Systematic opacity calculations for kilonovae

Abstract

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.

Observational constraints on the optical and near-infrared emission from the neutron star–black hole binary merger candidate S190814bv

“…Fernández & Metzger 2013;Just et al 2015). This justifies our choice of κ w = 5 cm 2 g −1 , which is appropriate for outflows with intermediate Y e ∼ 0.25 -0.35 (Tanaka et al 2020).…”

“…As a further simplification, we fix the average (root mean square) velocity v and the (grey) opacity κ of the remaining two components to plausible values, namely v t = 0.3c and κ t = 15 cm 2 g −1 for the tidal ejecta and v w = 0.1c and κ w = 5 cm 2 g −1 for the viscous disc wind, based on their expected velocity and composition: the tidal ejecta are typically expected to retain a very low electron frac- tion Y e < 0.2 (e.g. Fernández et al 2017) leading to efficient r-process nucleosynthesis of Lanthanides and hence a high opacity κ t > 10 cm 2 g −1 (Tanaka et al 2020); disc wind outflows feature a wider range of Y e , due to viscous heating and neutrino irradiation from the inner part of the disc. Differently from NS-NS mergers, though, the absence of shocks and of intense neutrino production by a meta-stable neutron star remnant are likely to cause the disc wind to remain significantly neutron-rich (e.g.…”

Progress in nuclear astrophysics of east and southeast Asia

“…Theoretical modeling of NSM r-process suggests that both the lighter and heavier r-process elements could be produced in the early dynamical ejecta and/or the later wind outflow from the remnant [225,243,244]. However, recent careful re-analysis of the spectra [245] and many other related studies (e.g., [246,247]) have indicated that the amount of lanthanoids is not as large as that expected in theoretical calculation for kilonova associated with GW170817. Meanwhile, the detection of the infrared emission at several tens of days has not yet been able to provide evidence for the presence of 254 Cf or αdecay nuclei.…”

Mergers of Binary Neutron Star Systems: A Multimessenger Revolution

“…While these effects can be controlled and deconvolved with the aid of a radiation transport model as it has been done for supernovae of all types (Mazzali et al, 2016;Hoeflich et al, 2017;Ergon et al, 2018;Hillier and Dessart, 2019;Ashall and Mazzali, 2020;Shingles et al, 2020), a more fundamental hurdle in modeling kilonova spectra consists in the much larger number of electronic transitions occurring in r-process element atoms than in the lighter ones that populate supernova ejecta, and in our extremely limited knowledge of individual atomic opacities of these neutron-rich elements, owing to the lack of suitable atomic data. First systematic atomic structure calculations for lanthanides and for all r-process elements were presented by Fontes et al (2020) and Tanaka et al (2020), respectively.…”