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Spectroscopic identification of r-process nucleosynthesis in a double neutron star merger

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posted on 01.05.2018, 13:30 by E. Pian, P. D'Avanzo, S. Benetti, M. Branchesi, E. Brocato, S. Campana, E. Cappellaro, S. Covino, V. D'Elia, J. P. U. Fynbo, F. Getman, G. Ghirlanda, G. Ghisellini, A. Grado, G. Greco, J. Hjorth, C. Kouveliotou, A. Levan, L. Limatola, D. Malesani, P. A. Mazzali, A. Melandri, P. Moller, L. Nicastro, E. Palazzi, S. Piranomonte, A. Rossi, O. S. Salafia, J. Selsing, G. Stratta, M. Tanaka, N. R. Tanvir, L. Tomasella, D. Watson, S. Yang, L. Amati, L. A. Antonelli, S. Ascenzi, M. G. Bernardini, M. Boer, F. Bufano, A. Bulgarelli, M. Capaccioli, P. G. Casella, A. J. Castro-Tirado, E. Chassande-Mottin, R. Ciolfi, C. M. Copperwheat, M. Dadina, G. D. Cesare, A. D. Paola, Y. Z. Fan, B. Gendre, G. Giuffrida, A. Giunta, L. K. Hunt, G. Israel, Z.-P. Jin, M. Kasliwal, S. Klose, M. Lisi, F. Longo, E. Maiorano, M. Mapelli, N. Masetti, L. Nava, B. Patricelli, D. Perley, A. Pescalli, T. Piran, A. Possenti, L. Pulone, M. Razzano, R. Salvaterra, P. Schipani, M. Spera, A. Stamerra, L. Stella, G. Tagliaferri, V. Testa, E. Troja, M. Turatto, S. D. Vergani, D. Vergani
The merger of two neutron stars is predicted to give rise to three major detectable phenomena: a short burst of gamma-rays, a gravitational wave signal, and a transient optical/near-infrared source powered by the synthesis of large amounts of very heavy elements via rapid neutron capture (the r-process). Such transients, named "macronovae" or "kilonovae", are believed to be centres of production of rare elements such as gold and platinum. The most compelling evidence so far for a kilonova was a very faint near-infrared rebrightening in the afterglow of a short gamma-ray burst at z = 0.356, although findings indicating bluer events have been reported. Here we report the spectral identification and describe the physical properties of a bright kilonova associated with the gravitational wave source GW 170817 and gamma-ray burst GRB 170817A associated with a galaxy at a distance of 40 Mpc from Earth. Using a series of spectra from ground-based observatories covering the wavelength range from the ultraviolet to the near-infrared, we find that the kilonova is characterized by rapidly expanding ejecta with spectral features similar to those predicted by current models. The ejecta is optically thick early on, with a velocity of about 0.2 times light speed, and reaches a radius of about 50 astronomical units in only 1.5 days. As the ejecta expands, broad absorption-like lines appear on the spectral continuum indicating atomic species produced by nucleosynthesis that occurs in the post-merger fast-moving dynamical ejecta and in two slower (0.05 times light speed) wind regions. Comparison with spectral models suggests that the merger ejected 0.03-0.05 solar masses of material, including high-opacity lanthanides.

Funding

This work is based on observations made with the ESO telescopes at the Paranal Observatory under programmes ID 099.D-0382 (Principal Investigator (PI): E. Pian), 099.D-0622 (PI: P.D’A.), 099.D-0191 (PI: A. Grado) and with the REM telescope at the ESO La Silla Observatory under programme ID 35020 (PI: S. Campana). Gemini observatory data were obtained under programme GS-2017B-DD-1 (PI: L. P. Singer). We thank the Gemini Observatory for performing these observations, the ESO Director General for allocating discretionary time and the ESO operation staff for support. We thank D. Fugazza for technical support with operating the REM telescope remotely and REM telescope director E. Molinari. We acknowledge INAF for supporting the project ‘Gravitational Wave Astronomy with the first detections of adLIGO and adVirgo experiments—GRAWITA’ (PI: E.B.) and support from ASI grant I/004/11/3. J.H. was supported by a VILLUM FONDEN Investigator grant (project number 16599). M.M.K. acknowledges support from the GROWTH (Global Relay of Observatories Watching Transients Happen) project funded by the National Science Foundation under PIRE grant number 1545949.

History

Citation

Nature, 2017, 551, pp. 67–70

Author affiliation

/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy

Version

AM (Accepted Manuscript)

Published in

Nature

Publisher

Nature Publishing Group

issn

0028-0836

eissn

1476-4687

Acceptance date

20/09/2017

Copyright date

2017

Available date

01/05/2018

Publisher version

https://www.nature.com/articles/nature24298

Language

en

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