TY - JOUR
T1 - Systematic opacity calculations for kilonovae
AU - Tanaka, Masaomi
AU - Kato, Daiji
AU - Gaigalas, Gediminas
AU - Kawaguchi, Kyohei
N1 - Funding Information:
This research was supported by JSPS Bilateral Joint Research Project, Inoue Science Research Award from Inoue Foundation for Science, the Grant-in-Aid for Scientific Research from JSPS (16H02183, 19H00694, 20H00158) and MEXT (17H06363), the NINS program of Promoting Research by Networking among Institutions (Grant Number 01411702). GG thanks the Research Council of Lithuania for funding his research (grant No. S-LJB -18-1).
Funding Information:
We thank Shinya Wanajo for providing the results of nucleosynthesis calculations, Michel Busquet for the generous support on the HULLAC code, and Brian Metzger for giving fruitful suggestions. MT and KK thank the Yukawa Institute for Theoretical Physics for support in the framework of International Molecule-type Workshop (YITP-T-18-06), where a part of this work has been done. Numerical simulations presented in this paper were carried out with Cray XC30 and XC50 at Center for Computational Astrophysics, National Astronomical Observatory of Japan. This research was supported by JSPS Bilateral Joint Research Project, Inoue Science Research Award from Inoue Foundation for Science, the Grant-in-Aid for Scientific Research from JSPS (16H02183, 19H00694, 20H00158) and MEXT (17H06363), the NINS program of Promoting Research by Networking among Institutions (Grant Number 01411702). GG thanks the Research Council of Lithuania for funding his research (grant No. S-LJB -18-1).
Publisher Copyright:
© 2020 The Author(s)
PY - 2020
Y1 - 2020
N2 - Coalescence of neutron stars (NSs) gives rise to kilonova, thermal emission powered by radioactive decays of freshly synthesized r-process nuclei. Although observational properties are largely affected by bound–bound opacities of r-process elements, available atomic data have been limited. In this paper, we study element-to-element variation of the opacities in the ejecta of NS mergers by performing systematic atomic structure calculations of r-process elements for the first time. We show that the distributions of energy levels tend to be higher as electron occupation increases for each electron shell due to the larger energy spacing caused by larger effects of spin–orbit and electron–electron interactions. As a result, elements with a fewer number of electrons in the outermost shells tend to give larger contributions to the bound–bound opacities. This implies that Fe is not representative for the opacities of light r-process elements. The average opacities for the mixture of r-process elements are found to be κ - 20–30 cm2 g−1 for the electron fraction of Ye ≤ 0.20, κ - 3–5 cm2 g−1 for Ye = 0.25–0.35, and κ - 1 cm2 g−1 for Ye = 0.40 at T = 5000–10 000 K, and they steeply decrease at lower temperature. We show that, even with the same abundance or Ye, the opacity in the ejecta changes with time by one order of magnitude from 1 to 10 d after the merger. Our radiative transfer simulations with the new opacity data confirm that ejecta with a high electron fraction (Ye 0.25, with no lanthanide) are needed to explain the early, blue emission in GW170817/AT2017gfo while lanthanide-rich ejecta (with a mass fraction of lanthanides -5 × 10−3) reproduce the long-lasting near-infrared emission.
AB - Coalescence of neutron stars (NSs) gives rise to kilonova, thermal emission powered by radioactive decays of freshly synthesized r-process nuclei. Although observational properties are largely affected by bound–bound opacities of r-process elements, available atomic data have been limited. In this paper, we study element-to-element variation of the opacities in the ejecta of NS mergers by performing systematic atomic structure calculations of r-process elements for the first time. We show that the distributions of energy levels tend to be higher as electron occupation increases for each electron shell due to the larger energy spacing caused by larger effects of spin–orbit and electron–electron interactions. As a result, elements with a fewer number of electrons in the outermost shells tend to give larger contributions to the bound–bound opacities. This implies that Fe is not representative for the opacities of light r-process elements. The average opacities for the mixture of r-process elements are found to be κ - 20–30 cm2 g−1 for the electron fraction of Ye ≤ 0.20, κ - 3–5 cm2 g−1 for Ye = 0.25–0.35, and κ - 1 cm2 g−1 for Ye = 0.40 at T = 5000–10 000 K, and they steeply decrease at lower temperature. We show that, even with the same abundance or Ye, the opacity in the ejecta changes with time by one order of magnitude from 1 to 10 d after the merger. Our radiative transfer simulations with the new opacity data confirm that ejecta with a high electron fraction (Ye 0.25, with no lanthanide) are needed to explain the early, blue emission in GW170817/AT2017gfo while lanthanide-rich ejecta (with a mass fraction of lanthanides -5 × 10−3) reproduce the long-lasting near-infrared emission.
KW - Opacity
KW - Radiative transfer
KW - Stars: neutron
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U2 - 10.1093/MNRAS/STAA1576
DO - 10.1093/MNRAS/STAA1576
M3 - Article
AN - SCOPUS:85091768652
VL - 496
SP - 1369
EP - 1392
JO - Monthly Notices of the Royal Astronomical Society
JF - Monthly Notices of the Royal Astronomical Society
SN - 0035-8711
IS - 2
ER -