New Research Sheds Light on Origin of Chemical Differences in Giant Binary Stars | Sci.News

An artist's impression of HD 138202 + CD-30 12303, a binary pair of giant stars located 1,707 light-years away in the constellation of Lupus. Image credit: NOIRLab / NSF / AURA / J. da Silva, Spaceengine / M. Zamani.

It is estimated that up to 85% of stars exist in binary star systems, some even in systems with three or more stars.

These stellar pairs are born together out of the same molecular cloud from a shared abundance of chemical building blocks, so astronomers would expect to find that they have nearly identical compositions and planetary systems. However, for many binaries that isn't the case.

While some proposed explanations attribute these dissimilarities to events occurring after the stars evolved, astronomers using the GHOST instrument confirmed that they can actually originate from before the stars even began to form.

"GHOST's extremely high-quality spectra offered unprecedented resolution, allowing us to measure the stars' stellar parameters and chemical abundances with the highest possible precision," said Dr. Carlos Saffe, an astronomer at the Institute of Astronomical, Earth and Space Sciences (ICATE-CONICET).

"These measurements revealed that one star had higher abundances of heavy elements than the other."

"To disentangle the origin of this discrepancy, we used a unique approach."

Previous studies have proposed three possible explanations for observed chemical differences between binary stars.

Two of them involve processes that would occur well into the stars' evolution: atomic diffusion, or the settling of chemical elements into gradient layers depending on each star's temperature and surface gravity; and the engulfment of a small, rocky planet, which would introduce chemical variations in a star's composition.

The third possible explanation looks back at the beginning of the stars' formation, suggesting that the differences originate from primordial, or pre-existing, areas of nonuniformity within the molecular cloud.

In simpler terms, if the molecular cloud has an uneven distribution of chemical elements, then stars born within that cloud will have different compositions depending on which elements were available at the location where each formed.

So far, studies have concluded that all three explanations are probable; however, these studies focused solely on main-sequence binaries.

The main-sequence is the stage where a star spends most of its existence, and the majority of stars in the Universe are main-sequence stars, including our Sun.

Instead, Dr. Saffe and his colleagues observed HD 138202 + CD-30 12303, a binary system consisting of two giant stars.

These stars possess extremely deep and strongly turbulent external layers, or convective zones.

Owing to the properties of these thick convective zones, the team was able to rule out two of the three possible explanations.

The continuous swirling of fluid within the convective zone would make it difficult for material to settle into layers, meaning giant stars are less sensitive to the effects of atomic diffusion — ruling out the first explanation.

The thick external layer also means that a planetary engulfment would not change a star's composition much since the ingested material would rapidly be diluted — ruling out the second explanation.

This leaves primordial inhomogeneities within the molecular cloud as the confirmed explanation.

"This is the first time astronomers have been able to confirm that differences between binary stars begin at the earliest stages of their formation," Dr. Saffe said.

The findings were published in the journal Astronomy & Astrophysics Letters.

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C. Saffe et al. 2024. Disentangling the origin of chemical differences using GHOST. A&A 682, L23; doi: 10.1051/0004-6361/202449263