The chemical signature of jet-driven hypernovae

Abstract

Hypernovae powered by magnetic jets launched from the surface of rapidly rotating millisecond magnetars are one of the leading models to explain broad-lined Type Ic supernovae (SNe Ic-BL), and have been implicated as an important source of metal enrichment in the early Universe. We investigate the nucleosynthesis in such jet-driven hypernovae using a parameterised, but physically motivated, approach that analytically relates an artificially injected jet energy flux to the power available from the energy in differential rotation in the proto-neutron star. We find ejected $^{56}\mathrm{Ni}$ masses of $0.05\,\mathrm{M}_\odot - 0.45\,\mathrm{M}_\odot$ in our most energetic models with explosion energy $>10^{52}\,\mathrm{erg}$. This is in good agreement with the range of observationally inferred values for SNe Ic-BL. The $^{56}\mathrm{Ni}$ is mostly synthesised in the shocked stellar envelope, and is therefore only moderately sensitive to the jet composition. Jets with a high electron fraction $Y_\mathrm{e}=0.5$ eject more $^{56}\mathrm{Ni}$ by a factor of 2 than neutron-rich jets. We can obtain chemical abundance profiles in good agreement with the average chemical signature observed in extremely metal-poor (EMP) stars presumably polluted by hypernova ejecta. Notably, $\mathrm{[Zn/Fe]} \gtrsim 0.5$ is consistently produced in our models. For neutron-rich jets, there is a significant r-process component, and agreement with EMP star abundances in fact requires either a limited contribution from neutron-rich jets or a stronger dilution of r-process material in the interstellar medium than for the slow SN ejecta outside the jet. The high $\mathrm{[C/Fe]}\gtrsim 0.7$ observed in many EMP stars cannot be consistently achieved due to the large mass of iron in the ejecta, however, and remains a challenge for jet-driven hypernovae based on the magneto-rotational mechanism.