Astronomers have identified a rare type of binary star system containing a rapidly spinning millisecond pulsar and a helium star companion, formed via common envelope evolution. Although such systems are rare, the authors of this new study predict that others do exist; they estimate there are 16 to 84 undiscovered examples in the Milky Way. Millisecond pulsars – rapidly spinning neutron stars that emit radio waves – achieve their extraordinary rotation rates by siphoning matter from a close stellar companion. The formation of these exotic binary systems is not fully understood, because it can involve a variety of complex processes. Theory predicts that binary systems can experience a "common envelope" phase, in which one stellar object orbits within the outer layers of its companion. If the companion object in this evolutionary scenario is a neutron star, theory predicts that the outer layers are rapidly ejected, leaving a binary system composed of a recycled pulsar and a stripped helium star. However, no such systems have previously been observed. ZongLin Yang and colleagues characterize the millisecond pulsar PSR J1928+1815 using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). They find that the pulsar resides in a tight binary system with a companion helium star, on a short orbital period of 3.6 hours. Yang et al. use stellar models to show that this system formed after an unstable transfer of mass from the companion star to the neutron star triggered the formation of a common envelope around both stars. The neutron star spiraled closer to the other star’s core, releasing energy that ejected the outer envelope and left behind a tightly bound binary system.
