Identification of strontium in the merger of two neutron stars

Abstract

Half of all of the elements in the Universe that are heavier than iron were created by rapid neutron capture. The theory underlying this astrophysical r-process was worked out six decades ago, and requires an enormous neutron flux to make the bulk of the elements1. Where this happens is still debated2. A key piece of evidence would be the discovery of freshly synthesized r-process elements in an astrophysical site. Existing models3,4,5 and circumstantial evidence6 point to neutron-star mergers as a probable r-process site; the optical/infrared transient known as a ‘kilonova’ that emerges in the days after a merger is a likely place to detect the spectral signatures of newly created neutron-capture elements7,8,9. The kilonova AT2017gfo—which was found following the discovery of the neutron-star merger GW170817 by gravitational-wave detectors10—was the first kilonova for which detailed spectra were recorded. When these spectra were first reported11,12, it was argued that they were broadly consistent with an outflow of radioactive heavy elements; however, there was no robust identification of any one element. Here we report the identification of the neutron-capture element strontium in a reanalysis of these spectra. The detection of a neutron-capture element associated with the collision of two extreme-density stars establishes the origin of r-process elements in neutron-star mergers, and shows that neutron stars are made of neutron-rich matter13.

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Fig. 1: Spectrum of the kilonova AT2017gfo, showing broad absorption features.
Fig. 2: Abundances of elements produced by the r-process.
Fig. 3: Thermal r-process-element transmission spectrum.
Fig. 4: Spectral series of AT2017gfo 1.5–4.5 days after the merger.

Data availability

Work in this paper was based on observations made with European Space Observatory (ESO) telescopes at the Paranal Observatory under programmes 099.D-0382 (principal investigator E. Pian), 099.D-0622 (principal investigator P. D’Avanzo), 099.D-0376 (principal investigator S. J. Smartt) and 099.D-0191 (principal investigator A. Grado). The data are available at http://archive.eso.org.

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Acknowledgements

We thank M. Tanaka for revisiting his previous analysis for us and for access to his spectra and line lists. We thank J. Hjorth and N. Rea for discussions. We thank the ESO Director General for allocating Director’s Discretionary Time to this programme, and the ESO operation staff for support. D.W., D.B.M., and J.S. are supported in part by Independent Research Fund Denmark grant DFF-7014-00017. The Cosmic Dawn Center is funded by the Danish National Research Foundation under grant number 140. C.J.H. acknowledges support from the ‘ChETEC’ COST Action (CA16117), supported by COST (European Cooperation in Science and Technology). A.A. is supported by the European Research Council (ERC) through ERC Starting Grant 677912 EUROPIUM. A.A. and A.B. are supported by the Sonderforschungsbereich SFB 1245 ‘Nuclei: From Fundamental Interactions to Structure and Stars’. A.B. and A.K. are supported by the Sonderforschungsbereich SFB 881 ‘The Milky Way System’ (subprojects A03, A05, A10 and A11) of the German Research Foundation (DFG). A.B. is supported by the ERC through ERC Starting Grant 759253 GreatMoves, and acknowledges support from the Klaus Tschira Foundation. S.C. acknowledges partial funding from Agenzia Spaziale Italiana-Istituto Nazionale di Astrofisica grant I/004/11/3. G.L. is supported by a research grant (19054) from Villum Fonden. K.E.H. acknowledges support by a Project Grant (162948-051) from The Icelandic Research Fund. A.J.L. acknowledges funding from the ERC under grant agreement 725246, and from the UK Science and Technologies Facilities Council (STFC) via grant ST/P000495/1. E.P. acknowledges funding from the Agenzia Spaziale Italiana (ASI) INAF grant I/088/06/0, and from the INAF project ‘Gravitational Wave Astronomy with the first detections of aLIGO and aVIRGO experiments’.

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Contributions

D.W., C.J.H. and J.S. were the primary drivers of the project; A.K., D.B.M., J.P.U.F. and A.C.A. were involved in discussions that developed the understanding of the physical processes. All authors contributed to discussions and to editing of the paper. D.W. did the initial blackbody with absorber fits to the first-epoch spectrum, created Figs. 1, 3 and Extended Data Fig. 3, 5, made the initial line identification, recognised the P Cygni profiles in the later epochs, wrote the LTE code, and was the primary author of the main text. C.J.H. computed the initial models and synthetic spectra with MOOG, and generated the MOOG spectra for HD 88609 and CS 22892−052. C.J.H. and A.K. produced the MOOG spectra from 3,000 Å to 20,000 Å for the kilonova template photosphere for all heavy elements. C.J.H. wrote the sections on MOOG spectrum synthesis and large parts of the text on nucleosynthesis. A.K. provided the line lists. J.S. reduced and processed all the X-shooter data, produced the P Cygni fitting codes and fit the P Cygni profiles to all epochs, as well as extending the TARDIS code to include the Kurucz line lists and implementing the TARDIS modelling. J.S. also produced Fig. 3 and Extended Data Figs. 1, 2, 6, and wrote the related Methods sections and a substantial part of the main text.

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Correspondence to Darach Watson.

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Extended data figures and tables

Extended Data Fig. 1 Synthetic r-process-element transmission spectra.

These spectra were generated using MOOG, in which the relative abundances are based on solar r-process abundances. The spectra were blueshifted, broadened and normalized as in Fig. 3. The solid black line is the total transmission spectrum for an atmosphere containing all the r-process elements (33As to 92U). The dashed black line is the same spectrum, but including only the light r-process elements (33As to 55Cs). The contributions from different subsets of species are also shown. The green dotted line shows the heavy r-process elements (56Ba to 92U); the blue dotted line shows the light r-process elements (33As−55Cs) excluding Sr, which is shown as a red dotted line. This plot shows how Sr stands out in absorption, regardless of the composition of the material. The normalization is arbitrary and different to the LTE equivalent in Fig. 3 for display reasons.

Extended Data Fig. 2 Synthetic r-process transmission spectra.

The spectra were generated with MOOG and are similar to those shown in Extended Data Fig. 1, except that all element contributions are displayed individually. The elements that contribute most at the reddest wavelengths are noted within the plotted line.

Extended Data Fig. 3 Thermal transmission spectra for r-process elements plotted individually.

The spectra are based on the lines formed in a gas in local thermal equilibrium. The abundances of elements are scaled to the solar r-process and the spectra are velocity broadened, blueshifted and normalized as in Fig. 3. The spectrum derived from the total solar r-process abundance mix is plotted as a thick black line. The contributions from Sr clearly dominate at around 8,000 Å, with no substantial contribution from any other element.

Extended Data Fig. 4 Evolution of the ejecta expansion velocity.

The velocities were determined independently from the P Cygni absorption line widths (blue points) and the blackbody radius (red points). Uncertainties shown are 1σ. The correspondence between the two independent estimates is striking.

Extended Data Fig. 5 Comparison of the expansion opacities at modest optical depths for Sr and Ce.

This calculation shows the potential of Sr to dominate the opacity at around 1 μm at low optical depths. The opacities are based on LTE calculations for a gas at a temperature of 5,000 K, a mean local density of 8.4 × 10−17 g cm−3 for Sr or Ce, an electron density of 7.6 × 108 cm−3, and a 1% atmospheric radius at 1.5 days after the explosion. Line lists used for Sr and Ce are from the Kurucz and VALD databases respectively.

Extended Data Fig. 6 Radiative transfer models from the first four epochs using the TARDIS code.

The blue line is the synthetic TARDIS spectrum using relative solar r-process abundances and including elements from 31Ga to 37Rb—that is, without Sr. The red line also includes 38Sr. The green line is a model including all elements from 31Ga to 92U. These models show that the spectra are well reproduced with elements around the first r-process abundance peak, specifically Sr.

Supplementary information

Supplementary information

Line lists used in the paper for the MOOG models. The line lists are tables of spectral lines compatible with the spectral synthesis code MOOG. Each file consists of an individual element's lines with the filename of the form ‘Z<atomic_number>.list’. The columns are: 1: Wavelength (Angstrom); 2: Atomic_number.charge; 3: Lower energy level of the transition (eV); 4: log(gf) of the line.

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Watson, D., Hansen, C.J., Selsing, J. et al. Identification of strontium in the merger of two neutron stars. Nature 574, 497–500 (2019). https://doi.org/10.1038/s41586-019-1676-3

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