Observations disprove theories of late changes in stellar composition
Binary stars born from the same parent cloud of collapsing gas and dust are not always identical twins. They may even have different types of planetary systems orbiting each of the pair's stars. But where do these differences come from? Looks like astronomers have finally found the answer.
According to scientists, about 85% of stars exist in binary or multiple systems with stellar companions. These binary stars are born from the same gas cloud, suggesting a similar chemical composition and, accordingly, the formation of similar planetary systems around each of the stars of the pair.
However, in practice this is not always the case. Using the powerful Gemini South telescope in northern Chile, a team of scientists discovered that the differences between binary stars result from irregularities in the chemical compounds of the parent molecular cloud from which they formed. This discovery helped researchers confirm the hypothesis that differences between twin stars may be laid down even before their formation begins.
«By demonstrating for the first time that there are real differences responsible for the dissimilarities between twin stars, we show that the processes of star formation and planet formation may be more complex than previously thought. The universe favors diversity», — noted Carlos Saffe, head of the research group from the Institute for Astronomical Research.
Before this study, scientists had three possible explanations for why stars born from the same cloud could differ in composition. Two of these hypotheses suggested that changes in stars occurred after they were formed.
One idea was that the movement of atoms in the interior of binary stars could lead to a process called “atomic diffusion.” This process is regulated by the temperature of the star and its gravity and leads to the settling of atoms in the layers of the star. For stars in binary systems with different masses and temperatures, this could explain the observed differences in chemical composition.
According to an alternative hypothesis, one of the stars in the binary system could have absorbed one of the rocky planets orbiting it, thereby assimilating some of the chemical elements of this planet and changing its composition.
However, there was a third hypothesis: differences in the composition of stars could arise due to chemical inhomogeneity in the molecular cloud that gave birth to the binary star system.
Scientists have found that all of these explanations are plausible. Their research focused on stars during their life on the main sequence — the stage in which stars such as the Sun convert hydrogen into helium in their cores and which makes up most of the star's life cycle.
To evaluate the validity of various hypotheses explaining differences in stellar composition, Saffe and his colleagues used the high-resolution optical spectrograph GHOST installed on the Gemini South telescope. With its help, they studied the spectral characteristics of the binary system HD 138202 + CD-30 12303, located approximately 1720 light years from Earth.
«The high-quality spectra obtained with GHOST provide unprecedented resolution, allowing us to measure stellar parameters and chemical composition with the highest possible accuracy», — noted Saffe.
The analysis showed that the stars HD 138202 and CD-30 12303 have deep and turbulent outer layers called «convective zones». This fact made it possible to exclude two of the previously proposed theories of the formation of differences. The fact is that constant mixing in convective zones prevents the settling of material due to atomic diffusion. In addition, the presence of thick outer layers means that the absorption of a planet is unlikely to seriously affect the composition of the star, since the absorbed material would quickly become diluted.
Thus, the most plausible hypothesis remains about primary chemical differences in the parent molecular cloud.
«This is the first time that astronomers have been able to confirm that the differences between binary stars are laid down in the very early stages of their formation», — emphasized Saffe.
In addition to revealing the secrets of the evolution of binary stars, the team's research also sheds light on the reasons for the formation of various planetary systems around them, where different stars influence the evolution of planets in different ways.
«Differences in planetary systems can mean very different types of planets — rocky, ice giants and gas giants rotating in different orbits around their stars. And the potential for the emergence of life on them can be completely different», — Saffe explained.
In addition, by demonstrating that stars with different chemical compositions can form from a single cloud of gas, the results of the study may prompt astronomers to reconsider methods for determining the origin of stars based on their chemical composition.
Scientists may also have to rethink their understanding of stars that were previously suspected of gobbling up planets. After all, these traces may simply be the result of the peculiarities of the birth of a star, and not a destructive collision with a planet.