Diversity of Early Kilonova with the Realistic Opacities of Highly Ionized Heavy Elements

Abstract

We investigate early ( t < 1 day) kilonova from a neutron star merger by deriving atomic opacities for all the elements from La to Ra ( Z = 57–88) ionized to the states V–XI. The opacities at high temperatures for the elements with open f -shells (e.g., lanthanides) are exceptionally high, reaching ${\kappa }_{\exp }\,\sim 3\times {10}^{3}\,{\mathrm{cm}}^{2}\,{{\rm{g}}}^{-1}$ at λ ≤ 1000 Å at T ∼ 70,000 K, whereas the opacities at the same temperature and wavelengths for the elements with open d -, p -, and s -shells reach ${\kappa }_{\exp }\,\sim 1$ , 0.1, and 0.01 cm ^2 g ^−1 , respectively. Using the new opacity data set, we derive early kilonovae for various compositions and density structures expected for neutron star merger ejecta. The bolometric luminosity of the lanthanide-rich ejecta shows distinct signatures and is fainter than that of the lanthanide-free ejecta. Early luminosity is suppressed by the presence of a thin outer layer, agreeing with the results of Kasen et al. and Banerjee et al. The early brightness in the Swift UVOT filters and in the optical g , r , i , and z filters for a source at 100 Mpc are about ∼22–19.5 and ∼21–20 mag, respectively, at t ∼ 0.1 day. Such kilonovae are ideal targets for the upcoming UV satellites, such as ULTRASAT, UVEX, and DORADO, and the upcoming surveys, e.g., the Vera Rubin Observatory. We suggest that the gray opacities that reproduce the bolometric light curves with and without lanthanides are ∼1–10 and ∼0.8 cm ^2 g ^−1 .

Keywords

• Neutron stars
• Nucleosynthesis
• R-process
• Stellar atmospheric opacity
• Radiative transfer
• Gravitational waves