Some Sun-like stars are 'earth-eaters' and while they are developing they ingest large amounts of the rocky material from which 'terrestrial' planets like Earth, Mars and Venus are made.
Trey Mack, a graduate student in astronomy at Vanderbilt University, has developed a model that estimates the effect that such a diet has on a star's chemical composition and has used it to analyze a pair of twin stars which both have their own planets.
Building on the work of co-author Simon Schuler of the University of Tampa, who expanded the examination of stars' chemical composition beyond their iron content, Mack looked at the abundance of 15 specific elements relative to that of the Sun.
He was particularly interested in elements like aluminum, silicon, calcium and iron that have melting points higher than 1,200 degrees Fahrenheit (600 degrees Celsius) as these are the refractory materials that serve as building blocks for Earth-like planets.
Mack, Schuler and Vanderbilt Professor of Astronomy Keivan Stassun, who supervised the study, decided to apply this technique to the planet-hosting binary pair designated HD 20781 and HD 20782. Both stars should have condensed out of the same cloud of dust and gas and so both should have started with the same chemical compositions. This particular binary pair is the first one discovered where both stars have planets of their own.
Both of the stars in the binary pair are G-class dwarf stars similar to the Sun. One star is orbited closely by two Neptune-size planets. The other possesses a single Jupiter-size planet that follows a highly eccentric orbit. The difference in their planetary systems make the two stars ideal for studying the connection between exoplanets and the chemical composition of their stellar hosts.
When they analyzed the spectrum of the two stars the astronomers found that the relative abundance of the refractory elements was significantly higher than that of the Sun. They also found that the higher the melting temperature of a particular element, the higher was its abundance, a trend that serves as a compelling signature of the ingestion of Earth-like rocky material.
They calculated that each of the twins would have had to consume an additional 10-20 Earth-masses of rocky material to produce the chemical signatures. Specifically, the star with the Jupiter-sized planet appears to have swallowed an extra ten Earth masses while the star with the two Neptune-sized planets scarfed down an additional 20.
The results support the proposition that a star's chemical composition and the nature of its planetary system are linked.
The results of the study have been published online in the Astrophysical Journal.
