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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3. Three-Dimensional Evolution of Early Solar Nebula
3. Three-Dimensional Evolution of Early Solar Nebula
Pages 31-43
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From page 31... ...
INITIAL CONDITIONS FOR PROTOSTELLAR COLLAPSE Mathematically speaking, solar nebula formation is an initial value problem. That is, it is believed that given the proper initial conditions and knowledge of the dominant physical processes at each phase, it should be possible to calculate the evolution of a dense molecular cloud core as it collapses to form a protosun and solar nebula.
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From page 32... ...
, where finite telescope resolution begins to limit the observations, molecular clouds are composed of centrally condensed cloud cores surrounded by cloud envelopes. The cloud cores are gravitationally bound and, should they begin to collapse upon themselves because of self-gravity, are quite likely to form stars (Myers and Benson 1983~.
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From page 33... ...
Finally, most three-dimensional calculations of protostellar collapse have ignored the possible importance of magnetic fields, in no small part because of the computational difficulties associated with their inclusion in an already formidable problem. OH Zeeman measurements of magnetic field strengths in molecular clouds yield values (~30pG)
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From page 35... ...
ANGULAR MOMENTUM TRANSPORT MECHANISMS Given the formation of a rotationally flattened, presolar nebula through the collapse of a cloud core that has avoided binary fragmentation, the next major dynamical problem is accumulating the protosun out of the disk
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From page 36... ...
Qualititativeh~r similar results hold when the initial cloud mass or initial angular velocity is decreased; binary formation is stifled. Diameter of region shown: (a)
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From page 37... ...
The remaining candidate for angular momentum transport is gravitational torques between nonaxisymmetric structures in the solar nebula. Possible sources of nonaxisymmetry include intrinsic spiral density waves (Larson 1984)
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From page 38... ...
(1981~. Extrapolated time scales for angular momentum transport, and hence nebula evolution, can be as short as ~103 years for strongly nonaxisymmetric models, or about ~106 - 107 years for less nonaxisymmetric models.
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From page 39... ...
Using a gas to dust ratio of 200:1 at 1 AU and 50:1 at 5 AU, these critical surface densities correspond to gas surface densities of 1500 g cm~2 at 1 AU and 750 g cm-2 at 5 AU. Similar minimum densities are inferred from reconstituting the planets to solar composition (Weidenschilling 1977~.
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From page 40... ...
The resulting nebulae become increasingly nonaxisymmetnc as the initial protosun mass is decreased; (b) forms trailing spiral arms that result in efficient transport of angular momentum, while (c)
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From page 41... ...
The most promising means for enhancing surface densities of the outer solar nebula appears to be through nebula evolution subsequent to formation. Viscous accretion disks can increase the surface density in the outer regions where the angular momentum is being deposited (tin and Bodenheimer 1982; Lissauer 1987~.
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From page 42... ...
1984b. Angular momentum transfer lay gravitational torques and the evolution of binary protostars.
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From page 43... ...
1985. Protostellar angular momentum transport by spiral density waves.
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