Role of the symmetry energy on the neutron-drip transition in accreting and nonaccreting neutron stars

Abstract

In this paper, we study the role of the symmetry energy on the neutron-drip transition in both nonaccreting and accreting neutron stars, allowing for the presence of a strong magnetic field as in magnetars. The density, pressure, and composition at the neutron-drip threshold are determined using the recent set of the Brussels-Montreal microscopic nuclear mass models, which mainly differ in their predictions for the value of the symmetry energy J and its slope L in infinite homogeneous nuclear matter at saturation. Although some correlations between on the one hand the neutrondrip density, the pressure, the proton fraction and on the other hand J (or equivalently L) are found, these correlations are radically different in nonaccreting and accreting neutron stars. In particular, the neutron-drip density is found to increase with L in the former case, but decreases in the latter case depending on the composition of ashes from x-ray bursts and superbursts. We have qualitatively explained these different behaviors using a simple mass formula. We have also shown that the details of the nuclear structure may play a more important role than the symmetry energy in accreting neutron-star crusts.

Experimental constraints on shallow heating in accreting neutron-star crusts

“…We shall also study such pycnonuclear reactions. As nuclei sink deeper into the crust, they become so neutron rich that they are unstable against neutron emission [37,38]. After capturing an electron, the nucleus (A, Z ) will thus transform into a nucleus (A − N, Z − 1) with the emission of N neutrons n and an electron neutrino ν e :…”

“…As discussed in Refs. [37,38], the first electron capture by the nucleus (A, Z ) may be accompanied by the emission of N > 0 neutrons. The corresponding pressure P drip and average baryon densityn drip are given by similar expressions as Eqs.…”

Heating in Magnetar Crusts from Electron Captures

“…≥ 0, such transitions occur at higher pressure P * β ≥ P β and density n * β (A, Z, B ) ≥ n β (A, Z, B ). As discussed in [39,42], the first electron capture by the nucleus (A, Z) may be accompanied by the emission of ∆N > 0 neutrons. The corresponding pressure P drip and baryon density n drip are given by similar expressions as Equations ( 23) and ( 24…”

“…As nuclei sink deeper into the crust, further compression may give rise to delayed neutron emission, thus marking the transition to the inner crust [20,39]:…”

Nuclear density functional theory

“…The symmetry energy also affects the composition of the outer crust (where less neutron-rich nuclei are present if the symmetry energy increases) [93,94], the transition density to the inner crust (the density at which neutrons spill out of nuclei) [95], and the proton fraction in the inner crust and outer core [94]. Last but not least, in the inner crust, neutrons are superfluid and the calculations must account for this exotic form of superfluidity [96].…”