first generationfixed star, that is, star family III.The first generation of stars refers to the first generation of stars that can be detected: it is the first generation of star members born in the Milky Way that have not yet evolved (such as stars with a life ≥ 14Gyr), have the chemical composition of the original gas;Or the first generation stars with zero metal abundance refer to stars with metal abundance [Fe/H] ≤ - 6.
Chinese name
First generation stars
Foreign name
First generation star
Discipline
astronomy
Meaning
Provide information for the formation of galaxies and black holes
The first generation of stars, that is, star family III.Astronomers have proposed at least four definitions of star family III according to the possible images of the formation of the first generation stars in the early universe.The first generation of stars refers to the first generation of stars that can be detected: it is the first generation of star members born in the Milky Way that have not yet evolved (such as stars with a life ≥ 14Gyr), have the chemical composition of the original gas;Or the first generation stars with zero metal abundance refer to stars with metal abundance [Fe/H] ≤ - 6.Although we have not yet found the first generation stars we defined, it is of great significance to its exploration and research.The first generation of star research is the early evolution of galaxies, the formation and primordial of massive black holesGas nebulaThe evolution of provides rich information.In addition, through the analysis of the spectral and photometric observation data of the first generation stars, many important information related to them can be extracted, mainly including:
one
The characteristics of the metal abundance distribution function (MDF) of the Milky Way halo are given a limiting condition, that is, what is the lower limit of the MDF?
two
It provides direct clues for the measurement of heavy metal element abundance and primary Li abundance of the second generation stars, the radiation of cosmic rays in the early Milky Way, the measurement of element productivity of the early H-type supernova explosion and the disclosure of its mass distribution;
three
To provide evidence for the astrophysical sites needed to produce r-processes and s-processes;
four
It provides evidence for the efficiency of the mixing process in the early Milky Way and the results of nuclear synthesis of the first generation AGB stars;
five
Using light and heavy elements as the standard for astronomical time determination, the minimum age limit of the Milky Way and the universe is estimated;It is proved that the existence of thick disk in the Milky Way and the distribution of metal abundance of thick disk in the low metal abundance have the characteristics of extension;
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Provide tools to measure the velocity ellipsoids of halos and thick disks in the Milky Way and their changes with the distance between the centers of the Milky Way;
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It provides kinematic constraints for examining the upper level silver halos with reverse rotation characteristics, and the substructures generated by the merging of the Milky Way with its smaller companion galaxy that have occurred or are occurring.
To sum up, the first generation stars are expected to carry information about the cosmic physical conditions in the early stages of galaxy evolution.Because the first generation stars formed in primitive gas without carbon and heavy elements, they are the oldest bright matter.If detected, these living and yet to evolve stars of Galaxy III will be the only evidence to reveal the chemical and dynamic evolution of galaxies.Since the discovery of cosmic microwave background radiation, the thermal big bang theory on the origin of the universe has been widely accepted.The calculation of element nuclear synthesis in the thermal big bang model shows that the original matter in the universe is composed of hydrogen, neon, helium and a small amount of filing elements.A basic assumption of stellar spectral MK classification is that the chemical composition of all stars is the same as that of the sun.However, in 1951 Chambelian and Aller found that the chemical composition of HD19445 and HD140283 stars was different from that of the sun, especially their metal abundance [Fe/He} was much lower than that of the sun. After the publication of the paper by Buridge et al., the view that stars had the same chemical composition as the sun was questioned.Assuming that other chemical elements in the universe except hydrogen, neon, helium and rasp are generated by supernova explosions, the simple chemical evolution model of galaxies predicts that the metal abundance of about 10% of low mass stars is 1/10 of that of stars formed today.However, there are too many metal poor stars predicted in the chemical evolution model of simple galaxies, and the proportion is inconsistent with the observation results near the sun, which is the so-called "G dwarf problem".originalGas nebulaIt is inevitable that the first generation stars with the same chemical composition as the matter after the hot big bang of the universe will evolve.However, there is no direct observational evidence for the existence of the first generation stars.When did the first generation stars form?First generation star formingInitial mass functionWhat is it?These are still very controversial issues[1]。
theoretical model
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Observational facts show that stars with extremely poor metal abundance have been found (for example, stars with [Fe/H]=- 4.1), but unfortunately, stars with zero metal abundance have not been observed. That is to say, it cannot be announced that the first generation of stars have been found.From the viewpoint of cosmology, the formation of the first generation stars is inevitable. Why can't such stars be observed?To explain this phenomenon, astronomers have proposed many theoretical models[2]。
Model 1
Doom et al. proposed in 1985 that if the formation process of the first generation stars is similar to the formation process of the current star consortia, then we have found the "real first generation stars".They are G dwarfs and K dwarfs with initial chemical elements that were formed before the completion of the evolution of massive O type stars and the formation of n-type supernovae, which led to the pollution of the surrounding interstellar medium (hereinafter referred to as ISM).If these G dwarfs and K dwarfs are "real first generation stars", there should be about 10.The initial mass function for forming these stars has the form of SalePtr's initial mass function, and its mass is between 0.8M and 0.9M.These stars are the main observation targets in the BPS sky survey.Therefore, in the process of star burst, if the low mass stars form first than the high mass stars, the existence of the real first generation stars cannot be completely ruled out.
Model II
Truran and Cameron proposed in 1971 that the cut-off mass of stars formed by the initial mass function in the environment with zero metal abundance is higher than 0.9M.Because the age of the main sequence of stars with an initial mass less than 0.9M is greater than the Hubble time, they have not yet left the main sequence to evolve into red giant stars.If the cut-off mass of the initial star formation is higher than 0.9M, it is impossible to have the first generation stars now.
Model III
Lin and Murray (hereinafter referred to as LM) proposed in 1992 that the reason why the first generation stars could not be detected was that the initial mass function in the environment with zero metal abundance determined that in the formation process of the first generation stars, the massive stars accounted for the majority (top heavy), and the number of low-quality first generation stars was very small.Therefore, according to LM's point of view, "the real first generation stars" will eventually be found.
Model IV
In 1986,Cayerl proposed the "dirty" star family III model to explain why no zero metal abundance stars have been observed. The model assumes that: in the original ISM, the gravitational collapse of the nebula first directly forms a massive star in its center, and then the cold gas that continues to fall relative to the shock wave generated after the supernova explosion forms a polluted low-quality star.These polluted low mass stars and high mass first generation stars are basically formed at the same time, called "dirty" star family III.
Model V
Yoshii (1955) and others put forward a completely different view from the above.We have observed the first generation stars, but their metal abundance is no longer zero.The model assumes that when the first generation star passes through the gravitational field of the Milky Way, it accumulates a certain amount of polluted (enriched) ISM from the surrounding environment, thus causing the surface of the first generation star to be polluted so that its metal abundance is not zero.
Model VI
Recently, the chemical evolution model of the galactic halo proposed by Tsujimoto, Shigeyama and Yoshii (hereinafter referred to as TSY) predicted that the first generation stars must exist.It is pointed out that if the IMF of the first generation star is taken as the IMF form of SalPeter.Estimated per
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A star family M can be found in the halo star sample.At the same time, TSY believes that constantly searching for stars with extremely poor metal abundance to increase BPS sky survey samples provides great hope for eventually finding the first generation stars.However, if the IMF of the original gas is different from the IMF form of SalPeter, but is conducive to the formation of massive stars, or the surface abundance of zero metal stars is polluted due to the accumulation of ISM, the possibility of observing the first generation stars will be reduced.
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When the first generation stars first formed, due to the lack of metal elements, they could only conduct nuclear reactions through the p-p chain at the beginning.The productivity of this reaction is low, so the stars continue to shrink and lead to higher central temperature. At such a high temperature, the 3 α reaction process of helium begins to synthesize a small amount of heavy elements, and then the stars can rely on the CNO cycle reaction of hydrogen to maintain their own stable main sequence stage. Therefore, the temperature of the first generation stars is higher, and the effective surface temperature is also high,As a result, the spectrum of the first generation stars is very hard, that is, compared with the same mass stars containing metals, the high energy part of the spectrum accounts for a larger proportion.The outcome of the first generation stars depends on their mass.If the influence of rotation is ignored, roughly speaking, stars with a mass between 10 and 40 solar masses will produce supernova explosions, and those with a mass between 40 and 140 solar masses will directly collapse into black holes. The first generation stars with a mass greater than 140 solar masses but less than 260 solar masses will pair unstable supernovae with positrons and negatives,In the form of PISN), metal is ejected around, and if the mass is greater than 260 solar mass, it will collapse directly into a black holePISN will produce and eject a large number of metals, and only one PISN is enough to increase the metal abundance of gas in its vicinity from 0 to above the critical abundance, so it may play an important role in the metal abundance of the universe and the transformation from the first generation stars to the first generation galaxies.However, the metal abundance produced by PISN has an obvious charge parity effect - that is, there are significantly more nuclides with even charges than nuclides with odd charges. However, this effect is not obvious in several extremely metal poor stars found in the silver halo, at least indicating that PISN has little contribution to the metal in the precursor gas to form such stars.In addition, if the first generation star has a fast rotation, the metal produced by nuclear combustion in the main sequence star stage will be redistributed in the star, thus changing the internal structure of the star.In a word, there are still many uncertainties in the quality and nature of the first generation stars and the characteristics of their metal abundance[3]。
The observation of the first generation stars is also very difficult. No first generation stars have been observed.Because the first generation stars predicted by the theory have very short lifetime and only exist in the universe with high redshift, it will be very difficult to observe them directly.The main directions for observing and studying the first generation stars are:
one
Look for stars with very low metal abundance in the Milky Way or nearby galaxies. These stars may not be the first generation stars themselves, but they may be formed in the gas polluted by the first generation stars only. Therefore, the properties of the first generation stars can be inferred from the content of different metal elements.
two
The first generation stars may produce intense gamma bursts and supernova bursts, especially PISN. Because PISN releases extremely high energy and has a long duration observed in the Earth's reference system, it is more likely to be identified.
three
The James Webb Space Telescope (JWST) to be built and the 30 meter scale ground-based optical telescope can be used to observe early re ionization galaxies, which may have a high proportion of PopIII stars.
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In the low-frequency radio band, the 21 cm red shift observation of SKA can outline the history of re ionization, which will also help us understand the first generation stars.In the future, we can also consider using the 21cm line to directly detect the ionization zone or Lymanalpha sphere around the first generation stars, both of which are much larger than the stars themselves.
research findings
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On June 7, 2023, the international academic journal Nature published an important achievement of the international team led by Zhao Gang, a researcher at the National Astronomical Observatory of the Chinese Academy of Sciences, online: the research team took the lead in discovering the chemical evidence for the existence of unstable supernovae (PISN) formed by the collapse of the first generation of super massive stars after their evolution.[4]
