Formation of Recycled Pulsars in Common Envelope Binaries

Abstract We present a systematic study of the evolution of low- and intermediate-mass X-ray binaries consisting of a 1.4 M ⊙ neutron star (NS) and a donor star of mass 1–8 M ⊙ . Using grids of detailed MESA simulations, we show that for donor masses of 2–8 M ⊙ , mass transfer may be dynamically unstable, leading to a common envelope (CE) phase. By adopting CE ejection efficiencies in the range α CE = 0.3–3.0, we find that post-CE binaries frequently experience a CE decoupling phase (CEDP), which plays a critical role in determining their final orbital and compositional properties. Systems with initial donor masses ≳3.5 M ⊙ predominantly evolve into NS binaries with carbon–oxygen or oxygen–neon white dwarfs (WDs) with masses between 0.5 and 1.4 M ⊙ . Comparison with the observed population of binary pulsars with a WD companion shows better agreement with higher CE ejection efficiencies ( α CE = 3.0). Furthermore, we demonstrate that NSs can accrete a sufficient amount of matter (≳0.01 M ⊙ ) during the CEDP and subsequent Case BA/BB/BC mass transfer phases to be effectively recycled into millisecond pulsars. We identify two distinct evolutionary channels capable of reproducing the observed characteristics of the millisecond pulsar PSR J1928+1815 with a helium-star companion. Our results highlight the importance of the CEDP in the formation of recycled pulsars and provide constraints on the CE ejection efficiency during binary evolution.