Binary stars form in the same nebula but are not identical. Now we know why.

It is logical that stars formed from the same cloud of material have the same metallicity. This fact underpins some avenues of astronomical research, such as the search for the brothers of the Sun. But for some binary stars, that’s not always true. Their composition can be different despite forming from the same reservoir of material, and the difference extends to their planetary systems.

New research shows that the differences can be traced back to their earliest stages of formation.

Binary stars are the norm, while solitary stars like our Sun are in the minority. Some estimates place the number of binary stars in the Milky Way as high as 85%. These pairs of stars form from the giant molecular clouds themselves. Each cloud has a certain abundance of metals, and this abundance should be reflected in the stars themselves.

But it’s not always like that.

Sometimes the metallicities of a pair of binary stars do not match. Astrophysicists have proposed three explanations for this.

Two explanations involve events that occur later in a star’s life after they have left the main sequence. One is atomic diffusion, where chemical elements settle in gradient layers in the star. The layers are determined by the star’s gravity and temperature. The second involves a nearby planet. As stars age, expand and become red giants, they engulf nearby planets. The planet would introduce new chemistry to the star, setting it apart from its binary partner.

The third explanation goes back in time to the formation of the binary pair. This explanation says that the giant molecular cloud that spawned the stars was not homogeneous. Instead, there were regional differences in cloud chemistry, and stars formed in different locations showed notable differences in their chemical composition.

A team of researchers wanted to delve deeper into this third explanation to verify its veracity. They used the Gemini South telescope and its Gemini High-Resolution Optical Spectrograph (GHOST) to examine the light from a pair of giant binary stars. The observations revealed significant differences in their spectra.

They presented their results in a paper titled “Unraveling the origin of chemical differences using GHOST.” It is published in the journal Astronomy and Astrophysics. The lead author is Carlos Saffe from the Institute of Astronomical, Earth and Space Sciences (ICATE-CONICET) in Argentina. The researchers examined a pair of giant binary stars called HD 138202 + CD?30 12303.

All three explanations for the chemical differences between binary stars come from studies of main-sequence stars. The main sequence is where stars spend most of their time, reliably fusing hydrogen into helium over billions of years.

But Saffe and his colleagues took a different approach. They used Gemini and GHOST to examine a pair of binary stars that had left the main sequence behind and become giant stars. These stars are different from main sequence stars.

“GHOST’s extremely high-quality spectra offer unprecedented resolution,” said Saffe, “allowing us to measure the stellar parameters and chemical abundances of the stars as precisely as possible.”

These stars experience dredges. Drags are when a star’s convection zone extends from the surface to where fusion is taking place. They are powerful convective currents that mix fusion products into the star’s surface layer when a main-sequence star becomes a red giant.

However, researchers say dredging and the atomic diffusion they lead to cannot explain the wide difference between the stars.

Convection currents would also rule out the second proposed explanation: planetary envelopment. With such strong currents, the chemicals on a engulfed planet are quickly diluted. “Giant stars are believed to be significantly less sensitive than main-sequence stars to entanglement events,” the authors write.

The authors went further and calculated how much planetary material a giant star would have to digest to cause the difference in metallicity between the stars. “We estimate that Star A should have ingested between 11.0 and 150.0 Jupiter masses of planetary material, depending on the mass of the convective envelope adopted and the metal content of the ingested planet,” explain the authors That’s a lot of material. They also explain that the planets must have extremely high metallicity for the low value of 11 Jupiter masses to cause the chemical differences.

This leaves only one explanation: inhomogeneities in the molecular cloud.

“This is the first time that astronomers can confirm that the differences between binary stars begin in the early stages of their formation,” Saffe said.

“Using the precision measurement capabilities provided by the GHOST instrument, Gemini South is now collecting observations of stars at the end of their lives to reveal the environment in which they were born,” said Martin Still, NSF program director. of the Gemini International Observatory. . “This gives us the ability to explore how the conditions under which stars form can influence their entire existence over millions or billions of years.”

The results go a long way toward explaining why a pair of binary stars can have different compositions. But they go even further than that. They also explain why a pair of binary stars can have such different planetary systems. “Different planetary systems could mean very different rocky, Earth-like, ice giant, gas giant planets orbiting their host stars at different distances and where the potential to support life could be very different,” he said. said Saffe.

But the results also present a challenge. Astronomers use chemical tagging to identify stars that associate with each other. Stars from the same stellar nursery are expected to have similar compositions. But this method seems unreliable in light of these findings.

The results also challenge the idea that differences in composition between binary stars can be explained by the planet’s envelope. Instead, these differences could stem from the early days of star formation.

“By showing for the first time that primordial differences are indeed present and responsible for the differences between twin stars, we show that star and planet formation could be more complex than originally thought,” Saffe said. “The Universe loves diversity!”

The only drawback of this study is the sample size of one. Small sample sizes are always precautionary: they may lead to an eventual conclusion, but they do not independently form reliable conclusions. The authors know it.

“We strongly encourage the study of giant-giant pairs,” the researchers conclude. “This new approach could help us distinguish the origin of the slight chemical differences observed in multiple systems.”