Saturday, July 26, 2014

Signature of a Giant Planet’s Rocky Core

Stars in binary systems generally have identical chemical compositions since they formed from the same natal cloud of material. Nevertheless, small differences in chemical composition can exist between a pair of stars in a binary system and one explanation is the process of planet formation. When planets form around a star, it can cause the star to be slightly depleted in heavy elements (i.e. elements heavier than hydrogen and helium) compared to its companion star.

Observations of 16 Cygni, a binary system comprised of two stars 16 Cygni A and 16 Cygni B (hereafter components A and B), reveal that component B has a giant planet with at least 1.5 times Jupiter’s mass. The giant planet, identified as 16 Cygni Bb, orbits its host star in a highly-eccentric 800-day orbit. At its minimum and maximum distances from its host star, the giant planet receives, respectively, 4.4 and 0.16 times the amount of insolation Earth gets from the Sun. Being a giant planet, 16 Cygni Bb is composed primarily of hydrogen and helium, much like Jupiter.

Figure 1: Artist’s impression of a giant planet with a system of moons around it.

Figure 2: Differences in heavy element abundance between components A and B versus condensation temperature. The dashed line is the average of the volatiles and the solid line is the average of the refractories. The dot dashed line is the mean trend for 11 Sun-like stars compared to the Sun. Maia et al. (2014).

A study by Maia et al. (2014) show a small difference in the abundance of heavy elements (i.e. metallicity) between components A and B of 16 Cygni. Component A has an overall metallicity (i.e. abundance of heavy elements) that is 0.047 ± 0.005 dex higher than the metallicity of component B. The abundance differences range from 0.03 dex for volatiles (i.e. elements such as carbon and oxygen), and up to 0.06 dex for refractories (i.e. elements such as iron, vanadium and magnesium). The lower abundance of heavy elements in component B is likely due to the formation of the giant planet that is presently in orbit around it, where the “missing” heavy elements were used to form the giant planet.

The higher deficiency in refractories compared to volatiles in component B means that the giant planet, 16 Cygni Bb, has a corresponding excess of refractories. This suggests that 16 Cygni Bb formed by the core accretion mechanism where an initial rocky core, comprised primarily of refractories, becomes massive enough to start accreting hydrogen, helium and other volatiles to form a giant planet. Estimates of the initial rocky core of 16 Cygni Bb places it at around 1.5 to 6 times the mass of Earth, consistent with Jupiter’s core mass. These findings validate the core accretion model for the formation of 16 Cygni Bb and offer yet another means to examine the relationship between a star and its planet.

Maia et al. (2014), “High precision abundances in the 16 Cyg binary system: a signature of the rocky core in the giant planet”, arXiv:1407.4132 [astro-ph.SR]