Favorable Conditions for Heavy Element Nucleosynthesis in Rotating Protomagnetar Winds

Abstract

The neutrino-driven wind cooling phase of proto-neutron stars (PNSs) follows successful supernovae. Wind models without magnetic fields or rotation fail to achieve the necessary conditions for production of the third r -process peak, but robustly produce a weak r- process in neutron-rich winds. Using 2D magnetohydrodynamic simulations with magnetar-strength magnetic fields and rotation, we show that the PNS rotation rate significantly affects the thermodynamic conditions of the wind. We show that high-entropy material is quasiperiodically ejected from the closed zone of the PNS magnetosphere with the required thermodynamic conditions to produce heavy elements. We show that maximum entropy S of the material ejected depends systematically on the magnetar spin period P _⋆ and scales as $S\propto {P}_{\star }^{-5/6}$ for sufficiently rapid rotation. We present results from simulations at a constant neutrino luminosity representative of ∼1–2 s after the onset of cooling for P _⋆ ranging from 5–200 ms and a few simulations with evolving neutrino luminosity where we follow the evolution of the magnetar wind until 10–14 s after the onset of cooling. We estimate at magnetar polar magnetic field strength B _0 = 3 × 10 ^15 G and 10 ^15 G that neutron-rich magnetar winds can, respectively, produce at least ∼1–5 × 10 ^−5 M _⊙ and ∼1–4 × 10 ^−7 M _⊙ of material with the required parameters for synthesis of the third r -process peak, within 1–2 s and 10 s, respectively, in that order after the onset of cooling. We show that proton-rich magnetar winds can have favorable conditions for production of p- nuclei, even at a modest B _0 = 5 × 10 ^14 G.

Keywords

• Neutron stars
• R-process
• Magnetars
• Supernovae
• Nucleosynthesis