Figure 1: Artist's rendition of massive, luminous first-generation stars in the Universe which would form a cluster. The most massive ones should have exploded and ejected material providing heavy elements in the surrounding gas clouds. A high resolution image is here (2.6 MB). (Credit: NAOC)
Astronomers have discovered a star on the outskirts of the Milky Way Galaxy with a chemical composition unlike anything they have ever seen. It matches theoretical expectations for the chemical footprint left behind by very massive, very early stars. This is the clearest evidence yet that the first stars included very massive star.
What is the nature of the first stars formed in the Universe? This is one of the most important questions in understanding how stars, galaxies, and the large-scale structures of the Universe formed after the Big Bang (Note 1). The first stars were born from gas clouds containing only hydrogen and helium, and nuclear fusion inside stars and supernova explosions have created new elements, the first steps in the formation of a diverse world of matter.
Theories predict that the first stars may have included many very massive stars that are rarely seen in the current Universe. Stars exceeding 140 times the mass of the Sun may have changed the environment of the Universe with intense ultraviolet radiation, and may have had a significant impact on the formation of the next generation of stars by very energetic supernovae (Pair-Instability Supernovae: PISNe) (Note 2).
However, there is a lack of clear observational evidence for the existence of such supernovae caused by very massive stars. Great efforts have been made to observe very old stars in the Milky Way Galaxy, along with observations of distant galaxies and intergalactic matter. Some of the old stars were born from gas clouds that captured elements ejected by the first stars, and their chemical compositions record the material produced by the first supernovae. Since PISNe caused by very massive stars produce chemical compositions that are very different from those of ordinary core-collapse supernovae, we can expect to identify the signature of very massive stars among old stars (Note 3).
A team of astronomers from the National Astronomical Observatory of Japan (NAOJ), the National Astronomical Observatories of China (NAOC), and other institutes have conducted studies using the Chinese survey telescope LAMOST to identify early generation stars in the Milky Way Galaxy and measured their detailed chemical compositions using the Subaru Telescope (Note 4). Among them, they have discovered LAMOST J101051.9+235850.2 (hereafter J1010+2358) with characteristic chemical compositions produced by a pair-instability supernova (Figures 2 and 3). This is the clearest trace of such supernovae found to date, and strongly supports the theory that stars that have masses more than 140 times larger than the mass of the Sun certainly formed in the early Universe.
What is the nature of the first stars formed in the Universe? This is one of the most important questions in understanding how stars, galaxies, and the large-scale structures of the Universe formed after the Big Bang (Note 1). The first stars were born from gas clouds containing only hydrogen and helium, and nuclear fusion inside stars and supernova explosions have created new elements, the first steps in the formation of a diverse world of matter.
Theories predict that the first stars may have included many very massive stars that are rarely seen in the current Universe. Stars exceeding 140 times the mass of the Sun may have changed the environment of the Universe with intense ultraviolet radiation, and may have had a significant impact on the formation of the next generation of stars by very energetic supernovae (Pair-Instability Supernovae: PISNe) (Note 2).
However, there is a lack of clear observational evidence for the existence of such supernovae caused by very massive stars. Great efforts have been made to observe very old stars in the Milky Way Galaxy, along with observations of distant galaxies and intergalactic matter. Some of the old stars were born from gas clouds that captured elements ejected by the first stars, and their chemical compositions record the material produced by the first supernovae. Since PISNe caused by very massive stars produce chemical compositions that are very different from those of ordinary core-collapse supernovae, we can expect to identify the signature of very massive stars among old stars (Note 3).
A team of astronomers from the National Astronomical Observatory of Japan (NAOJ), the National Astronomical Observatories of China (NAOC), and other institutes have conducted studies using the Chinese survey telescope LAMOST to identify early generation stars in the Milky Way Galaxy and measured their detailed chemical compositions using the Subaru Telescope (Note 4). Among them, they have discovered LAMOST J101051.9+235850.2 (hereafter J1010+2358) with characteristic chemical compositions produced by a pair-instability supernova (Figures 2 and 3). This is the clearest trace of such supernovae found to date, and strongly supports the theory that stars that have masses more than 140 times larger than the mass of the Sun certainly formed in the early Universe.
Figure 2: An optical image of LAMOST J101051.9+235850.2 taken from SDSS. This star exists in the direction of the constellation Leo with a distance of 3000 light-years. This is a main-sequence star with a mass slightly smaller than the mass of the Sun. (Credit: SDSS/NAOJ)
Figure 3: Elemental abundances of LAMOST J101051.9+235850.2 (red circles) and predictions from supernova models (lines). The upper panel shows a comparison with the supernova model for a progenitor with 10 solar masses, which does not explain the observation. The middle shows a comparison with a 85 solar mass model, which is still insufficient to explain elements like Na, Mg, Mn and Co. The bottom shows a comparison with the pair-instability model for a very massive star with 260 solar masses, which best explains the observation. (Credit: NAOC)
"The peculiar odd-even variance, along with deficiencies of sodium and α-elements in this star, are consistent with the prediction of primordial PISN from first-generation stars with 260 solar masses," says Dr. XING Qianfan, first author of the study.
The discovery of J1010+2358 is direct evidence of the hydrodynamical instability due to electron–positron pair production in the theory of very massive star evolution. The creation of electron–positron pairs reduces thermal pressure inside the core of a very massive star and leads to a partial collapse.
"It provides an essential clue to constraining the initial mass function in the early universe," says Prof. ZHAO Gang, corresponding author of the study. "Before this study, no evidence of supernovae from such massive stars has been found in the metal-poor stars."
Professor AOKI Wako of the National Astronomical Observatory of Japan, who has been leading the observing programs with the Subaru Telescope in the collaboration, says, "Our team has been working for nearly 10 years to study stars found by LAMOST in detail using the Subaru Telescope. Searching for evidence of the existence of very massive stars, which is thought to be unique to first stars, has been a challenge we have been working on for many years. We have achieved this major goal by this study."
What percentage of the first stars were very massive? This is the next big question that needs to be answered, and to do so, we need to explore many more stars and measure their chemical compositions.
The discovery of J1010+2358 is direct evidence of the hydrodynamical instability due to electron–positron pair production in the theory of very massive star evolution. The creation of electron–positron pairs reduces thermal pressure inside the core of a very massive star and leads to a partial collapse.
"It provides an essential clue to constraining the initial mass function in the early universe," says Prof. ZHAO Gang, corresponding author of the study. "Before this study, no evidence of supernovae from such massive stars has been found in the metal-poor stars."
Professor AOKI Wako of the National Astronomical Observatory of Japan, who has been leading the observing programs with the Subaru Telescope in the collaboration, says, "Our team has been working for nearly 10 years to study stars found by LAMOST in detail using the Subaru Telescope. Searching for evidence of the existence of very massive stars, which is thought to be unique to first stars, has been a challenge we have been working on for many years. We have achieved this major goal by this study."
What percentage of the first stars were very massive? This is the next big question that needs to be answered, and to do so, we need to explore many more stars and measure their chemical compositions.
Movie: Message from Professor AOKI Wako of the National Astronomical Observatory of Japan, who has been leading the observing programs with the Subaru Telescope in the collaboration. (Credit: NAOJ)
These results appeared as Xing et al. "A metal-poor star with abundances from a pair instability supernova" in Nature on June 7, 2023.
Notes:
(Note 1) Starting from initial non-uniform distributions (i.e., inhomogeneities) in the dark matter density that existed in the Universe after the Big Bang, matter increasingly gathers in areas of high density due to the effect of gravitational forces. The first stars would have formed in regions with the highest density of matter.
(Note 2) Previous observations with the Subaru Telescope have also found stars with peculiar compositions that cannot be explained by ordinary core collapse supernovae (Note 3), suggesting the existence of very massive stars, but there were still problems that could not be explained by theoretical models of supernovae. (Subaru Telescope August 21, 2014 Press Release)
(Note 3) At the end of their evolution, massive stars with masses dozens of times greater than the mass of the Sun undergo a supernova explosion with the collapse of the central part to form a black hole or neutron star (core collapse supernova). They eject a wide variety of elements ranging from carbon to iron. On the other hand, massive stars with masses 140 times greater than the mass of the Sun become so hot at their centers that they collapse to form electron-positron pairs, which explode in runaway nuclear fusion (pair-instability supernovae). Furthermore, when the mass of a star exceeds 300 times the mass of the Sun, even runaway nuclear fusion cannot stop the star’s gravitational collapse, and it is believed that the star becomes a black hole without an explosion.
(Note 4) The team searched for low metal stars using the survey telescope. LAMOST J101051.9+235850.2 is one of the stars that had been suggested to have a unique composition during the LAMOST search. The detailed chemical composition was successfully determined from the high-resolution spectrum obtained with HDS on the Subaru Telescope.
(Note 2) Previous observations with the Subaru Telescope have also found stars with peculiar compositions that cannot be explained by ordinary core collapse supernovae (Note 3), suggesting the existence of very massive stars, but there were still problems that could not be explained by theoretical models of supernovae. (Subaru Telescope August 21, 2014 Press Release)
(Note 3) At the end of their evolution, massive stars with masses dozens of times greater than the mass of the Sun undergo a supernova explosion with the collapse of the central part to form a black hole or neutron star (core collapse supernova). They eject a wide variety of elements ranging from carbon to iron. On the other hand, massive stars with masses 140 times greater than the mass of the Sun become so hot at their centers that they collapse to form electron-positron pairs, which explode in runaway nuclear fusion (pair-instability supernovae). Furthermore, when the mass of a star exceeds 300 times the mass of the Sun, even runaway nuclear fusion cannot stop the star’s gravitational collapse, and it is believed that the star becomes a black hole without an explosion.
(Note 4) The team searched for low metal stars using the survey telescope. LAMOST J101051.9+235850.2 is one of the stars that had been suggested to have a unique composition during the LAMOST search. The detailed chemical composition was successfully determined from the high-resolution spectrum obtained with HDS on the Subaru Telescope.
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Source: Subaru Telescope
