Circumplanetary Discs: Truncation and Outbursts

Astronomy and Astrophysics – Astronomy

Scientific paper

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Scientific paper

After a planet with a mass greater than that of Nepture forms it opens a gap in the circumstellar disc. Material continues to flow on to the planet and a circumplanetary disc forms. We model circumplanetary discs as accretion discs subject to the tidal forces of the central star. The tidal torques remove the disc angular momentum near the disc outer edge and permit the accreting disc gas to lose angular momentum at the rate appropriate for steady accretion. Circumplanetary discs are truncated near the radius where periodic ballistic orbits cross, where tidal forces on the disc are strong. This radius occurs at approximately 0.4 times the planet Hill radius. During the T Tauri stage of disc accretion, the disc is fairly thick with aspect ratio H/r>0.2. We model the disc structure using one-dimensional time-dependent and steady-state models and also two-dimensional SPH simulations. The circumplanetary disc structure depends on the variation of the disc turbulent viscosity with radius and is insensitive to the angular distribution of the accreting gas. If the disc is turbulent throughout, the predicted disc structure near the location of the regular Jovian and Saturnian satellites is smooth with no obvious feature that would favor formation at their current locations. Dead zones form in accretion discs where the temperature is too low for the disc to be fully thermally ionised. They are even more favorable in circumplanetary discs than in circumstellar discs because the surface densities are high while the temperature remains low enough to avoid thermal ionisation. We consider the conditions under which dead zones form and the resulting unsteady accretion on to the planet. Outbursts in the accretion rate on to the planet have observational implications for planet detection.

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