Collapse and Fragmentation of Molecular Cloud Cores. X. Magnetic Braking of Prolate and Oblate Cores

The collapse and fragmentation of initially prolate and oblate, magnetic molecular clouds is calculated in three dimensions with a gravitational, radiative hydrodynamics code. The code includes magnetic field effects in an approximate manner: magnetic pressure, tension, braking, and ambipolar diffusion are all modelled. The parameters varied for both the initially prolate and oblate clouds are the initial degree of central concentration of the radial density profile, the initial angular velocity, and the efficiency of magnetic braking (represented by a factor $f_{mb} = 10^{-4}$ or $10^{-3}$). The oblate cores all collapse to form rings that might be susceptible to fragmentation into multiple systems. The outcome of the collapse of the prolate cores depends strongly on the initial density profile. Prolate cores with central densities 20 times higher than their boundary densities collapse and fragment into binary or quadruple systems, whereas cores with central densities 100 times higher collapse to form single protostars embedded in bars. The inclusion of magnetic braking is able to stifle protostellar fragmentation in the latter set of models, as when identical models were calculated without magnetic braking (Boss 2002), those cores fragmented into binary protostars. These models demonstrate the importance of including magnetic fields in studies of protostellar collapse and fragmentation, and suggest that even when magnetic fields are included, fragmentation into binary and multiple systems remains as a possible outcome of protostellar collapse.Comment: 20 pages, 8 figures. Astrophysical Journal, in pres

On the Formation of Gas Giant Planets on Wide Orbits

A new suite of three dimensional radiative, gravitational hydrodynamical models is used to show that gas giant planets are unlikely to form by the disk instability mechanism at distances of ~100 AU to ~200 AU from young stars. A similar result seems to hold for the core accretion mechanism. These results appear to be consistent with the paucity of detections of gas giant planets on wide orbits by infrared imaging surveys, and also imply that if the object orbiting GQ Lupus is a gas giant planet, it most likely did not form at a separation of ~100 AU. Instead, a wide planet around GQ Lup must have undergone a close encounter with a third body that tossed the planet outward to its present distance from its protostar. If it exists, the third body may be detectable by NASA's Space Interferometry Mission.Comment: 13 pages, 4 figures. in press, ApJ Letter

Evolution of the Solar Nebula. IX. Gradients in the Spatial Heterogeneity of the Short-Lived Radioisotopes 60^{60}Fe and 26^{26}Al and the Stable Oxygen Isotopes

Short-lived radioisotopes (SLRI) such as $^{60}$Fe and $^{26}$Al were likely injected into the solar nebula in a spatially and temporally heterogeneous manner. Marginally gravitationally unstable (MGU) disks, of the type required to form gas giant planets, are capable of rapid homogenization of isotopic heterogeneity as well as of rapid radial transport of dust grains and gases throughout a protoplanetary disk. Two different types of new models of a MGU disk in orbit around a solar-mass protostar are presented. The first set has variations in the number of terms in the spherical harmonic solution for the gravitational potential, effectively studying the effect of varying the spatial resolution of the gravitational torques responsible for MGU disk evolution. The second set explores the effects of varying the initial minimum value of the Toomre $Q$ stability parameter, from values of 1.4 to 2.5, i.e., toward increasingly less unstable disks. The new models show that the basic results are largely independent of both sets of variations. MGU disk models robustly result in rapid mixing of initially highly heterogeneous distributions of SLRIs to levels of $\sim$ 10% in both the inner ($$ 10 AU) disk regions, and to even lower levels ($\sim$ 2%) in intermediate regions, where gravitational torques are most effective at mixing. These gradients should have cosmochemical implications for the distribution of SLRIs and stable oxygen isotopes contained in planetesimals (e.g., comets) formed in the giant planet region ($\sim$ 5 to $\sim$ 10 AU) compared to those formed elsewhere.Comment: 37 pages, 1 table, 19 figures, ApJ accepte

Gas Giant Protoplanets Formed by Disk Instability in Binary Star Systems

We present a suite of three dimensional radiative gravitational hydrodynamics models suggesting that binary stars may be quite capable of forming planetary systems similar to our own. The new models with binary companions do not employ any explicit artificial viscosity, and also include the third (vertical) dimension in the hydrodynamic calculations, allowing for transient phases of convective cooling. The calculations of the evolution of initially marginally gravitationally stable disks show that the presence of a binary star companion may actually help to trigger the formation of dense clumps that could become giant planets. We also show that in models without binary companions, which begin their evolution as gravitationally stable disks, the disks evolve to form dense rings, which then break-up into self-gravitating clumps. These latter models suggest that the evolution of any self-gravitating disk with sufficient mass to form gas giant planets is likely to lead to a period of disk instability, even in the absence of a trigger such as a binary star companion.Comment: 52 pages, 28 figure