Carbon-Rich Planet Formation in a Solar Composition Disk

Abstract

The C-to-O ratio is a crucial determinant of the chemical properties of planets. The recent observation of WASP 12b, a giant planet with a C/O value larger than that estimated for its host star, poses a conundrum for understanding the origin of this elemental ratio in any given planetary system. In this paper, we propose a mechanism for enhancing the value of C/O in the disk through the transport and distribution of volatiles. We construct a model that computes the abundances of major C and O bearing volatiles under the influence of gas drag, sublimation, vapor diffusion, condensation and coagulation in a multi-iceline 1+1D protoplanetary disk. We find a gradual depletion in water and carbon monoxide vapors inside the water's iceline with carbon monoxide depleting slower than water. This effect increases the gaseous C/O and decreases the C/H ratio in this region to values similar to those found in WASP 12b's day side atmosphere. Giant planets whose envelopes were accreted inside the water's iceline should then display C/O values larger than those of their parent stars, making them members of the class of so-called "carbon-rich planets".

Gas composition of the main volatile elements in protoplanetary discs and its implication for planet formation

“…Öberg et al (2011) showed that these variations could be due to the different locations of condensation of H 2 O, CO, and CO 2 , resulting in an increase in the C/O ratios during condensation. This trend was subsequently reported by other studies (see, e.g., Ali-Dib et al 2014). The main shortcoming of the studies of Öberg et al (2011) and Ali-Dib et al (2014) is that they did not take into account the planet migration during their formation or the gas phase evolution, which can both significantly change the C/O ratio in the atmosphere of growing planets.…”

“…Its presence is very likely, however, given the composition of planetary and cometary bodies, and has been detected in the disc of GV Tau N (Gibb & Horne 2013). The inclusion of CH 4 in the models indicates an increase in the C/O ratio that has not been observed in previous studies (Öberg et al 2011;Ali-Dib et al 2014). This increase implies an enrichment of the C/O ratio by a factor of 3 to 4 compared to the solar value at distances where CH 4 and CO are the only C-and O-bearing species in the gas phase.…”

“…2. The diffusion is hereby neglected in contrast to Ali-Dib et al (2014). However, the disc structure and planet formation is computed as a function of time, whereas Ali-Dib et al (2014) have a more realistic diffusion model, but only computed snapshots of the disc in time.…”

Excess C/O and C/H in Outer Protoplanetary Disk Gas

“…Diffusion has been shown to have a large effect on elemental ratios: Ali-Dib et al (2014) recently showed that the faster diffusion of water compared to CO interior to the water snowline results in an increased C/O ratio in the inner disk. In general diffusion rates depend on concentration gradients, and are therefore especially efficient for species X close to the X condensation line.…”

“…We adopt a 30 AU sublimation radius and then spread the CO vapor inward and outward, simulating the effect of diffusion (e.g. Ros & Johansen 2013;Ali-Dib et al 2014;Owen 2014). The diffusion efficiency is not well known and probably varies between disks, and during disk lifetimes.…”