Astronomers have identified the star Pic2-503, over 13 billion years old and with a high carbon content, possibly originating from the explosion of the first supernovae.
The star Pic2-503 was initially identified in the Mapping the Ancient Galaxy in CaHK (Magic) survey, which conducts dedicated observations of dwarf galaxies, such as Pictor 2 where the star is located, using the Victor Blanco telescope at the Cerro Tololo Observatory in Chile.
The discovery of a star with more carbon than expected could enrich our understanding of the Universe’s evolution. Located in Pictor 2, a tiny satellite galaxy of the Milky Way, Pic2-503 shows a carbon enrichment three thousand times greater than our Sun, unlike other stars of the same type—known as “ultra metal-poor” stars.
“In astronomy, ‘metal’ means any element with an atomic number greater than 2 on the periodic table, that is, heavier than hydrogen and helium. Therefore, an ultra metal-poor star is one that has 0.01% of the heavy metal fraction that the Sun has, and they are studied for being direct descendants of the first stars,” explains Guilherme Limberg, a PhD in astronomy from the Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG) at USP and one of the study’s authors.
According to the researchers who identified the star, the carbon content is possibly associated with the explosion of some of the Universe’s first supernovae, which released large amounts of the chemical element. All this material was abundantly retained by smaller galaxies with less gravitational attraction, like Pictor 2, about 13 billion years ago.
The description of the star Pic2-503 is provided by an international group of scientists, including Brazilian participation, in an article in the scientific journal Nature Astronomy. “The term ultra-poor refers to very old stars that are direct descendants of the first generation of stars in the Universe and are poor, relative to the Sun, in chemical elements other than hydrogen and helium,” Limberg tells the Jornal da USP.
“These stars are rare to find. We know of only about 40 to date, most in our galaxy, the Milky Way. The crucial question of the research is whether ultra metal-poor stars born in different environments, for example, galaxies of different sizes, have similar or different chemical compositions.”
“After the Big Bang, the Universe was composed only of hydrogen and helium, the lightest elements on the periodic table. Thus, the first stars must also have this composition, being responsible for producing, over time, the first heavy elements of the Universe, including those fundamental to life on Earth, such as carbon, nitrogen, oxygen, etc.”
“This is the novelty of the study: the star, discovered in a very small galaxy, has a gigantic enrichment in carbon, about 3,000 times that of the Sun,” says Guilherme Limberg.
According to the researcher, it is not possible to observe the first stars, as they have all “died” in supernova explosions, which is what happens at the end of their life cycle. “Still, we can observe the direct descendants, which should be those stars with the smallest amounts of metals today; these are the ultra metal-poor,” he highlights.
The age of the Universe is estimated at 13.8 billion years. “The first stars form only about 200 million years after the Big Bang and die quickly in supernova explosions. The second generation, which includes Pic2-503, is born soon after. Therefore, this star undoubtedly has more than 13 billion years,” says Guilherme Limberg, explaining that, in this way, by studying the ultra metal-poor, one can infer properties of the first stars and understand how the Universe’s evolution occurred.
“For some years now, we have been trying to expand the study to other galaxies, including the Large Magellanic Cloud and other satellites of the Milky Way. The main observational property of these stars is that almost all have large amounts of carbon relative to the Sun, generally about five times or more,” Limberg points out. “However, unlike those found in the Milky Way, the ultra metal-poor stars discovered so far in the Large Magellanic Cloud have so little carbon that we can’t even detect this element in observations. This raises the question about the relationship between the environment and the chemical enrichment from the first stars.”
Limberg reports that the star found, called Pic2-503, is located in the southern celestial hemisphere, observed in the Southern Hemisphere of Chile, in a dwarf galaxy called Pictor 2, which has only 2,000 solar masses of stellar mass, equivalent to 1 millionth (0.0001%) of the Large Magellanic Cloud and 1 hundred-millionth (0.000001%) of the Milky Way.
“Initially, it was identified in the Mapping the Ancient Galaxy in CaHK (Magic) survey, of which I am part, which uses the photometry technique to estimate the chemical composition for stars in a large area of the sky, but also conducts dedicated observations of dwarf galaxies, conducted from the 4-meter Victor Blanco telescope at the Cerro Tololo Observatory in Chile,” he describes.
“We confirmed that it was indeed an ultra metal-poor star with another technique, spectroscopy, which provides more precise and detailed data on the chemical composition, from observations of two other telescopes, the 6.5-meter Magellan Baade in Las Campanas, and the 8.2-meter Very Large Telescope (VLT) in Cerro Paranal, both also in Chile.”
According to the researcher, one of the great questions of astronomy is precisely about the origin of the excess carbon in ultra metal-poor stars, with several competing hypotheses. “Therefore, the star Pic2-503 and its gigantic enrichment in carbon relative to the Sun demonstrate that the carbon enrichment mechanism in the primordial Universe seems to be different for galaxies of different sizes,” he observes. “Among the current hypotheses, the only one that fits this new observation, and also with the carbon-free ultra metal-poor stars in the Large Magellanic Cloud, establishes that carbon enrichment depends on the explosion energy of the supernovae associated with the first stars.”
According to Limberg, the idea is that “low-energy” supernovae (in relative terms, as every supernova is an extremely energetic event) inject large amounts of carbon into the host galaxy when they explode. “Meanwhile, higher-energy supernovae enrich the medium preferentially with other elements. Thus, tiny dwarf galaxies like Pictor 2, which have very little mass, can retain the material ejected only from low-energy supernovae, those that produce excess carbon,” he emphasizes. “On the other hand, larger galaxies like the Large Magellanic Cloud are also able to retain the relatively carbon-poor material from high-energy supernovae due to their immense gravitational potential, causing the ultra metal-poor stars in this galaxy to have a relatively lower amount of this element.”
“This understanding has huge implications for galaxy formation models and chemical evolution in the primordial Universe, as it is now clear that heavy element enrichment can be dramatically different in different galaxies, even if the first stars are all identical and composed only of hydrogen and helium,” concludes the researcher. “This should influence, for example, the way we interpret observations of distant galaxies with the James Webb Space Telescope, which can already detect galaxies located less than 500 million years after the Big Bang, remembering that the Universe is about 13.8 billion years old.”
The main authors of the work are Anirudh Chiti from Stanford University, coordinator of the MAGIC survey, Vinicius M. Placco from NSF NOIRLab, also a PhD from IAG, Andrew B. Pace from the University of Virginia, Alexander Ji and Guilherme Limberg from the University of Chicago, Deepthi Prabhu from the University of Arizona, William Cerny from Yale University, Guy Stringfellow from the University of Colorado, and Alex Drlica-Wagner from Fermilab and the University of Chicago, USA. The work is also signed by Kaia Atzberger and Nitya Kallivayalil from the University of Virginia (USA), Clecio Bom from the Brazilian Center for Physics Research (CBPF), Julio Carballo-Bello from the University of Tarapacá (Chile), Yumi Choi and Clara Martínez-Vázquez from NSF NOIRLab (USA), Denija Crnojević from the University of Tampa (USA), Peter Ferguson from the University of Washington (USA), David James from Applied Materials (USA), Gustavo E. Medina from the University of Toronto (Canada), Noelia Noël from the University of Surrey (UK), Alexander Riley from the University of Durham (UK), David Sand from the University of Arizona (USA), Joshua D. Simon from Carnegie Observatories (USA), Katherine Vivas and Alistair Walker from the Cerro Tololo Inter-American Observatory (Chile).
The article Enrichment by the first stars in a relic dwarf galaxy is available here.
More information: limberg@uchicago.edu, with Guilherme Limberg.
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Original published at O Cafezinho.