3,097 research outputs found
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 or ). 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 Fe and Al and the Stable Oxygen Isotopes
Short-lived radioisotopes (SLRI) such as Fe and 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 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 10% in both the inner ( 10 AU) disk
regions, and to even lower levels ( 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 ( 5 to 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
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