'At the edge of what we thought possible': Astronomers find extremely rare star from ancient universe

Scientists have adopted the role of "cosmic archaeologists" to discover a rare, iron-deficient second-generation star — essentially a fossil record of our universe's chemical evolution. Just as uncovering artifacts here on Earth teaches us about lost generations of humans, this observation provides hard 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 around 150,000 light-years from Earth in the constellation Pictor, using the Dark Energy Camera (DECam) mounted atop Víctor M. Blanco 4-meter Telescope. Designated PicII-503, the star has only 1/40,000th of the iron contained within the sun, which is a third-generation, or (somewhat confusingly) POP I, star. The fact that PicII-503 has the lowest concentration of iron ever seen beyond the Milky Way makes it one of the most primordial stars ever discovered.

That deficit isn't the most extraordinary about PicII-503, however. The team also found that this POP II star has a massive overabundance of carbon, with its ratio of carbon-to-iron over 1,500 times greater than the same ratio in the sun. This overabundance mirrors the unique carbon signature of low-iron stars found in the nebulous outer halo of the Milky Way.

"Discoveries like this are cosmic archaeology, uncovering 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 didn't extend beyond hydrogen, helium, and a smattering of heavier elements, which astronomers collectively call "metals. "This meant that these POP III stars were also dominated by hydrogen with just a little helium and very little in terms of metals. These stars forged the first carbon and iron in their cores, material that was distributed into the interstellar medium when these stars went supernova and exploded at the end of their lives.

Interstellar clouds of gas and dust enriched with these metals eventually cooled and collapsed to birth the second generation of stars, stars that were more metal-rich thanks to the donation of heavy elements from their predecessors. That makes POP II akin to time capsules, recording an important stage in the chemical enrichment of the universe.

"Discovering a star that unambiguously preserves the heavy metals from the first 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 statement. "With the lowest iron abundance ever derived in any ultra-faint dwarf galaxy, PicII-503 provides a window into initial element production within a primordial system that is unprecedented."

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 MAGIC (Mapping the Ancient Galaxy in CaHK) survey. This 54-night observing endeavor was developed with the explicit purpose of identifying the oldest and most chemically primitive stars in the Milky Way and its dwarf galaxy companions.

"Without data from MAGIC, it would have been impossible to isolate this star among the hundreds of other stars in the vicinity of the Pictor II ultra-faint dwarf galaxy," Chiti said.

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

One possible explanation for the shockingly low iron-to-carbon ratio of PicII-503 is that when POP III stars went supernova, these explosions were relatively low in energy. That would have meant that while lighter elements like carbon were blasted into the interstellar medium, heavy elements like iron fell back into the wreckage of the supernova.

The fact that PicII-503 is found in one of the smallest dwarf galaxies ever seen, with a correspondingly low gravitational influence, supports the idea of POP III stars dying in low-energy supernovas.

"What excites me the most is that we have observed an outcome of the very initial element production in a primordial galaxy, which is a fundamental observation!” Chiti said. "It also cleanly connects to the signature that we have seen in the lowest-metallicity Milky Way halo stars, tying together their origins and the first-star-enriched nature of these objects."

The team's research was published on Monday (March 16) in the journal Nature Astronomy.