Accretion Outbursts on to Young Stars and Planets
Space Telescope Science Institute
Understanding angular momentum transport in accretion discs is fundamentalto explaining the formation of systems on all scales, from satellitesystems, to planetary systems to the growth of supermassive blackholes. Turbulence, driven by the magneto-rotational instability (MRI),transports angular momentum outwards allowing accretion on to the centralobject. However, the MRI may be suppressed by a low ionisation fractionand dead zones can form at the disc midplane. Dead zones are regionswith no turbulence that prevent the free flow of material throughthe disc. Material accumulates in the dead zone and the disc becomesgravo-magneto unstable. Accretion on to the central object occurs inlarge outbursts involving a large fraction of the disc mass. We explainthe outbursts as transitions between steady state solutions, one that isfully turbulent and a second that is self-gravitating. Outbursts havebeen observed in young stellar objects such as FU Orionis systems andwe find they should also occur in circumplanetary discs. When a planetforms in a circumstellar disc, if it has sufficient mass it opens a gapin the disc. Accretion through the gap forms a circumplanetary disc inwhich satellites are thought to form. We find that these discs are alsounstable to the gravo-magneto instability and the increased luminosityof the circumplanetary disc during the outbursts may aid planet detectionduring the early stages of planet formation.
Date: Thursday, 3 November 2011 Time: 13:30 Where: McGill University Ernest Rutherford Physics Building, Board Room (room 105) Contact: Robert Rutledge