Neutrino-heated winds from millisecond protomagnetars as sources of the weak r-process

Abstract

We explore heavy element nucleosynthesis in neutrino-driven winds from rapidly-rotating, strongly magnetized proto-neutron stars ("millisecond proto-magnetars") for which the magnetic dipole is aligned with the rotation axis, and the field is assumed to be a static force-free configuration. We process the proto-magnetar wind trajectories calculated by Vlasov et al. (2014) through the r-process nuclear reaction network SkyNet using contemporary models for the evolution of the wind electron fraction during the proto-neutron star cooling phase. Although we do not find a successful second or third peak r-process for any rotation period P, we show that proto-magnetars with P ∼ 1 − 5 ms produce heavy element abundance distributions that extend to higher nuclear mass number than from otherwise equivalent spherical winds (with the mass fractions of some elements enhanced by factors of ∼ > 100 − 1000). The heaviest elements are synthesized by outflows emerging along flux tubes which graze the closed zone and pass near the equatorial plane outside the light cylinder. Due to dependence of the nucleosynthesis pattern on the magnetic field strength and rotation rate of the proto-neutron star, natural variations in these quantities between core collapse events could contribute to the observed diversity of the abundances of weak r-process nuclei in metal-poor stars. Further diversity, including possibly even a successful third-peak r-process, could be achieved for misaligned rotators with non-zero magnetic inclination with respect to the rotation axis. If proto-magnetars are central engines for GRBs, their relativistic jets should contain a high mass fraction of heavy nuclei of characteristic mass numberĀ ≈ 100, providing a possible source for ultra-high energy cosmic rays comprised of heavy nuclei with an energy spectrum that extends beyond the nominal GZK cut-off for protons or iron nuclei.

A Magnetar Origin for the Kilonova Ejecta in GW170817

“…The 1D solutions of Metzger et al (2008c) were calculated along flow lines which emerge along the equatorial plane, near the last closed field line. Because these lowlatitude lines cover a large solid angle and carry most of the total mass flux of the wind, the results are similar to the total mass-loss rate and mass-averaged value of the outflow Y e integrated across the full open magnetosphere (Vlasov et al 2014(Vlasov et al , 2017. Nevertheless, the wind velocity and composition do show moderate variation with polar latitude θ of the outflow launching point; for instance, Vlasov et al (2014) found that for P ≈ 1 ms and L ν ≈ 2 × 10 52 ergs s −1 , Y e varied from ≈ 0.40 to ≈ 0.32 as θ increased from 0.35 to ≈ 0.5 (flow lines which carry ≈ 20% and ≈ 80% of the total mass loss, respectively).…”

Correlations of r-Process Elements in Very Metal-Poor Stars as Clues to their Nucleosynthesis Sites

“…They suggest that the wind develops only in the lowest-mass cases. (ii) The above mentioned investigations for magneto-rotational supernovae predict dynamical ejecta of the order 0.01-0.1 M , while the wind ejecta of Vlasov et al (2017) for fast rotating magnetized protoneutron stars (which should follow such dynamical ejecta) are of the order 10 −4 to 10 −3 M . Here one might need to include both such ejecta (the dynamical as well as the wind ejecta), which we will discuss when considering case (c).…”

“…In the literature one can find further suggestions for weak rprocess sites, i.e. predictions from neutrino winds of magnetized neutron stars (Vlasov et al 2017) and from neutrino winds of regular CCSNe following the explosions (e.g. Wanajo 2013).…”

“…5 3 in Fischer et al (2020a), in comparison to the observed ratios in low metallicity stars of regimes 1 and 2. If one would add late wind contributions of Vlasov et al (2017) to the MHD models, they would at most add 10% to the Sr and Eu values presented in Table 9. The listed sources are the ones expected to be explained via category I and II events.…”

FRIB and the GW170817 Kilonova

“…1). SkyNet has been used for r-process nucleosynthesis calculations in different scenarios by various authors: [3][4][5][6][7][8][9].…”