Finding Thorne-Zitkow objects: strange red giant stars that have engulfed a neutron star.
Thorne-Zitkow objects are a type of star that contain a neutron star core surrounded by large envelop star such as a red giant star. Thorne-Zitkow objects were first hypothesized by two scientists, Thorne and Zitkow in 1975. They proposed that the star contains a neutron star of one solar mass with radius of 10 km and a large diffuse outer layer which looks similar to a red giant star. The neutron star and the diffuse mass was proposed to be separated by a thin, 40-meter-thick layer where all the gravitational energy release occurred. Over decades theories have been developed about the properties, evolution and life span of Thorne-Zitkow objects, and also, how to find or detect them. However, research articles as recently as 2022 describe these objects as hypothetical objects. This is because the Thorne-Zitkow objects look similar to the red giant stars.
Martin Rees and his group have estimated that the birth rate of Thorne-Zitkow objects in our galaxy could be around 2×10-4 per year, i.e. one Thorne-Zitkow object can be formed in about 5000 years in our galaxy. It has also been estimated that a Thorne-Zitkow object’s lifetime is about a hundred thousand to one million years. Combining the formation rate and the lifespan it is estimated that our galaxy might contains 20 to 200 Thorne-Zitkow objects at present. The Thorne-Zitkow object phase ends when the envelop mass (red-giant part) decreases below 14 solar masses. The envelop material may collapse down on the neutron star which would lead to the formation of a black hole.
Certain elements are expected to increase Thorne-Zitkow objects because of the conditions present at the margin where the neutron star core interacts with the outer red giant star. Dr. Levesque and her group first observed the star (HV2112) which is situated in the Small Magellanic Cloud. They found evidence of Rubidium, Lithium and Molybdenum elements in higher amounts than expected in a red giant star. This observation is an indication that this star can be a Thorne-Zitkow object. Christopher Tout’s group after just few months reiterated this claim by citing the higher calcium content found in HV2112. Dr. Levesque recently has also proposed that gravitational waves could be used to detect the neutron core of the Thorne-Zitkow objects more reliably than the detection of elements from the spectra.
In summary, the Thorne-Zitkow objects are still postulated objects and we might just be able to confirm their existence with new technologies.
