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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4. Formation and Evolution of the Protoplanetary Disk
4. Formation and Evolution of the Protoplanetary Disk
Pages 44-60
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From page 44... ...
This value is highly dependent on the distribution of angular momentum within the cloud. This paper discusses a disk formation model during collapse of the protosolar nebula with J ~ 1052g cm2s-i, yielding a low-mass protoplanetary disL The disk begins to form at the growth stage of the stellar-like core and expands during accretion to the present dimensions of the solar system.
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From page 45... ...
The latter include the thermal pressure gradient, magnetic pull, and centrifugal forces. Data from infrared and radioastronomy tell us that the stars are formed in molecular clouds: low temperature regions (~ 10 K)
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From page 46... ...
IRAS discovered in the envelopes of stars c' Lyr, c' PsA, and ,5 Pic that the central region with a radius of 30 AU is dust-free (Beckman 1987~. This empty region could not have been retained after the star's formation stage, since the dust grains from the surrounding envelope shift inside under the Poynting-Robertson effect and fill up the empty space over a time scale of < 105 years.
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From page 47... ...
It holds for a core which has separated from the homogeneous rotating medium where the specific angular momentum of each cloud element is conserved. The second corresponds to a solid-state rotating, singular isothermal sphere.
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From page 48... ...
system is formed, the bulk of a cloud's angular momentum is concentrated in the orbital movement of stars relative to each other. The formation of a single star with a disk is an alternative and additional route by which a forming star expels excess angular momentum.
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From page 49... ...
Consequently, dissipation of rotational energy, accompanied by removal of the angular momentum to the periphery and its concentration in a low amount of mass, is necessary to form a single star with ~ protoplanetary disk as a cloud contracts. Effectiveness of angular momentum redistribution is the key issue of protoplaneta~y disk formation.
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From page 50... ...
With disruption of the core's axial symmetry, angular momentum removal to the periphery may also be carried out by the spiral density wave generated in the envelope (Yuan and Cassen 1985~. Consequently, the formation of a single, stellar-like core appears to be sufficient for the formation of a single star.
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From page 51... ...
1989~. EARLY EVOLUTION OF THE PROTOPLANETARY DISK Let us consider the stage of protosolar nebula contraction when a single, stellar-like core and a compact embryonic disk are formed in the center, both surrounded by an accretive shell.
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From page 52... ...
Near-sound turbulence produces disk growth to 103 AU. It is noteworthy that the process of disk growth occurs inside the protosolar nebula as it continues to contract.
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From page 53... ...
TEMPERATURE CONDITIONS AND CONVECTION IN THE PROTOPI~ETARY DISK The question of temperature distribution and fluctuation in the disk is important for an understanding of the physical and chemical evolution of preplanetary matter. Temperature greatly affects the kinetics of chemical reactions, matter condensation, and vaporization, the efficiency rate at which dust grains combine during collision, and the conditions within planetesimals.
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From page 54... ...
These models considered the further evolution of the protoplanetary disk after protosolar nebula matter has stopped precipitating on it. A large portion of disk mass is transported inside and accretes to the Sun at this stage.
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From page 55... ...
At the accretion stage of contraction of a protosolar nebula with J ~ 1052g cm2 s-i, energy is emitted at the shock front of the protosolar core's surface and the portion of the disk with Rk ~ 10~2 centimeters nearest to it (which is only several times greater than the core radius and two to three orders less than the radius of disk RD by the end of the accretion stage)
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From page 56... ...
Even where a developed convection is present, the flow of radiant energy inside the protoplanetary disk F is approximately three times greater than the convective value (tin and Papaloizou 1985~. This is similar to what occurs in hot accretion disks.
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From page 57... ...
(1985) for a protoplanetary disk taking into account the chemical composition and dimensions of dust grains, can be approximated as a function TV with various values of ~ in different temperature intervals.
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From page 58... ...
105 years. This model of protoplanetary disk formation predicts a significantly different solid matter thermal history: vaporization and subsequent condensation of dust grains in the internal region of the solar system as it forms, and the conservation of interstellar dust (including organic compounds and ices)
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From page 59... ...
1981. On the role of magnetic fields and turbulence in the evolution of the presolar nebula.
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From page 60... ...
1985. Protostellar angular momentum transport by spiral density waves.
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