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  Physical conditions for the r-process I. radioactive energy sources of kilonovae

Wanajo, S. (2018). Physical conditions for the r-process I. radioactive energy sources of kilonovae. Astrophysical Journal, 868(1): 65. doi:10.3847/1538-4357/aae0f2.

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Item Permalink: http://hdl.handle.net/21.11116/0000-0002-9766-1 Version Permalink: http://hdl.handle.net/21.11116/0000-0002-EE46-4
Genre: Journal Article

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 Creators:
Wanajo, Shinya1, Author              
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1Computational Relativistic Astrophysics, AEI-Golm, MPI for Gravitational Physics, Max Planck Society, ou_2541714              

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Free keywords: Astrophysics, High Energy Astrophysical Phenomena, astro-ph.HE
 Abstract: Radioactive energies from unstable nuclei made in the ejecta of neutron star mergers play principal roles in powering kilonovae. In previous studies power-law-type heating rates (e.g., ~ t^-1.3) have frequently been used, which may be inadequate if the ejecta are dominated by nuclei other than the A ~ 130 region. We consider, therefore, two reference abundance distributions that match the r-process residuals to the solar abundances for A >= 69 (light trans-iron plus r-process elements) and A >= 90 (r-process elements). Nucleosynthetic abundances are obtained by using free-expansion models with three parameters: expansion velocity, entropy, and electron fraction. Radioactive energies are calculated as an ensemble of weighted free-expansion models that reproduce the reference abundance patterns. The results are compared with the bolometric luminosity (> a few days since merger) of the kilonova associated with GW170817. We find that the former case (fitted for A >= 69) with an ejecta mass 0.06 M_sun reproduces the light curve remarkably well including its steepening at > 7 days, in which the mass of r-process elements is ~ 0.01 M_sun. Two beta-decay chains are identified: 66Ni -> 66Cu -> 66Zn and 72Zn -> 72Ga -> 72Ge with similar halflives of parent isotopes (~ 2 days), which leads to an exponential-like evolution of heating rates during 1-15 days. The light curve at late times (> 40 days) is consistent with additional contributions from the spontaneous fission of 254Cf and a few Fm isotopes. If this is the case, the event GW170817 is best explained by the production of both light trans-iron and r-process elements that originate from dynamical ejecta and subsequent disk outflows from the neutron star merger.

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 Dates: 2018-08-112018-09-112018
 Publication Status: Published in print
 Pages: 15 pages, 5 figures, accepted for publication in ApJ
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 Identifiers: arXiv: 1808.03763
DOI: 10.3847/1538-4357/aae0f2
URI: http://arxiv.org/abs/1808.03763
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Title: Astrophysical Journal
Source Genre: Journal
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Pages: - Volume / Issue: 868 (1) Sequence Number: 65 Start / End Page: - Identifier: -