X-Ray Ionization and the Influence of Magnetic Fields on the Growth of Gas Giant Planets

Author: Kim, Stacy Yeonchi

Year: 2013

Degree: Senior thesis (Major)

Advisor: Turner, Neal J.

Committee Member: None, None

Option: Physics

DOI: 10.7907/D9BC-AG81

Abstract

The past two decades have witnessed an explosion in the number of known planets outside our Solar System. The exoplanets' number and diversity are surprising, with many planetary systems quite unlike our own. Naturally, the question of their origins arise. Yet our knowledge of how planetary systems form and evolve are far from complete, and even for our own Solar System, many fundamental questions remain. One puzzle concerns the formation of gas giants with masses intermediate between Jupiter (MJ = 320M⊕, where M⊕ = 6 x 1027 g equals the mass of the Earth) and Neptune (MN = 17M⊕ ), which our Solar System provides a prime laboratory for study: Saturn (MS = 95M⊕)· According the popular core accretion scenario, gas giants form through the growth of an ice and rock core that, upon becoming sufficiently massive, accretes a gaseous envelope from the surrounding protoplanetary disk. If the mass of the envelope reaches the mass of the core, runaway gas accretion commences, with terminal masses typically a Jupiter mass or more. The combined mass of the envelope and core at the critical point when runaway accretion begins is about a Neptune mass. Saturn, being a factor of a few times more massive than Neptune, achieved this critical mass, yet appears not to have undergone runaway accretion to an extent similar to Jupiter, posing a problem for the core accretion hypothesis.

In this thesis, we investigate one possible explanation for Saturn's low mass: magnetic fields. If the gas surrounding a protoplanet is sufficiently ionized, it can be well coupled to the ambient magnetic field; as magnetized gas accretes onto the protoplanet, magnetic pressure and tension forces would grow, prematurely halting the flow of gas. To investigate this possibility, a preexisting Monte Carlo transport code was extended to calculate x-ray transfer. The code was applied to calculate ionization rates for a proto-Saturn still embedded in the protosolar disk. The results were inputted into a simplified chemical reaction network, which takes into account recombination and charge transfer and returns the ionization fraction of the disk. Magnetic diffusivities in proto-Saturn's vicinity can then be calculated, allowing us to infer whether the magnetic field was coupled well enough to the gas surrounding proto-Saturn to affect gas dynamics.

We find that for a disk depleted in small grains and possessing magnetic fields of about 0.1 G, magnetic fields were likely well-coupled to the gas near proto-Saturn and may have influenced its gas accretion rate.

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