Discovery of a new ultra-short period binary

Using the Survey Telescopes (TMTS) at Tsinghua-Ma Huateng University, an international team of astronomers has discovered a new binary known as TMTS J052610.43+593445.1. The new system is an ultrashort orbital period binary composed of a sub-dwarf star and a white dwarf companion. The discovery was reported in a paper published Dec. 21 on the preprint server. arXiv.

According to the theory of binary evolution, the second common envelope ejection can produce low-mass sub-dwarf B-type (sdB) stars inside ultrashort orbital period binaries. The orbital periods of these detached sdB binary systems can be as short as 20 minutes. However, to date only four such binaries with orbital periods less than one hour have been found.

Now, a team of astronomers led by Jie Lin of Tsinghua University in Beijing, China, reports the detection of a new sdB binary with an extremely short orbital period. They used TMTS to search for unusual short-period objects in our galaxy, the Milky Way. As a result, they discovered a dozen such objects, and one of them, which received the TMTS designation J052610.43+593445.1 (or J0526 for short), turned out to be an sdB binary with a period orbital of approximately 20.5 minutes.

Lin’s team discovered that J0526 consists of a visible sdB star (designated J0526B), about seven times larger than Earth, with a mass of about 0.33 solar masses, which is tidally distorted by an invisible carbon-oxygen white dwarf companion (J0526A). .

Observations indicate that the white dwarf is about 10 times smaller than the sun, has a mass of about 0.735 solar masses, and its effective temperature is 25,400 K. The system has an orbital period of about 20.506 minutes and is estimated be located some 2,760 light years from Earth.

Astronomers assume that the J0526 system is currently detached. They predict that after about 1.5 million years, J0526B will overflow its Roche lobe and transfer mass to J0526A over an orbital period of about 14 minutes. This will lead to the formation of an AM CVn star via the helium-star channel.

The researchers noted that J0526B will begin a transition to a degenerate state, which could lead to the formation of a helium-core white dwarf. They added that when electron degeneracy pressure becomes dominant, J0526B will reach the minimum orbital period of about nine minutes and begin to expand with its mass loss. This will likely lead to an increase in the orbital period, as predicted by the theory of binary evolution.

Summarizing the results, the authors of the paper pointed out that J0526 could be the single degenerate binary with the shortest orbital period. Further studies of this system could be essential for the study of binary evolutionary paths from the second common envelope ejection to the formation of AM CVn stars.

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