An Etymological Dictionary of Astronomy and Astrophysics
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A theoretical class of → stellar objects in which a → neutron star core is surrounded by a large and diffuse envelope. TŻOs are expected to form as a result of the evolution of two → massive stars in a → close binary, with the neutron star forming when the more massive star explodes as a → supernova. During subsequent evolution of the system, the expanding envelope of the companion may lead to a common envelope state and the spiral-in of the neutron star into the core of its companion. Alternately, a TŻO may be produced when a newly-formed neutron star receives a supernova "kick" velocity in the direction of its companion and becomes embedded. Supergiant TŻOs are predicted to be almost identical in appearance to → red supergiants (RSGs). The best features that can be used at present to distinguish TŻOs from the general RSG population are the unusually strong → heavy element and → lithium lines present in their spectra, products of the star's fully → convective envelope linking the → photosphere with the extraordinarily hot burning region in the vicinity of the neutron star core. These objects are thought to be extremely rare, with as few as 20-200 of them predicted to exist in the Galaxy at present, though some authors have doubted whether such an object could survive the merger with the envelope intact. A candidate is HV 2112 (Levesque et al., 2014, MNRAS, arXiv:1406.0001; Beasor et al., 2018, MNRAS, arXiv:1806.07399).

Thorne K. S., Żytkow A., 1975, ApJ 199, L19.