"Our research results challenge the classical theories in the field of astrophysics!"
On January 19, Nature published a major result of cooperation between the team of Liu Chao, a researcher at the National Astronomical Observatory of the Chinese Academy of Sciences, and researchers from Beijing Normal University, Nanjing University, and the Purple Mountain Observatory. The study believes that the "stellar initial mass distribution law" will change significantly with the content and age of stellar metal elements, which challenges the classical theory that "the initial mass distribution law of stars does not change" and refreshes human understanding of this basic concept.
Professor Pafer Kurumpa, an international authority on the initial mass function of stars and a professor at the University of Bonn in Germany, commented: "This study, based on high-quality data obtained from large-sample observations, reveals that the initial mass function of stars in the Milky Way is related to the evolutionary history and environment of the Milky Way. This is important for a deeper understanding of the nature of star formation in different environments and at different times in the Milky Way. ”
Resolved a 30-year-old controversy
New stars are born all the time in the vast universe, and many new stars of different masses will form in batches in the same star-forming region. The law of the distribution of the initial mass of stars, known in astronomy as the "initial mass function of stars", describes the proportion of stars of different masses in a group of stars at the time of birth.
"The initial mass function of a star is universal and constant throughout the universe and at all stages of evolution." Since it was proposed in 1955, this concept has been widely used as a basic hypothesis in many astrophysical research fields such as galaxy formation and evolution, star cluster structure and evolution, binary star evolution, and even extrasolar planets and gravitational waves, and has become a "classic hypothesis" in astrophysics textbooks.
"There are some indications that the proportion of massive stars is higher in star-forming environments, which means that the initial mass function of the star may not be universal." Zhang Zhiyu, a co-author of the paper and a professor in the Department of Astronomy at Nanjing University, said.
But because no direct and strong observational evidence could be found, the controversy existed for 30 years.
"What bothered us at the time was that there were so few stars near the Sun that could be analyzed that there was a lot of uncertainty in the statistics, and the interpretation of key signals from extragalactic galaxies relied heavily on physical models and could be subject to systematic bias." Liu Chao said.
The original results published this time give full play to the advantages of the ultra-large spectral data samples of the Guo Shoujing Telescope (LAMOST), a major scientific and technological infrastructure on the mainland, and combine the data of the Gaia satellite of the European Space Agency. LAMOST currently holds the world record for the largest sample of stellar spectral data, and Gaia's performance complements LAMOST's, and the combination of the two is very helpful for star classification, and can single out "old" stars.
It is this advantage that the research team was able to "carefully select" samples of 90,000 stars within 1,000 light-years of the sun. Minimize reliance on the physical model while ensuring that the error is sufficiently small.
After the advancement of observation methods, Liu Chao's team for the first time used the most intuitive stellar counting method, that is, "counting stars", to count stars with different metal content and age, and directly obtained the initial mass function of stars that hardly depends on any model from the observation perspective.
With so many stars, how to "count"? Liu Chao said that based on physical characteristics such as the brightness of stars, they built a model to reverse how many stars these samples can represent.
Then, according to the different masses and proportions of metals contained in each star, Liu Chao divided the stars into different groups for statistics. The distribution of stars within 1,000 light-years is uneven, so the characteristics of spatial distribution should also be taken into account.
"Of these 90,000 stars, 1/4 are not single stars, there will be a companion star next to it that is not very clear, and even a small number of multi-star systems, which must be taken into account statistics." Liu Chao said.
When all circumstances are taken into account, the task of "counting the stars" is completed. They observed that the proportion of young small-mass stars was significantly higher than that of older stars, and that the higher the metal content, the greater the proportion of small-mass stars in the stellar family.
"This is the first time that astronomers have observed so clearly that the initial mass distribution law of stars changes significantly with the metal content and age of stars, which directly leads to the basic assumption that the initial mass distribution law of stars is universal and unchanged in the universe no longer holds, ending the controversy in the astronomical community about whether the initial mass distribution law of stars changes." Liu Chao said.
"What is taken for granted is probably the most worthy of reflection"
Doing basic research requires a deep heart, and challenging classical theories is the "foundation of the foundation". In fact, this research, important as it is, is only being done by "a small number of people" in the field of astrophysics.
From 2015 to the official publication of the paper, why was Liu Chao able to persist in this research for 8 years? "My research pitch tends to be more fundamental, hoping to do the textbook change' thing," he said. Some of the theories we take for granted may be the most worthy of reflection. ”
In July 2015, Liu Chao came to a conclusion similar to the current result. Excited, he took the initiative to contact the editor of Nature.
"However, there were no 90,000 stars at that time, and we found that the data had a selective effect problem, and it was not rigorous enough in selectivity and sampleability." Liu Chao said.
It took another two years, and in 2017 Liu Chao solved the problem of the selection effect. In the following two years, Liu Chao successively optimized the method of measuring the spatial distribution of stars and solved the problem of binary stars.
Until 2019, the early technical difficulties and obstacles were basically swept away, and all that remained was to launch a "breakthrough" on research. Since 2020, Liu Chao and Li Jiadong, the first author of the paper and a doctoral student at the National Astronomical Observatory, began intense data analysis and processing, as well as model building. At the beginning of 2021, the results came out!
In the Spring Festival of 2022, they opened the submission system of Nature and sent the article to the editorial office. Under normal circumstances, "Nature" only has two rounds of review, and I never expected that Liu Chao and the reviewers experienced 5 back and forth "pulls".
"If these 90,000 stars are not grouped, can they reproduce the previous results?" "I hope to re-detect your method from the perspective of the brightness distribution of stars"... Faced with the research results of this challenge classical theory, the three reviewers asked nearly 70 questions and asked the team to make different verifications.
It was not until October 26, 2022 that Liu Chao received a notice of acceptance of papers. "After 9 months of review, we have repeatedly verified and become more and more confident in this result. The reviewers' questions are pointed, they don't pick bones in the egg, they make our results more reliable. Liu Chao said.
It's just "breaking doors" work
This breakthrough has influenced several studies in the field of astrophysics. Whether it is measuring the mass of dark matter and baryon matter in galaxies at different stages of the universe, building a model of the chemical evolution of galaxies, understanding the process of star formation, analyzing the physical mechanism of binary star evolution, detecting extrasolar planets, and even studying a series of cutting-edge astrophysical problems such as star-level gravitational wave events, it will be challenged by the change of the initial mass function of stars.
"This is like a 'ruler' that changes with the environment, you can't use the same 'ruler' to measure different parts of the universe, astronomers need to change the appropriate 'ruler' to get the correct measurement results." For example, the early universe cannot be measured using the current 'ruler' of the Milky Way. Liu Chao said.
Unfortunately, although this study challenges classical theory, it does not give better parameters for changing the initial mass function of stars. "It's just 'breaking the door' work, and we'll follow that line of thought to drive further research." Liu Chao said.
Fortunately, Continental will launch the China Space Station Engineering Sky Survey Telescope (CSST) in the near future, and Liu Chao's team will complete more observations with CSST.
Liu Chao said that CSST can observe most of the Milky Way and see billions of stars. In the past, stars with masses below 8 thousandths of the mass of the Sun were "failed stars". With CSST, astronomers can observe even objects as small as one-thousandth of the mass of the sun and obtain a more accurate mass function of such objects. (Gao Yali)
Source: China Science News