Nearly 4 decades after it was first proposed by physicist
Kip Thorne and astronomer Anna Zytkow, a paper by E. M. Levesque et al. (2014)
reports on the discovery of a possible Throne-Zytkow object in the Small
Magellanic Cloud - a satellite galaxy of the Milky Way. A Throne-Zytkow object
is an exotic type of hybrid star consisting of a red supergiant with a neutron
star tucked inside its core. Basically, it is two stars in one. This possible
Throne-Zytkow object is a star called HV 2112 and it is probably the first of
its kind to be found.
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There are 2 ways through which a Throne-Zytkow object might
form. The first involves 2 massive stars in a tight binary system. The more
massive star explodes as a supernova and leaves behind a neutron star.
Subsequently, the companion stars evolves and expands to form a red supergiant,
engulfing the neutron star in the process. The neutron star then spirals into
the core of the red supergiant, producing a Throne-Zytkow object.
Alternatively, a Throne-Zytkow object can form when a neutron star produced
from an asymmetric supernova receives a ‘natal’ kick in the direction of its
companion and becomes embedded in the core of its companion.
A Throne-Zytkow object is nearly indistinguishable from a
typical red supergiant. However, the extremely high temperature at the surface
of the neutron star deep within the core of the Throne-Zytkow object supports
nuclear processes that produce various unique heavy elements. The completely
convective envelop surrounding the neutron star brings these elements to the
surface of the Throne-Zytkow object where they become detectable.
Indeed, observations of the light emitted by HV 2112 show
that it is enriched with the elements rubidium, lithium and molybdenum,
consistent with theoretical predictions. Although red supergiants produce these
elements as well, they do not produce them in the high abundances observed for
HV 2112. Nevertheless, more work is still needed to confirm whether HV 2112 is
truly a bona fide Throne-Zytkow object.
Reference:
E. M. Levesque et al. (2014), “Discovery of a Thorne-Zytkow
object candidate in the Small Magellanic Cloud”, arXiv:1406.0001 [astro-ph.SR]