Astronomers find extremely rare star from ancient universe

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Scientists have adopted the role of “cosmic archaeologists” to discover a rare, second-generation, iron-deficient star – essentially a fossil record of the chemical evolution of our universe. Just as the discovery of artifacts here on Earth teaches us about lost human generations, this observation provides tangible evidence of how the first generation of stars died to chemically enrich their successors.

The second generation, or POP II, star was discovered in the dwarf galaxy Pictor II, located approximately 150,000 light years from Earth in the constellation Pictor, uses the Dark Energy Camera (DECam) mounted atop the 4-meter Víctor M. Blanco telescope. Designated PicII-503, the star contains only 1/40,000th of the iron contained in the sun, which is a third generation star, or (somewhat confusingly) POP I. The fact that PicII-503 has the lowest concentration of iron ever observed beyond Milky Way in fact one of the most primordial stars ever discovered.

This deficit, however, is not the most extraordinary of PicII-503. The team also discovered that this POP II star has a massive overabundance of carbon, with a carbon-to-iron ratio more than 1,500 times that of the Sun. This overabundance reflects the unique carbon signature of low-iron stars found in the nebulous outer halo of the Milky Way.

“Discoveries like this are cosmic archaeology, unearthing rare stellar fossils that preserve the fingerprints of the universe’s first stars,” Chris Davis, National Science Foundation program director for NOIRLab. said in a statement.

A kind of magic

The first stars in the universe, or POP III stars, were born when the chemical abundance of the cosmos did not extend beyond hydrogenhelium and a handful of heavier elements, which astronomers collectively call “metals.” This meant that these POP III stars were also hydrogen dominated with just a little helium and very little in terms of metals. These stars forged the first carbon and iron in their cores, material which was distributed in the interstellar medium when these stars disappeared. supernova and exploded at the end of their life.

The interstellar clouds of gas and dust enriched with these metals eventually cooled and collapsed to give rise to the second generation of stars, stars richer in metals thanks to the donation of heavy elements from their predecessors. This makes POP II a sort of time capsule, recording an important step in the chemical enrichment of the universe.

“Discovering a star that unambiguously preserves heavy metals from early stars was at the edge of what we thought possible, given the extreme rarity of these objects,” team leader Anirudh Chiti of Stanford University said in the release. “With the lowest iron abundance ever found in an ultra-faint dwarf galaxy, PicII-503 opens a window into the initial production of elements within an unprecedented primordial system.”

The first confirmed example of a POP II star found in a faint dwarf galaxy, PicII-503 was highlighted as an extremely metal-poor star in data collected by DECam’s Mapping the Ancient Galaxy in CaHK (MAGIC) survey. This 54-night observing effort was developed with the explicit goal of identifying the oldest and most chemically primitive stars in the Milky Way and its dwarf galaxy companions.

“Without MAGIC data, it would have been impossible to isolate this star among the hundreds of other stars located near the ultra-faint dwarf galaxy Pictor II,” Chiti said.

Chiti and colleagues combined the MAGIC data with observations from Very large telescope (VLT) in the Atacama Desert region of northern Chile and the Baade Magellan Telescope to discover the low iron and calcium abundances of PicII-503, the lowest observed beyond our home galaxy. This revealed that PicII-503 was the first record of chemical enrichment found in a dwarf galaxy.

One possible explanation for PicII-503’s incredibly low iron-to-carbon ratio is that when POP III stars went supernova, these explosions were relatively low in energy. This would have meant that while lighter elements like carbon were thrown into the interstellar medium, heavier elements like iron fell back into the supernova wreckage.

The fact that PicII-503 lies in one of the smallest dwarf galaxies ever observed, with correspondingly weak gravitational influence, supports the idea of ​​POP III stars dying in low-energy supernovas.

“What excites me the most is that we observed the result of the first ever production of elements in a primordial galaxy, which is a fundamental observation!” Chiti said. “This also clearly connects to the signature we saw in the less metallic Milky Way halo stars, thus linking their origins and the early star-enriched nature of these objects.”

The team’s research was published Monday March 16 in the journal Natural astronomy.