This article on Thorne Zytkow objects is a guest article by Raghav Kavimandan, a computer science student from Amravati, Maharashtra, India.

The concept of stars and supernovae explosions has always been a fascinating one for astronomers as well as normal people. Over the course of many years, astronomers have discovered many weird and rare stars that are lurking in our universe and some even in our native galaxy “Milky Way”.

In this article we are going to look at one of the weirdest stars, called the Thorne Zytkow objects, that are claimed to have been detected by a team of astronomers.

What actually is a Thorne Zytkow object (TZO)?

Artistic impression of a Thorne Zytkow objects (TZO)
Image courtesy: Astronomy magazine
Artistic impression of a Thorne Zytkow object (TZO)
Image courtesy: Astronomy magazine

A Thorne Zytkow object (TZO) is defined as a star system containing two members:

When a neutron star is present inside a red supergiant as its core, the resulting combination is termed as a TZO. These types of stars originally are binary systems but their interactions result in the formation of a TZO. These types of binary systems are generally found in globular clusters of stars. Due to the constituents present inside a TZO, they are oftentimes referred to as “Hybrid Stars”.

Historical Analysis Thorne Zytkow objects (TZO’s):

Astronomers around the world have been working on discovering new stars and studying their impact on life on earth as well as on the whole universe for many years. One such team of astronomers, including Kip Thorne and Anna Zytkow, hypothesized a peculiar type of stars in 1977 where a red supergiant contains a neutron star at its core (A star within a star!).

A Star Within A Star: Understanding The Science Behind Thorne Zytkow Objects. 1
kip Thorne
A Star Within A Star: Understanding The Science Behind Thorne Zytkow Objects. 2
Anna Zytkow
Image Courtesy: Colorado arts and sciences magazine archive

After nearly 50 years, another astronomer Dr. Emily Levesque claimed to have found a candidate for being a TZO. She along with her collaborators studied many red supergiant stars in our galaxy as well as in a nearby galaxy called Small Magellanic Cloud and found a star with different behavior than others. This star was named as HV-2112 and has been quoted as the best candidate for a TZO.

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Formation of Thorne Zytkow objects (TZO’s)

The Thorne Zytkow objects are definitely one of the weirdest stars in the universe and the reason for that is buried in their modes of formation. There are basically 2 accepted theories describing their formation as follows:

1. Engulfing :

In a binary star system containing a red supergiant and a main-sequence star, engulfing is the mode of formation. When the red supergiant star explodes into a supernova, a neutron star is formed which remains in the gravitationally bound orbit. But as time goes by (thousands and even millions of years) the main sequence star evolves into a red supergiant, puffs outwards and it engulfs the neutron star. When it enters the red supergiant, the drag between the neutron star and the diffused outer layers of the supergiant results in the decay of the orbit of the binary system.

Due to this, the neutron star spirals into the core and merges with it, and eventually, Thorne Zytkow objects are formed. If the combined mass of the core exceeds the Tolman-Oppenheimer-Volkoff limit, then the combined core collapses into a black hole. If the companion of a neutron star is a white dwarf then a TZO is formed with properties of an R Coronae Borealis Variable.

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2. Collision :

When a red supergiant present in a binary system explodes into an asymmetric supernova, the mode of formation is a collision. Due to the asymmetric explosion, the neutron star gets a “kick” towards one side due to disturbances in the binding energy of the binary system. Many times, the neutron star collides with its companion and enters it.

Properties of Thorne Zytkow objects:

TZ0’s possess numerous unique properties with some of them being listed as follows:

1. The surface temperature of these stars is very high. The temperature is high due to the compression of the gas because of the strong gravity of the neutron star.

2. Very unusual nuclear processes occur due to the high temperature resulting in the formation of rare elements like lithium, rubidium, molybdenum, etc.

A Star Within A Star: Understanding The Science Behind Thorne Zytkow Objects. 3

3. When atoms of the diffused layers of the supergiant reach down near the surface of the neutron star, a process called Rapid Proton Bombardment takes those atoms to the maximum elemental limit and then again the atoms reach upper surface and decay and then again go down and repeat the cycle. Due to this cycle, elements like Li, Ru, Mo, etc. which generally are not present in normal red supergiant stars are formed in Thorne Zytkow objects. This abundance of elements like Li, Ru, Mo, etc. is used to differentiate TZO’s from normal red supergiants, as from the outside they look very similar to Red supergiants.

4. It is estimated that nearly 20-200 of these Thorne Zytkow objects are present in our universe.

Detection of Thorne Zytkow objects:

Red supergiants and TZO’s are very hard to distinguish from outside hence, advanced spectral techniques are used to identify them. Spectral techniques use the light given off by a star, sort it into different colors and identify the elements present in that star. If the concentration of elements like Li, Ru, Mo, etc. is found greater than a normal limit, then it may be a candidate for a TZO.

A Star Within A Star: Understanding The Science Behind Thorne Zytkow Objects. 4
Image: Digital Sky Survey/Center de Données astronomiques de Strasbourg

In the spectral graphs of Thorne Zytkow objects, many dips in the light are observed that result due to the absorption by the proposed elements. The team of astronomers of Dr. Emily Levesque and her collaborators found such similar dips in the spectral graphs of the star HV-2112. Since then, they have been working extensively to gather more proofs for proving their discovery, but haven’t achieved success in that yet. In 2018 they faced a major setback as some other team of astronomers denied their claim by stating that the levels of the proposed elements are not correct.

Final Words: Despite all the challenges and some setbacks, the work on the Thorne Zytkow objects is still going strong and we are getting closer to the truth day by day. Let’s hope that in our lifetimes, we discover the truth behind these weird yet fascinating cosmic wonders.

Before you go, make sure you also read:

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