Conf. Ser. 328, 012009 (2011)

    Article  Google Scholar 

  12. Van Eck, S. & Jorissen, A. The Henize sample of S stars. I. The technetium dichotomy. Astron. Astrophys. 345, 127–136 (1999)

    ADS  CAS  Google Scholar 

  13. Jorissen, A., Frayer, D. T., Johnson, H. R., Mayor, M. & Smith, V. V. S stars: infrared colors, technetium, and binarity. Astron. Astrophys. 271, 463–481 (1993)

    ADS  CAS  Google Scholar 

  14. Bao, Z. Y. et al. Neutron cross sections for nucleosynthesis studies. At. Data Nucl. Data Tables 76, 70–154 (2000)

    Article  ADS  CAS  Google Scholar 

  15. Tagliente, G. et al. The 93Zr(n,γ) reaction up to 8 keV neutron energy. Phys. Rev. C 87, 014622 (2013)

    Article  ADS  Google Scholar 

  16. Merrill, P. W. Spectroscopic observations of stars of class S. Astrophys. J. 116, 21–26 (1952)

    Article  ADS  CAS  Google Scholar 

  17. Smith, V. V. & Wallerstein, G. Quantitative technetium and niobium abundances in heavy-element stars. Astrophys. J. 273, 742–748 (1983)

    Article  ADS  CAS  Google Scholar 

  18. Mathews, G. J., Takahashi, K., Ward, R. A. & Howard, W. M. Stellar technetium and niobium abundances as a measure of the lifetime of AGB stars in the third dredge-up phase. Astrophys. J. 302, 410–414 (1986)

    Article  ADS  CAS  Google Scholar 

  19. Vassiliadis, E. & Wood, P. R. Evolution of low- and intermediate-mass stars to the end of the asymptotic giant branch with mass loss. Astrophys. J. 413, 641–657 (1993)

    Article  ADS  Google Scholar 

  20. Schröder, K.-P. & Cuntz, M. A new version of Reimers’ law of mass loss based on a physical approach. Astrophys. J. 630, L73–L76 (2005)

    Article  ADS  Google Scholar 

  21. Van Eck, S., Jorissen, A., Udry, S., Mayor, M. & Pernier, B. The HIPPARCOS Hertzsprung-Russell diagram of S stars: probing nucleosynthesis and dredge-up. Astron. Astrophys. 329, 971–985 (1998)

    ADS  Google Scholar 

  22. Joint IRAS Science Working Group. IRAS Catalogue of Point Sources, Version 2.0 (IPAC, 1986)

  23. Kupka, F., Piskunov, N., Ryabchikova, T. A., Stempels, H. C. & Weiss, W. W. VALD-2: Progress of the Vienna atomic line data base. Astron. Astrophys. 138, 119–133 (1999)

    ADS  CAS  Google Scholar 

  24. Alvarez, R. & Plez, B. Near-infrared narrow-band photometry of M-giant and Mira stars: models meet observations. Astron. Astrophys. 330, 1109–1119 (1998)

    ADS  CAS  Google Scholar 

  25. de Laverny, P. et al. Chemical analysis of carbon stars in the Local Group. Astron. Astrophys. 446, 1107–1118 (2006)

    Article  ADS  CAS  Google Scholar 

  26. Lambert, D. L., Smith, V. V., Busso, M., Gallino, R. & Straniero, O. The chemical composition of red giants. IV. The neutron density at the s-process site. Astrophys. J. 450, 302–317 (1995)

    Article  ADS  CAS  Google Scholar 

  27. Siess, L. Evolution of massive AGB stars. I. Carbon burning phase. Astron. Astrophys. 448, 717–729 (2006)

    Article  ADS  CAS  Google Scholar 

  28. Asplund, M., Grevesse, N. & Sauval, A. J. in Cosmic Abundances as Records of Stellar Evolution and Nucleosynthesis (eds Barnes, T. G. III & Bash, F. N. ) 25–38 (Astonomical Society of the Pacific, 2005)

    Google Scholar 

  29. Goriely, S. & Siess, L. S-process in hot AGB stars: a complex interplay between diffusive mixing and nuclear burning. Astron. Astrophys. 421, L25–L28 (2004)

    Article  ADS  CAS  Google Scholar 

  30. Garcìa-Hernàndez, D. A. et al. Rubidium-rich asymptotic giant branch stars. Science 314, 1751–1754 (2006)

    Article  ADS  Google Scholar 

  31. Takahashi, K. & Yokoi, K. Beta-decay rates of highly ionized heavy atoms in stellar interiors. At. Data Nucl. Data Tables 36, 375–409 (1987)

    Article  ADS  CAS  Google Scholar 

  32. Wallerstein, G., Balick, B., Alcolea, J., Bujarrabal, V. & Vanture, A. D. Carbon isotopic abundance ratios in S-type stars. Astron. Astrophys. 535, A101 (2011)

    Article  ADS  Google Scholar 

  33. Savina, M. R. et al. Extinct technetium in silicon carbide stardust grains: implications for stellar nucleosynthesis. Science 303, 649–652 (2004)

    Article  ADS  CAS  Google Scholar 

  34. Kashiv, Y. et al. Extinct 93Zr in single presolar SiC Grains from low mass asymptotic giant branch stars and condensation from Zr-depleted gas. Astrophys. J. 713, 212–219 (2010)

    Article  ADS  CAS  Google Scholar 

  35. Stephenson, C. B. A General Catalogue of Galactic S Stars 2nd edn, Publications of the Warner & Swasey Observatory 3 (1984)

    Google Scholar 

  36. Biémont, E., Grevesse, N., Hannaford, P. & Lowe, R. M. Oscillator strengths for Zr I and Zr II and a new determination of the solar abundance of zirconium. Astrophys. J. 248, 867–873 (1981)

    Article  ADS  Google Scholar 

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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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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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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).

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