Speaker
Description
Radioactive decay in material ejected from a binary neutron star (BNS) merger powers a thermal transient known as a kilonova. Following the detection of the gravitational-wave event GW170817, the associated kilonova AT2017gfo was observed. This event provided the first opportunity for spectroscopic studies of freshly synthesized $r$-process material. Although several absorption and emission features have recently been identified, the elemental abundances inferred from the spectra remain uncertain. The key limitation of previous analyses is the assumption of local thermodynamic equilibrium (LTE), thereby neglecting non-thermal ionization by high-energy electrons from radioactive decay.
We develop a non-LTE ionization model including non-thermal ionization by high-energy electrons to improve abundance estimates for the early-phase spectra of AT2017gfo. Focusing on the prominent absorption feature around $1\,\mu\mathrm{m}$, we investigate the required abundances of helium (He) and strontium (Sr), which have been proposed as the candidate contributors to this feature. Our modeling indicates that the spectral feature requires either He or Sr with a mass fraction of $1\mathrm{-}10\%$ in the ejecta moving at $\sim 0.15\mathrm{c}$. Comparison with nucleosynthesis calculations suggests that the inferred abundances favor relatively neutron-rich conditions ($Y_\mathrm{e} \lesssim 0.35$) for the $r$-process nucleosynthesis in GW170817 because both He and Sr are overproduced at higher electron fractions due to $\alpha$-rich freeze-out. Within this range, there are two regimes: intermediate electron fractions ($0.15 \lesssim Y_\mathrm{e} \lesssim 0.35$) and even lower electron fractions ($Y_\mathrm{e} \lesssim 0.15$). In the former, the inferred Sr abundance is reproduced, in agreement with the solar $r$-process pattern. In the latter, Sr production is suppressed while a sufficient amount of He is produced through $\alpha$-decays of trans-Pb nuclei. Interestingly, if He is the dominant contributor to the absorption feature, this would provide an indirect signature of the production of elements beyond the third $r$-process peak.
| Category | Theory |
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