Possible Existence of Extremely Neutron-Rich Superheavy Nuclei in Neutron Star Crusts Under a Superstrong Magnetic Field

Abstract

We investigate outer crust compositions for a wide range of magnetic field strengths, up to $B\simeq 4\times10^{18}$ G, employing the latest experimental nuclear masses supplemented with various mass models. The essential effects of the magnetic field are twoholds: 1) Enhancement of electron fraction, which is connected to that of protons via the charge neutrality condition, due to the Landau-Rabi quantization of electron motion perpendicular to the field, namely, neutron-richness is suppressed for a given pressure. As a result, 2) nuclei can exist at higher pressures without dripping out neutrons. By exploring optimal outer-crust compositions from all possible nuclei predicted by theoretical models, we find that neutron-rich heavy nuclei with neutron magic numbers 50, 82, 126, as well as 184, with various proton numbers emerge for $B\gtrsim 10^{17}$ G. Moreover, we show that superheavy nuclei with proton numbers $Z\ge104$, including unknown elements such as $Z=119, 120, 122$, and/or $124$, depending on mass models, may emerge as an equilibrium composition at bottom layers of the outer crust for $B\gtrsim 10^{18}$ G, which are extremely neutron-rich ($N\approx 260$-$287$, i.e., $N/Z\approx 2.2$-$2.4$). We point out that those extremely neutron-rich superheavy nuclei locate around the next neutron magic number $N=258$ after $N=184$, underlining importance of nuclear structure calculations under such really exotic, extreme conditions. We demonstrate how the superstrong magnetic field substantially alters crustal properties of neutron stars, which may have detectable consequences.