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

Pian, E and D'Avanzo, P and Benetti, S and Branchesi, M and Brocato, E and Campana, S and Cappellaro, E and Covino, S and D'Elia, V and Fynbo, JPU and Getman, F and Ghirlanda, G and Ghisellini, G and Grado, A and Greco, G and Hjorth, J and Kouveliotou, C and Levan, A and Limatola, L and Malesani, D and Mazzali, PA and Melandri, A and Møller, P and Nicastro, L and Palazzi, E and Piranomonte, S and Rossi, A and Salafia, OS and Selsing, J and Stratta, G and Tanaka, M and Tanvir, NR and Tomasella, L and Watson, D and Yang, S and Amati, L and Antonelli, LA and Ascenzi, S and Bernardini, MG and Boër, M and Bufano, F and Bulgarelli, A and Capaccioli, M and Casella, P and Castro-Tirado, AJ and Chassande-Mottin, E and Ciolfi, R and Copperwheat, CM and Dadina, M and De Cesare, G and Di Paola, A and Fan, YZ and Gendre, B and Giuffrida, G and Giunta, A and Hunt, LK and Israel, GL and Jin, Z-P and Kasliwal, MM and Klose, S and Lisi, M and Longo, F and Maiorano, E and Mapelli, M and Masetti, N and Nava, L and Patricelli, B and Perley, DA and Pescalli, A and Piran, T and Possenti, A and Pulone, L and Razzano, M and Salvaterra, R and Schipani, P and Spera, M and Stamerra, A and Stella, L and Tagliaferri, G and Testa, V and Troja, E and Turatto, M and Vergani, SD and Vergani, D (2017) Spectroscopic identification of r-process nucleosynthesis in a double neutron-star merger. Nature, 551 (7678). ISSN 1476-4687

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Abstract

The merger of two neutron stars is predicted to give rise to three major detectable phenomena: a short burst of γ-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 γ-ray burst at redshift 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 GW170817 and γ-ray burst GRB 170817A associated with a galaxy at a distance of 40 megaparsecs 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 to 0.05 solar masses of material, including high-opacity lanthanides.

Item Type: Article
Uncontrolled Keywords: MD Multidisciplinary
Subjects: Q Science > QB Astronomy
Divisions: Astrophysics Research Institute
Publisher: Nature Publishing Group
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Date Deposited: 14 Nov 2017 14:19
Last Modified: 14 Nov 2017 14:19
DOI or Identification number: 10.1038/nature24298
URI: http://researchonline.ljmu.ac.uk/id/eprint/7550

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