Planetary Sciences American and Soviet Research/Proceedings from the U.S.-U.S.S.R. Workshop on Planetary Sciences (1991) / Chapter Skim
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2. Numerical Two-Dimensional Calculations of the Formation of the Solar Nebula
2. Numerical Two-Dimensional Calculations of the Formation of the Solar Nebula
Pages 17-30
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From page 17... ...
The initial mass distribution and angular momentum distribution in the core of a molecular cloud determine whether a binary system or a single star is formed. A relatively slower rotating and centrally condensed cloud is likely to collapse to a disk-like structure out of which planets can form.
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From page 18... ...
The close physical proximity of such cores with T Tauri stars, with imbedded infrared sources which presumably are protostars, and with sources with bipolar outflows, presumably coming from stars in a very early stage of their evolution, lends support to this hypothesis (Myers 1987~. by The specific angular momenta 0)
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From page 19... ...
suggests that single star formation occurs in initial clouds with j ~ 102i cm2 set. After reaching a rotationally supported equilibrium that is stable to fragmentation, the cloud becomes unstable to nonaxisymmetric perturbations, resulting in angular momentum transport and collapse of the central regions.
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From page 20... ...
A further goal is to predict the observational properties of the system at various times during the collapse. A full treatment would include a large number of physical effects: the hydrodynamics, in three space dimensions, of a collapsing rotating cloud tenth a magnetic field; the equation of state of a dissociating and ionizing gas of solar composition, cooling from molecules and grains in optically thin regions; frequency-dependent radiative transfer in optically thick regions; molecular chemistry; the generation of turbulent motions as the disk and star approach hydrostatic equilibrium; and the properties of the radiating accretion shock which forms at the edge of the central star and on the surfaces of the disk (Shu et al.
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From page 21... ...
The remaining infalling material passes though accretion shocks at the boundaries of the core and disk; most of the infall kinetic energy is converted into radiation behind the shock The surrounding infalling material is optically thick, and the object radiates in the infrared, with a peak at around 6()
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From page 22... ...
Possible transport processes in the disk include turbulent (convectively driven) viscosity, magnetic fields, and gravitational torques driven by gravitational instability in the disk or by non-axisymmetric instabilities in the initially rapidly rotating central star.
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From page 24... ...
Over a time scale of 3 x 104 years the temperature of material with the same specific angular momentum as that of the orbit of Mercury ranged from 400 600 K The predicted temperature for Jupiter remained fairly constant at 100 K, while that for Pluto approximated 15 K In the inner region of the nebula these temperatures are slightly cooler than those generally thought to exist during planetary formation or those calculated in evolving models of a viscous solar nebula (Ruden and Lin 1986~. A further calculation was made by Tscharnuter (1987)
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From page 25... ...
core, a circumstellar disk, and a surrounding infalling, dusty, and optically thick envelope. The radiation produced at the accretion shocks at the core and disk is reprocessed in the envelope, and emerges at the dust photosphere, primarily in the midinfrared.
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From page 26... ...
The initial density distribution is assumed to be a power law, the temperature is assumed to be isothermal at 20 K, and the angular velocity is taken to be uniform with a total angular momentum of 1053 g cm2 sol. Because the cloud is already optically thick at the initial state, the temperature increases rapidly once the collapse starts.
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From page 27... ...
Small, initial non-axisymmetric perturbations grow during the collapse, so that the central regions, on a scale of 10 AU, become significantly nonaxisymmetric even before a quasi-equilibrium configuration is reached. The deduced time scales for angular momentum transport depend on the initial conditions but range from 103 to 106 years for systems with a total mass of 1 Me.
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From page 28... ...
One possibility is the turbulence generated in the surface layers of the disk caused by the shear between disk matter and infalling matter. Another possibility is that the initial cloud had higher j, and the rapidly spinning central object is braked through a centrifugally driven magnetic wind which can remove the angular momentum relatively quickly (Shu et al.
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From page 29... ...
AMERICAN AND SOVIET RESEARCH 29 Bodenheimer, P., M Rozyczka, H.~!
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From page 30... ...
. Fundamental Problems in the Theory of Stellar Evolution (IAU Symposium 93)
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