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.
Science
A pulsar-helium star compact binary system formed by common envelope evolution
22-May-2025