Infrared spectral signatures of light r-process elements in kilonovae

TL;DR

This work investigates whether neutron star mergers produce the full r-process, focusing on light r-process elements (Z ≤ 40) whose solar abundances imply substantial ejecta mass in kilonovae. Using a 1D NLTE radiative-transfer framework (SUMO) with two ejecta models (A-low and B-high), the authors predict infrared diagnostics for Z=31–40, plus Te, over 10–80 days, and compare to AT2017gfo and AT2023vfi data. The results indicate that several IR lines (Ge, As, Se, Br, Kr, Zr) are promising diagnostics, with the 2.1 μm feature arising from a blend of Kr–Te–Se depending on ionization and velocity structure; Se can explain Spitzer 4.5 μm emission in AT2017gfo, while Kr and Br signatures are not clearly seen in AT2023vfi, suggesting subsolar light r-process production or asymmetric ejecta. The study emphasizes the need for JWST data beyond 5 μm and improved atomic data (e.g., collision strengths and recombination rates) to robustly constrain the origin of light r-process elements in kilonovae and distinguish emission from slow disk winds vs. fast proto-NS outflows.

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.

Figures (21)

• Figure 1: The solar r-process number abundances in the $Z=31-40$ range, according to the Prantzos2020 estimation. The value for Te ($Z=52$, not plotted) is 2.3%.
• Figure 2: Formation regimes for [Kr II] 1.86 $\mu$m (top) and [Ge I] 17.94 $\mu$m (bottom), in model A-low at 10d (circles), 40d (triangles) and 80d (squares). Each color corresponds to a zone, with purple the innermost zone. Zone 1 has $v\sim 0.02$c, zone 10 has $v\sim0.06c$. The stated temperature-dependencies are for the limit $T \gg T_{exc}$.
• Figure 3: Physical conditions in model A-low; radioactive deposition (top), temperature (middle), electron fraction (bottom), at 10d (black, solid), 40d (blue, dashed) and 80d (red, dash-dotted). The $\gtrsim$80% deposition region is marked gray.
• Figure 4: Escape probability for a photon emitted at the centre of the nebula at the modelled epochs (10d; black, solid, 40d; blue, dashed, 80d; red, dash-dotted) for model A-low.
• Figure 5: Spectrum of model A-low at 10d. The observed (ground-based) spectrum of AT2017gfo at +10.4d Pian2017 is plotted in blue. Overplotted is also a 2400 K blackbody (black, dashed) - lines forming in the optically thick LTE regime have peak fluxes at or close to this curve.