By David Yong & Fan Liu
[email protected] & [email protected]
Binary stars (like the famous scene from Star Wars, below) are prime examples of pairs of stars born from the same gas cloud. They share the same age and initial chemical composition. A variety of astrophysical processes, however, can result in differences in the chemical composition between the two stars in a binary system. Those astrophysical processes can include: (1) the formation and engulfment of planets; (2) mixing processes within the stars themselves; (3) whether the gas clouds from which the two stars formed were fully mixed; and/or (4) the formation, evolution and ingestion of dust grains.
These effects, however, can be subtle and extremely difficult to detect as the chemical content of the stars needs to be measured with errors as low as 2%. Unfortunately, the vast majority of binary star systems are simply too faint for astronomers to be able to measure the chemical compositions at the required precision level. Only 30 or so binary star systems have been analysed in this way, and most studies only consider a handful of binary star systems. Thanks to the Gaia space observatory of the European Space Agency, we can now identify millions of wide binary stars where the two stars can be separated by as much as a million times the distance from the Earth to the Sun.
In our study published recently in MNRAS (which even includes “C3PO” in the title!), we made use of data from the Ultraviolet and Visual Echelle Spectrograph (UVES) at the European Southern Observatory’s 8m Very Large Telescope in Chile as well as spectrographs on the Magellan and Keck telescopes. We studied 125 pairs of wide binaries (250 stars) and achieved extremely high precision measurements of the iron content and other quantities of the stars such as their temperatures and densities. To our knowledge, this is the largest high precision homogeneous analysis of binaries ever undertaken. We were able to classify the stars into two groups based on their iron content; (a) pairs of stars that were born together from the same gas cloud and (b) pairs of stars that were not born from the same gas cloud. We found that a spatial separation of 106 AU (a million times the distance from the Earth to the Sun) is an approximate boundary between the two groups.
The first key result from this study is that the fraction of binary star systems with differences in their iron content increases with increasing stellar temperature. This trend matches a “toy model” of planet engulfment. The second key result from this study is a new sample of 56 bright wide binary star systems with chemical abundance differences at or below the 5% level, such as the co-moving pair shown below. Such chemical differences are lower than what is found within star clusters. In a future study, we will use these wide binary star systems to search for chemical signatures of the engulfment of planetary material by considering 20–25 chemical elements.
