Astrophysics with Radioactive Isotopes

r-Process Radioisotopes from Near-Earth Supernovae and Kilonovae

Not scheduled

20m

Oral Presentation

Speaker

Xilu Wang (Institute of High Energy Physics, Chinese Academy of Sciences)

Description

The astrophysical sites where $r$-process elements are synthesized remain mysterious: it is clear that neutron-star-mergers (kilonovae, KNe) contribute, and some classes of core-collapse supernovae (SNe) are also possible sources of at least the lighter $r$-process species. The discovery of $^{60}$Fe on the Earth and Moon implies that one or more astrophysical explosions have occurred near the Earth within the last few Million years (Myr), probably SNe. Intriguingly, $^{244}$Pu has now been detected, mostly overlapping with $^{60}$Fe pulses. However, the $^{244}$Pu flux may extend to before 12Myr ago, pointing to a different origin. Motivated by these observations and difficulties for $r$-process nucleosynthesis in SN models, we propose that ejecta from a KN enriched the giant molecular cloud that gave rise to the Local Bubble where the Sun resides. Accelerator Mass Spectrometry (AMS) measurements of $^{244}$Pu and searches for other live isotopes could probe the origins of the $r$-process and the history of the solar neighborhood, including triggers for mass extinctions, e.g., at the end of the Devonian epoch, motivating the calculations of the abundances of live $r$-process radioisotopes produced in SNe and KNe that we present here. Given the presence of $^{244}$Pu, other $r$-process species such as $^{93}$Zr, $^{107}$Pd, $^{129}$I, $^{135}$Cs, $^{182}$Hf, $^{236}$U, $^{237}$Np and $^{247}$Cm should be present. Their abundances and well-resolved time histories could distinguish between SN and KN scenarios, and we discuss prospects for their detection in deep-ocean deposits and lunar regolith. We show that AMS $^{129}$I measurements in Fe-Mn crusts already constrain a possible nearby KN scenario. Thus, we urge searches for $r$-process radioisotopes in deep-ocean Fe-Mn crusts, and in the lunar regolith samples brought to Earth recently by the Chang’e-5 lunar mission and upcoming missions including Artemis.