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What Are Thorne Zytkow Objects? Hybrid Stars (or the Star Within a Star)

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.

What is a Thorne Zytkow object?

What is a Thorne Zytkow object?

A New Theory

Kip Thorne (of late known for his role in developing Interstellar) and Anna Zytkow were both working at the California Institute of Technology in 1977 on binary star theories. Most stars exist in such a system, but not all of them behave the same way. Particularly, they were interested in the behavior of a massive star in such a system, for the bigger a star is the quicker it burns through its fuel and thus the shorter its life is. That ending is typically a supernova if the star is massive enough. And if you have the right combo, you can have a neutron star (one of several possible outcomes of a supernova) with a red supergiant as its binary companion (Cendes 52, University of Colorado).

And we know many such pairs exist, based on X-ray flares from the neutron star as it reacts to infalling material from the red supergiant. But what would happen if the system was unstable? That is what Thorne and Zytkow investigated. If the pair was unstable enough, they could be flung apart (because of a gravitational slingshot) or they could begin to spiral towards their barycenter, or common point of orbit until they merged. The product would look like a red supergiant but would contain a neutron star at its center. This is what is known as a Thorne Zytkow object (TZO), and according to their work up to 1% of red supergiants could be TZOs (Cendes 52, University of Colorado).

A star within a star

A star within a star

Weird Physics Ensue

Okay, now how would such an object even work? Is it as simple as two stars coexisting in one space? Sadly, it is not as simple as that but the possible mechanism that actually occurs is way cooler. In fact, because of the bizarre internal happenings, strange forms of matter that are heavy (on the bottom side of the periodic table) could be created there. The secret here is what the neutron star does to the red supergiant. Normal stars are powered through nuclear fusion, building up smaller elements into larger and larger ones. But the neutron star is a hot object, and through this exchange of heat it actually causes convection to occur. It is a thermonuclear reactor! And through convection, those heavy elements can be brought to the surface and therefore can be seen. Since normal red supergiants wouldn’t make these, we now have a way to spot one by looking for their signatures in the EM spectrum! (Cendes 52, Levesque).

Of course, it would be lovely if things were that simple. Unfortunately, red supergiants have a dirty spectrum because of all the elements that are present in them, and distinguishing individual elements can prove to be a challenge. This makes positively identifying one extremely difficult, but Zytkow kept looking as the years wore on, with the knowledge that if you take the expected percentage of existence into account with the elements they produce, it would produce the necessary heavy elements seen in the universe. In fact, because of these heavy elements, the interruption in the irp-process (aka the interrupted rapid proton process), and the high level of convection from the hot material rising, the following spectrum lines should be more pronounced: Rb I, Sr I and Sr II, Y II, Zr I, and Mo I (Cendes 54-5, Levesque).

But something that the theory is unsure of is what the destiny of a TZO is. It could possibly collapse into a black hole or be torn apart by the convection the neutron star produces. If the latter happens, then a neutron star would remain, but what would it appear? Maybe like 1F161348-5055, a supernova remnant from 200 years ago that is now an X-ray object. It is suspected to be a neutron star but completes a rotation in 6.67 hours, way too slow for a neutron star of its age. But if it had been a TZO that was torn apart, then the outer less dense layer of the neutron star could have been ripped off too, lowering the angular momentum and thus slowing it down (Cendes 55).

Found One?

It may have taken 40 years since the initial theory was founded, but recently the first Thorne Zytkow object was found (possibly). Work done by Emily Levesque (from the University in Boulder, Colorado) and Phillip Massey (from Lowell Observatory) found an unusual red supergiant in the Magellanic Clouds. HV 2112 first stood out because it was unusually bright for a star of that type. In fact, its hydrogen line was exceptionally strong, in fact within the limits predicted by Thorne and Zytkow. Further analysis of the spectrum also showed high levels of lithium, molybdenum, and rubidium, also something predicted by the theory. HV 2112 has the highest levels of these elements ever seen in a star, but it is not definitive proof that it is a TZO. Follow-up observations by a separate team a few years later didn't show the same elemental readings save for lithium. It looks like HV 2112 isn't the smoking gun we all thought it was, but the same team did offer a potential new candidate: HV 11417, whose spectrum does seem to match our hypothetical object (Cendes 50, 54-5; Levesque, University of Colorado, Betz).

Work Cited

Betz, Eric. "Thorne-Żytkow objects: When a supergiant star swallows a dead star." Kalmbach Publishing Co., 02 Jul. 2020. Web. 24 Aug. 2020.

Cendes, Yvette. “The Weirdest Star in the Universe.” Astronomy Sept. 2015: 50, 52-5. Print.

Levesque, Emily and Philip Massey, Anna N. Zytkow, Nidia Morrell. “Discovery of a Thorne-Zytkov Object Candidate in the Small Magellanic Cloud.” arXiv 1406.0001v1.

University of Colorado, Boulder. “Astronomers Discover First Thorne-Zytkow Object, a Bizarre Type of Hybrid Star.” Kalmbach Publishing Co., 09 Jun. 2014. Web. 28 Jun. 2016.

© 2017 Leonard Kelley