Shulou(Shulou.com)11/24 Report--

White dwarfs are the most common star fossils, and they are one of the final stages of stellar evolution. When a star runs out of nuclear fuel, it becomes a white dwarf. The mass of a white dwarf is about the mass of the sun, but its volume is about the same as that of the earth, so its density is very high.

When white dwarfs and another star form a binary system, they can show different physical phenomena and evolution processes. For example, if a white dwarf accumulates material from a companion star, causing its mass to exceed the Chandra Seka limit, a type 1a supernova will be formed. If two white dwarfs are close to each other, gravitational waves may occur and merge and radiate outward.

In June 2023, an international team of astronomers published a paper in the journal Nature Astronomy reporting on a very special white dwarf binary system: J191213.72-441045.1 (hereinafter referred to as J1912-4410). The system consists of an M-type dwarf and a fast-spinning white dwarf with an orbital period of 4.03 hours. More surprisingly, the system emits 5.30-minute pulses over a wide range of bands, from radio to X-rays.

This kind of pulse signal is very rare, and there is only another known example: AR Scorpii (hereinafter referred to as AR Sco). AR Sco is also made up of an M-type dwarf and a fast-spinning white dwarf with an orbital period of 3.56 hours. It emits 1.97-minute pulses in a wide range of bands from radio to X-rays. Because this pulse signal is similar to that of a neutron star pulsar (pulsar), AR Sco is called a white dwarf pulsar.

What is a white dwarf pulsar? How do they generate pulses? How did they form and evolve? These questions are difficult problems that astronomers are currently exploring and solving. The discovery of J1912-4410 provides new clues and evidence for these problems.

First of all, J1912-4410 confirmed that white dwarf pulsars are not an isolated case, but a category. This means that there may be other similar systems in the Milky way that have not yet been discovered or identified. This also means that the formation and evolution of white dwarf pulsars are not accidental, but have certain laws and mechanisms.

Secondly, J1912-4410 provides a new constraint and test for the pulse mechanism of white dwarf pulsars. At present, there are several different hypotheses about the pulse mechanism of white dwarf pulsars, including the coronal rings of M-type dwarfs, the magnetosphere of white dwarfs, the bow shock waves related to white dwarfs and so on. These hypotheses need to explain why the pulse period of a white dwarf pulsar is the difference between the rotation period and the orbital period of two stars.

The interaction between binary stars plays an important role in the pulse mechanism. There is a similar proportional relationship between the pulse period and the orbital period of J1912-4410 and AR Sco, which may imply a general physical mechanism. At the same time, there are some differences in the pulse signals between J1912-4410 and AR Sco in different wave bands, which may reflect the influence of different system parameters (such as magnetic field intensity, rotation velocity, orbit inclination, etc.) on the pulse signal.

Finally, J1912-4410 provides new support and verification for the formation and evolution model of white dwarf pulsars. In order to form a white dwarf pulsar, two conditions need to be met: one is that the white dwarf should have a strong magnetic field, and the other is that the white dwarf should have a fast rotation speed, which are not easy to achieve.

The magnetic field of a white dwarf may be caused by the conservation or amplification of the magnetic field during the evolution of the star. The rotation speed of a white dwarf star needs to be produced by increasing angular momentum by accreting matter from the companion star. However, there is a paradox between these two processes: if there is too much accretion, it will cause the white dwarf to explode a supernova and disappear; if there is too little accretion, it will not be able to make the white dwarf rotate fast enough. Therefore, a special accretion mode is needed, which can not only make white dwarfs rotate rapidly, but also avoid supernova explosions.

At present, there are several possible models to explain this model, including symbiotic binary model, co-rotating disk model, resonance locking model and so on. J1912-4410 and AR Sco are consistent with the parameter range predicted by these models, which provides observational evidence for these models.

In a word, J1912-4410 is an exciting and mysterious white dwarf binary system, which provides new perspectives and information for us to understand the novel phenomenon of white dwarf pulsars. In the future, more observational and theoretical work will uncover the mysteries behind white dwarf pulsars.

This article comes from the official account of Wechat: Vientiane experience (ID:UR4351), author: Eugene Wang

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