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In the early morning of April 30th, Beijing time, two articles published in the world's top academic journal Nature staged a dialogue from a distance. The topic of debate is: The Liu Jifeng team of the National Astronomical Observatory of Chinese Academy of Sciences announced last year that the mass of the largest stellar black hole is 70 times that of the sun. Is the conclusion correct?

These two articles, one from Hugues Sana 'a, an associate professor in the Department of Physics and Astronomy of the University of Leuven, are entitled "On the Characteristics of Black Holes with 70 times the Solar Mass in the LB- 1 System"; The other is from Liu Jifeng's team, entitled "Recovering the characteristics of black holes with 70 times the mass of the sun in the LB- 1 system".

In fact, before this, some international teams questioned that the key premise used by the Liu Jifeng team was wrong. LB- 1 The mass of a black hole is far from so great, and it is not even a black hole at all, but a neutron star or star.

In reply, Liu Jifeng team admitted that the actual situation was more complicated than originally thought, but the newly added factors were not enough to overturn the premise. At present, they are reanalyzing from a concise angle that was discovered later. The preliminary results are consistent with the previous ones, which can reach 65 times the mass of the sun and remain the largest stellar black hole in history.

They also expect the Gaia telescope of the European Space Agency to provide decisive data for this academic debate.

Before understanding the views of both sides, we might as well go back to the starting point of the discussion: a black hole that has been evaluated as "impossible to exist".

"impossible" black hole

2019165438+128 October, Nature published an important discovery made by the National Astronomical Observatory and Zhang's research team. Relying on the Guo Shoujing telescope, the team found a black hole with a mass about 70 times that of the sun in the LB- 1 system.

Many media reports describe it as "impossible". Liu Jifeng himself expressed "disbelief" when he first discovered it, and thought that such a black hole "should not" exist in the Milky Way.

The artistic imagination of LB- 1

In fact, there is a supermassive black hole at the center of the galaxy, and the threshold is several million times the mass of the sun. Why can't LB- 1 black hole be 70 times the mass of the sun?

It turned out that the research team judged that the LB- 1 black hole belongs to the "stellar black hole" and is the smallest type of black hole in astrophysics. Its mass is usually between three times and several hundred times that of the sun, which is formed by the explosion of stars.

Generally speaking, the richer the metal a star contains, the stronger the stellar wind that "scrapes" the matter outward, and a lot of mass is lost in advance. According to this inference, massive stellar black holes can only be formed in environments with low metal abundance. According to the chemical composition of our Milky Way galaxy, there will be no more than 30 times of stellar black holes.

The stellar black hole discovered by astronomers in the Milky Way galaxy is indeed less than 20 times the mass of the sun.

The LB- 1 metal discovered this time is similar to the sun, but its mass is close to 70 times that of the sun, which is obviously "super-class" and directly challenges the existing theory of stellar evolution.

For this reason, David Ray, director of the American Laser Interference Gravitational Wave Observatory, gave a high evaluation at that time: "This extraordinary achievement will be combined with the double black holes detected by LIGO and Virgo in the past four years to promote the revival of black hole astrophysical research."

Two key premises

How did Liu Jifeng find LB- 1?

Their tool is the Guo Shoujing Telescope of Xinglong Observatory, which focuses on monitoring an area scanned by Kepler telescope and tracking binary systems.

They innovatively use the "radial velocity method" to find silent black holes. When the celestial body approaches or moves away from the observation direction, its spectrum will shift blue or red regularly, just as the siren tone of an ambulance becomes higher when accelerating and lower when accelerating away.

They came across a star with a B-spectrum, which was very striking. The characteristic absorption line of the star fluctuates with time, but at the same time there is an almost static hydrogen emission line, which runs in anti-phase with the B-type star.

This shows that it is rotating around an invisible thing, and the mass of that celestial body is very large, which is very different, causing the star to rotate quickly, while the invisible guy rotates very little.

According to the calculation of Liu Jifeng group, the mass of type B star is about 8 times that of the sun. In proportion, invisible celestial bodies are as high as 70 times the mass of the sun, so they can only be black holes.

From the above derivation, we find that there are two key preconditions to reach the heavy conclusion of "70 times the mass of the sun":

1, the hydrogen emission line of "invisible guy" is only almost static, not really static. It needs to swing from side to side with a very small amplitude to form a shape similar to a wine bottle on the image. Otherwise, its mass will exceed the sky according to the swing ratio, which is difficult to explain.

2. The visible B-type star is indeed eight times the mass of the sun. In proportion, the greater the mass of type B star, the greater the mass of "invisible guy".

Is there really a slight swing in the hydrogen emission line?

Although the most advanced optical telescopes on earth, including the10m Keck telescope in Hawaii and the10.4m GTC telescope in Spain, were also confirmed at the time of publication, there have been incessant international doubts since then.

On February 9, 65438, El badri, a graduate student at the University of California, Berkeley, published an article without peer review on the preprint website, arguing that the slight swing of hydrogen emission line was caused by the absorption of the atmosphere around B-type star, but it was completely static. The source of this hydrogen emission line can't be a black hole.

65438+1October 10, a team of astronomers from Germany-Nuremberg University and Potsdam University wrote in Astronomy and Astrophysics that this celestial body may not be a black hole at all, but a massive neutron star or even an ordinary star.

Their argument is mainly based on premise 2. Some chemical elements show that the visible type B star may have been stripped of its shell, leaving only 1. 1 times the mass of the sun. Based on this calculation, the mass of the invisible guy is only 2-3 times that of the sun.

This time, the commentary of Sana 'a team of Leuven University in Belgium was published in the form of "pending issues". Their main point of view is similar to that of badri, that is, the slight swing of hydrogen emission line is caused by the absorption part of B-type star.

They believe that from the current data, the lower mass limit of type B star is four times that of the sun. If two stars with four times the mass of the sun form a binary system, and the rotation speed is fast enough, the other star may not be detected.

Reply: The latest observation and analysis still reaches 65 times the mass of the sun.

Continuation is a natural section launched at the end of 20 18. If there are particularly interesting and timely peer comments or explanations on the papers previously published in Nature, they can be published in the form of "sequel". At the same time, give the original author a chance to reply. If the editorial department thinks that the reply "constructively advances the discussion", it will publish the reply article at the same time as the article on "Continuing Matters".

In the reply article, Liu Jifeng's team combined the latest spectral observation data and analysis to confirm that the key premise is still valid. The mass of a B-type star is likely to be between 5 and 8 times that of the sun, and the corresponding black hole mass is 23 to 65 times that of the sun.

In view of the slight swing of the hydrogen emission line, they think that although B-star absorption can indeed move the hydrogen emission line, it cannot be concluded that the hydrogen emission line is static.

In fact, the hydrogen emission line is also influenced by many other factors, among which B-type star radiation and star wind will have a "hedge" effect on B-type star absorption.

In addition, it seems that the shape and width of the hydrogen emission line are not generated in the surrounding environment of the binary system.

Nevertheless, the author's team agrees that the problem of hydrogen emission line is more complicated than originally thought. They found that the orbital motion of "invisible celestial bodies" can be measured more simply and clearly with Paschen emission line.

Liu Jifeng et al. introduced that relevant papers are already in preparation. The preliminary conclusion of this study is that the peak point of Paschen emission line clearly shows the anti-phase orbital motion relative to the B-type star, which once again shows that it is emitted by another invisible celestial body, not the surrounding environment of the binary star system.

According to the preliminary calculation, the mass ratio of visible and invisible objects is between 4.6_8. 1, which is consistent with the results of hydrogen emission line measurement.

Liu Jifeng's article also responded to the problem of "peeling off the shell" of stars raised by the German team. They believe that there is no observation support for this speculation at present, and the life span of the "stripped" star is very short, and the possibility of just being hit is very low.

At the end of the article, it is mentioned that the final judge of this academic debate may be the Gaia telescope. By combining telescope data with Kepler's third law, the total mass of visible and invisible celestial bodies can be calculated.

If you want to know more about the Debate of Nature, Chinese scientists have discovered the largest stellar black hole, please continue to pay attention to the deep space science and technology information column, and Bian Xiao will continue to update you with more science and technology news.

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