Binary neutron stars have been found in the Milky Way as millisecond pulsars and twice outside the galaxy through gravitational-wave emission. In a simple variation, the excellent envelope of the mass-transferring star– the donor– bloats and engulfs the whole binary, producing a new system comprised of an inner compact binary, and a shared “common” envelope. In a recent study, we focussed on the common-envelope stage scenario of a donor star with a neutron star buddy. We emulated the common-envelope stage by getting rid of the envelope of the donor star, either partially or entirely. Our outcomes imply that a star can be stripped without experiencing Roche lobe overflow right away after the common envelope, a most likely condition for an effective envelope ejection.
An artists impression of gravitational waves produced by binary neutron stars. Credit: R. Hurt/Caltech-JPL
Binary neutron stars have actually been discovered in the Milky Way as millisecond pulsars and twice outside the galaxy via gravitational-wave emission. To date, one of the strong competitors to discuss this transition is the highly-complex phase of binary outstanding advancement known as the common-envelope stage.
The common-envelope stage is a particular outcome of a mass transfer episode. It begins with the Roche-lobe overflow of (at least) one of the stars, and its triggered by a dynamical instability. In a basic version, the stellar envelope of the mass-transferring star– the donor– bloats and engulfs the whole binary, developing a new system made up of an inner compact binary, and a shared “common” envelope. The interaction of the inner binary with the common envelope leads to drag, and the dissipated gravitational energy is transferred onto the common envelope, which can lead to its ejection. A successful ejection suggests that a compact binary can form. But what does a “successful ejection” indicate?
Illustration of binary neutron stars. Credit: Carl Knox, OzGrav-Swinburne University
To explore the common-envelope phase with three-dimensional hydrodynamical models, we tried to attend to the most likely outcomes of common-envelope development by thinking about the reaction of a one-dimensional stellar design to envelope elimination. In a current study, we concentrated on the common-envelope stage situation of a donor star with a neutron star buddy. We imitated the common-envelope stage by getting rid of the envelope of the donor star, either partly or totally. After the star was removed, we followed its radial development. The most extreme situations resulted as expected: If you remove all the envelope, the removed star remains compact. If you leave most of the envelope, the stripped star consequently expands a lot. The question is: what occurs in between the extreme cases?
This recommends that a star does not requires to be stripped all the way to the core to prevent an imminent stellar merger. The quantity of energy needed to partly remove the envelope is less than the one required to completely remove it.
This research study is an advance in the understanding of the typical envelope stage and the development of double neutron star binaries. Our results imply that a star can be removed without experiencing Roche lobe overflow immediately after the common envelope, a likely condition for an effective envelope ejection. It also recommends that removed stars retain a few solar masses of strange, hydrogen-poor material in their surface area. While this amount of hydrogen is not extreme, it may be observable in the spectra of a star and can contribute at the end of its life when it takes off into a supernova. While the full understanding of the common-envelope phase stays elusive, we are linking the dots of the development and fate of systems that have experienced a common-envelope event.
Composed by OzGrav scientist Alejandro Vigna-Gómez from the Niels Bohr Institute (University of Copenhagen).
