The formation of the Galilean moons and Titan in the Grand Tack scenario

In the Grand Tack (GT) scenario for the young solar system, Jupiter formed beyond 3.5 AU from the Sun and migrated as close as 1.5 AU until it encountered an orbital resonance with Saturn. Both planets then supposedly migrated outward for several $10^5$ yr, with Jupiter ending up at ~5 AU. The initial conditions of the GT and the timing between Jupiter's migration and the formation of the Galilean satellites remain unexplored. We study the formation of Ganymede and Callisto, both of which consist of ~50% H$_2$O and rock, in the GT scenario. We examine why they lack dense atmospheres, while Titan is surrounded by a thick N$_2$ envelope. We model an axially symmetric circumplanetary disk (CPD) in hydrostatic equilibrium around Jupiter. The CPD is warmed by viscous heating, Jupiter's luminosity, accretional heating, and the Sun. The position of the H$_2$O ice line in the CPD, which is crucial for the formation of massive moons, is computed at various solar distances. We assess the loss of Galilean atmospheres due to high-energy radiation from the young Sun. Ganymede and Callisto cannot have accreted their H$_2$O during Jupiter's supposed GT, because its CPD (if still active) was too warm to host ices and much smaller than Ganymede's contemporary orbit. From a thermal perspective, the Galilean moons might have had significant atmospheres, but these would probably have been eroded during the GT in < $10^5$ yr by solar XUV radiation. Jupiter and the Galilean moons formed beyond 4.5 (+/- 0.5) AU and prior to the proposed GT. Thereafter, Jupiter's CPD would have been dry, and delayed accretion of planetesimals should have created water-rich Io and Europa. While Galilean atmospheres would have been lost during the GT, Titan would have formed after Saturn's own tack, because Saturn still accreted substantially for ~$10^6$ yr after its closest solar approach, ending up at about 7 AU.Comment: A&A Letter, 4 pages, 2 colored figure

Magnetohydrodynamics of turbulent accretion discs around black holes

The Cyg X-1 X-ray source is believed to be comprised of an accretion disc around a central black hole. We apply the methods of Mean Field Electrodynamics to the study of magnetic processes in such an accretion disc. By decomposing the magnetic field in the disc into mean and fluctuating components, the observed X-ray properties of this system may be accounted for. It is found that intense, short lived magnetic fluctuations may occur which give rise to solar-like flares on the surfaces of the accretion disc. The energy releases and time scales of such flares is found to provide a physical basis for the observed shot-noise like character of the X-ray emission from the system. It is demonstrated that a rather strong, large scale magnetic field can be generated by turbulent dynamo action in the accretion disc. This result is the reason why magnetic fields may play a vital role in these systems. The long time averaged structure of the accretion disc is determined by the Maxwell-stress due to the mean field, and is in agreement with the "standard" cool accretion disc models. We prove that on intermediate time and length scales, the Maxwell stresses due to the magnetic fluctuations remove the known instability of "standard" accretion disc models to ring- like "clumping" and subsequent heating of the gas. This result shows that the hard X-ray emission of the Cyg X-1 source must arise from either a hot corona, or intense solar-type flares above the disc surfaces. If the hard X-ray emission arises from non-thermal electron populations accelerated in the flares, it is found that this emission must occur in a rapid "flash-phase" on submillisecond time scales. These flares occur well away from the inner disc boundaries so that we believe that submillisecond variations of the Cyg X-1 source need not be a test of the rotation of the central black hole.Science, Faculty ofPhysics and Astronomy, Department ofGraduat