A NEW WAY TO MAKE THORNE-ZYTKOW OBJECTS

We have found a new way to make Thorne-Zytkow objects, which are massi ve stars with degenerate neutron cores. The asymmetric kick given to t he neutron star.formed when the primary of a massive tight binary syst em explodes as a supernova sometimes has the appropriate direction and amplitude to place the newly formed neutron star into a bound orbit w ith a pericenter distance smaller than the radius of the secondary. Co nsequently, the neutron star becomes embedded in the secondary. Thorne -Zytkow objects are expected to look like extreme M-type supergiants, assuming that they can avoid a runaway neutrino instability. Accretion onto the embedded neutron star will produce either an isolated, spun- up neutron star (possibly a short-period pulsar) or a black hole. Whet her neutron star or black hole remnants predominate depends on the lif etime of Thorne-Zytkow objects, the accretion rates involved, and the maximum neutron star mass, none of which are definitively understood.