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 (L/IMXBs) consisting of a $1.4M_{\odot }$ neutron star (NS) and a donor star of mass $1-8M_{\odot }$. Using grids of detailed MESA simulations, we show that for donor masses of $2-8M_{\odot }$, mass transfer may be dynamically unstable, leading to a common envelope (CE) phase. By adopting CE ejection efficiencies in the range ${\alpha }_{\mathrm{CE}}=0.3-3.0$, 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 $\gtrsim 3.5M_{\odot }$ predominantly evolve into NS binaries with carbon-oxygen or oxygen-neon white dwarfs (WDs) with masses between $0.5M_{\odot }$ and $1.4M_{\odot }$. Comparison with the observed population of binary pulsars with a WD companion shows better agreement with higher CE ejection efficiencies (${\alpha }_{\mathrm{CE}}=3.0$). Furthermore, we demonstrate that NSs can accrete a sufficient amount of matter ($\gtrsim 0.01M_{\odot }$) 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.

13 pages, 8 figures, accepted for publication in ApJ