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arXiv · 2510.12410

Infrared spectral signatures of light r-process elements in kilonovae

Anders Jerkstrand·Quentin Pognan·Smaranika Banerjee·Nicholas Sterling·Jon Grumer·Niamh Ferguson·Keith Butler·James Gillanders·Stephen Smartt·Kyohei Kawaguchi·Blanka Vilagos
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Abstract

A central question regarding neutron star mergers is whether they are able to produce all the r-process elements, from first to third peak. The high abundances of first-peak elements (atomic number $Z \sim 31-40$) in the solar composition means they may dominate the ejecta mass in kilonovae. We here study theoretical infrared signatures of such light elements with spectral synthesis modelling. By combining state-of-the-art NLTE physics with new radiative and collisional data for these elements, we identify several promising diagnostic lines from Ge, As, Se, Br, Kr and Zr. The models give self-consistent line luminosities and indicate specific features that probe emission volumes at early phases ($\sim$10d), the product of ion mass and electron density in late phases ($\gtrsim$75d), and in some cases direct ionic masses at intermediate phases. Emission by [Se I] 5.03 \mum\ + [Se III] 4.55 \mum\ can produce satisfactory fits to the Spitzer photometry of AT2017gfo. However, the models show consistently that with a Kr/Te and Se/Te ratio following the solar r-process pattern, Kr + Se emission is dominant over Te for the blend at 2.1 \mum\ observed in both AT2017gfo and AT2023vfi. The somewhat better line profile fit with [Te III] may suggest that both AT2017gfo and AT2023vfi had a strongly sub-solar production of the light r-process elements. An alternative scenario could be that Kr + Se in an asymmetric morphological distribution generates the feature. Further JWST spectral data, in particular covering the so far unobserved $>5$ \mum\ region, holds promise to determine the light r-process production of kilonovae, and in particular whether the light elements are made in a slow disk wind or in a fast proto-NS outflow. We identify specific needs for further atomic data on recombination rates and collision strengths for $Z=31-40$ elements.

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Anders Jerkstrand, Quentin Pognan, Smaranika Banerjee, Nicholas Sterling, Jon Grumer, Niamh Ferguson, Keith Butler, James Gillanders, Stephen Smartt, Kyohei Kawaguchi, Blanka Vilagos. 2025-10-14. Infrared spectral signatures of light r-process elements in kilonovae. https://arxiv.org/abs/2510.12410

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