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Metal-poor star is rich in data to understand the evolution of the Universe
Astronomers have identified the star Pic2-503, over 13 billion years old and with high carbon content, possibly originating from the explosion of the first supernovae
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 – Photo: NOIRLab
The discovery of a star with more carbon than expected may enrich knowledge about the evolution of the Universe. Located in Pictor 2, a tiny satellite galaxy of the Milky Way, the star Pic2-503 shows carbon enrichment 3,000 times greater than that of our Sun, unlike other stars of the same type—the so-called “ultra metal-poor” stars.
“In astronomy, ‘metal’ means any element with atomic number greater than 2 in the periodic table, that is, heavier than hydrogen and helium. Thus, an ultra metal-poor star is one that has 0.01% of the fraction of heavy metals found in the Sun, and they are studied because they are direct descendants of the first stars”, explains Guilherme Limberg, PhD in astronomy from USP’s Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG) and one of the study’s authors.
According to the researchers who identified the object, the carbon content is possibly associated with the explosion of some of the first supernovae in the Universe, which released large quantities of this chemical element.
All this material was retained in abundance by smaller galaxies with lower gravitational attraction, such as Pictor 2, around 13 billion years ago.
The description of the star Pic2-503 is presented by an international group of scientists, with Brazilian participation, in an article published 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 that are poor, relative to the Sun, in chemical elements other than hydrogen and helium”, Limberg explains.
“These stars are rare to find. We know of only about 40 to date, most of them in our galaxy, the Milky Way. The crucial question of the research is whether ultra metal-poor stars formed in different environments, for example galaxies of different sizes, have similar or different chemical compositions”, he adds. “After the Big Bang, the Universe was composed only of hydrogen and helium, the lightest elements in the periodic table. Thus, the first stars must also have had this composition, being responsible for producing [over time] the first heavy elements in 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 carbon enrichment, on the order of 3,000 times that of the Sun”
Guilherme Limberg
According to Limberg, it is not possible to observe the first stars, since all of them have already “died” in supernova explosions, which is what happens at the end of their life cycle. “Even so, we can observe their direct descendants, which should be those stars with the lowest amounts of metals today; these are the ultra metal-poor ones”, he emphasizes.
The age of the Universe is estimated to be 13.8 billion years. The first stars formed only about 200 million years after the Big Bang and died quickly in supernova explosions. The second generation, which includes Pic2-503, was formed shortly afterward. “Therefore, this star is undoubtedly more than 13 billion years old”, says Guilherme Limberg, explaining that, in this way, by studying ultra metal-poor stars, it is possible to infer properties of the first stars and understand how the evolution of the Universe took place.
“In recent years, we have tried 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 basically all of them have large amounts of carbon relative to the Sun, generally on the order of 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 cannot even detect this element in observations. This raises questions about the relationship between environment and chemical enrichment from the first stars”.
Carbon excess
Limberg reports that the star found, called Pic2-503, is located in the southern celestial hemisphere and is observed from the Southern Hemisphere of Chile, in a dwarf galaxy called Pictor 2, which has only 2,000 solar masses of stellar mass, equivalent to one millionth (0.0001%) of the Large Magellanic Cloud and one hundredth of a millionth (0.000001%) of the Milky Way.
“At first, 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 chemical composition for stars over a large area of the sky, but also conducts dedicated observations of dwarf galaxies, carried out using the four-meter Victor Blanco telescope at the Cerro Tololo Observatory in Chile”, he describes.
“We confirmed that it was indeed an ultra metal-poor star using another technique, spectroscopy, which provides more precise and detailed data on chemical composition, based on observations from two other telescopes: the 6.5-meter Magellan Baade, at Las Campanas, and the 8.2-meter Very Large Telescope (VLT), at Cerro Paranal, both also in Chile”.
According to the researcher, one of the major questions in astronomy concerns the origin of excess carbon in ultra metal-poor stars, with several competing hypotheses. “Therefore, the star Pic2-503 and its enormous carbon enrichment relative to the Sun demonstrate that the mechanism of carbon enrichment in the primordial Universe appears to differ for galaxies of different sizes”, he observes. “Among the existing hypotheses, the only one that fits this new observation, and also the ultra metal-poor stars without carbon in the Large Magellanic Cloud, establishes that carbon enrichment depends on the explosion energy of the supernovae associated with the first stars”.
Location of the star Pic2-503, within the dwarf galaxy Pictor 2, estimated to be 13 billion years old, which has a high carbon content, possibly retained after the explosions of the first supernovae in the Universe – Images: NOIRLab
According to Limberg, the idea is that “low-energy” supernovae (in relative terms, since every supernova is an extremely energetic event) inject large amounts of carbon into the host galaxy when they explode. “Meanwhile, higher-energy supernovae preferentially enrich the environment with other elements. Thus, tiny dwarf galaxies like Pictor 2, which have very little mass, are only able to retain material ejected from low-energy supernovae, those that produce excess carbon”, he explains. “On the other hand, larger galaxies such as the Large Magellanic Cloud are also able to retain relatively carbon-poor material from high-energy supernovae due to their immense gravitational potential, causing ultra metal-poor stars in that galaxy to have relatively lower amounts of this element”.
“This understanding has enormous implications for models of galaxy formation and chemical evolution in the primordial Universe, since it is now clear that enrichment in heavy elements can be dramatically different in different galaxies, even if the first stars are all identical and composed only of hydrogen and helium”, the researcher concludes. “This should influence, for example, how we interpret observations of distant galaxies with the James Webb Space Telescope, which is already capable of detecting galaxies located less than 500 million years after the Big Bang, considering 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, in the United States. Also signing the work are Kaia Atzberger and Nitya Kallivayalil, from the University of Virginia (United States); 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 (United States); Denija Crnojević, from the University of Tampa (United States); Peter Ferguson, from the University of Washington (United States); David James, from Applied Materials (United States); Gustavo E. Medina, from the University of Toronto (Canada); Noelia Noël, from the University of Surrey (United Kingdom); Alexander Riley, from Durham University (United Kingdom); David Sand, from the University of Arizona (United States); Joshua D. Simon, from Carnegie Observatories (United States); and 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 on the Nature Astronomy website.
More information: limberg@uchicago.edu, with Guilherme Limberg
English version: Nexus Traduções, edited by Denis Pacheco
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A reprodução de matérias e fotografias é livre mediante a citação do Jornal da USP e do autor. No caso dos arquivos de áudio, deverão constar dos créditos a Rádio USP e, em sendo explicitados, os autores. Para uso de arquivos de vídeo, esses créditos deverão mencionar a TV USP e, caso estejam explicitados, os autores. Fotos devem ser creditadas como USP Imagens e o nome do fotógrafo.