2021/03/29 by Nanae Domoto, Masaomi Tanaka, Shinya Wanajo +1
Physics and Astronomy · #Abundance of the chemical elements #Astronomy #Astrophysics #Ejecta #Electron #Electron capture #Gamma-ray bursts and supernovae #Kilonova #Neutron star #Nuclear physics #Nucleosynthesis #Physics #Pulsars and Gravitational Waves Research #Spectral line #Stars #Stellar, planetary, and galactic studies #Supernova #astro-ph.HE #astro-ph.SR #r-process
paper · pdf · doi:10.3847/1538-4357/abf358
16 pages, 14 figures, accepted for publication in ApJ
arxiv created 2021/03/29 · openalex publication_date 2021/05/01 · arxiv updated 2021/06/02 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/06
Abstract Binary neutron star (NS) mergers have been expected to synthesize r -process elements and emit radioactively powered radiation, called kilonovae. Although r -process nucleosynthesis was confirmed by the observations of GW170817/AT2017gfo, no trace of individual elements has been identified except for strontium. In this paper, we perform systematic calculations of line strength for bound–bound transitions and radiative transfer simulations in NS merger ejecta toward element identification in kilonova spectra. We find that Sr ii triplet lines appear in the spectrum of a lanthanide-poor model, which is consistent with the absorption feature observed in GW170817/AT2017gfo. The synthetic spectrum also shows the strong Ca ii triplet lines. This is natural because Ca and Sr are coproduced in the material with relatively high electron fraction and their ions have similar atomic structures with only one s -electron in the outermost shell. The line strength, however, highly depends on the abundance distribution and temperature in the ejecta. For our lanthanide-rich model, the spectra show the features of doubly ionized heavy elements, such as Ce, Tb, and Th. Our results suggest that the line-forming region of GW170817/AT2017gfo was lanthanide-poor. We show that the Sr ii and Ca ii lines can be used as a probe of physical conditions in NS merger ejecta. Absence of the Ca ii line features in GW170817/AT2017gfo implies that the Ca/Sr ratio is <0.002 in mass fraction, which is consistent with nucleosynthesis for electron fraction ≥0.40 and entropy per nucleon (in units of Boltzmann constant) ≥25.