Discovery of a 760 nm P Cygni line in AT2017gfo: Identification of yttrium in the kilonova photosphere

Neutron star mergers are believed to be a major cosmological source of rapid neutron-capture elements. The kilonovae associated with neutron star mergers have to date yielded only a single well-identified spectral signature: the P Cygni line of Sr + at about 1 μm in the spectra of the optical transient, AT2017gfo. Such P Cygni lines are important because they provide significant information not just potentially on the elemental composition of the merger ejecta, but also on the velocity, geometry, and abundance stratification of the explosion. In this paper, we show evidence for a previously unrecognised P Cygni line in the spectra of AT2017gfo that emerges several days after the explosion, located at λ ≈ 760 nm. We show that the feature is well-reproduced by 4d 2 –4d5p transitions of Y + , which have a weighted mean wavelength of around 760–770 nm, with the most prominent line at 788.19 nm. While the observed line is weaker than the Sr + feature, the velocity stratification of the new line provides an independent constraint on the expansion rate of the ejecta, similar to the constraints from Sr + .

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