The role of turbulent convection in the primitive solar nebula II. Results

Details The role of turbulent convection in the primitive solar nebula II. Results The role of turbulent convection in the primitive solar nebula II. Results

Mar 1987

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987icar...69..423c&link_type=abstract

Icarus, Volume 69, Issue 3, p. 423-457.

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Numerical results from a new model of the primordial solar nebula are presented in which convection is assumed to be the sole source of turbulence that causes the nebula to evolve. We introduce a new model of convective turbulence (described in detail in Paper I of this series) and new grain opacities computed from an improved physical model. The nebula is assumed to be in a stage prior to planetesimal formation in which gas and dust grains are mixed homogeneously, but in a stage after significant infall of matter from the outer cloud. Vertical structures for a thin nebular disk are calculated for different radii and accretion rates assuming vertical hydrostatic and thermal equilibrium; radial sequences of vertical solutions are constructed for constant accretion rates to represent quasistatic disk structures. Some aspects of our results differ markedly from those done previously by Lin and co-workers. Our values for the turbulent efficiency α (10-2 to 10-4) are much lower and much more sensitive to opacity and surface density. Our low values of α result in (1) small turbulent speeds (<=1% of sound speed), which will alter prior computations of grain coagulation and sedimentation rates; (2) a more massive disk (> 0.1Msolar) that becomes gravitationally unstable at outer (super-Uranian) orbits; (3) a lower ``best value'' of the accretion rate (~1018.5 g sec-1) and (4) a longer characteristic dispersal time for the disk (> 2 × 106years), which may greatly exceed that inferred from young stellar objects. The high sensitivity of α on surface density produces an inverse accretion rate-surface density relationship, which implies that the Lightman-Eardley diffusive instability develops througout a steady disk structure in the radial direction, causing the disk, at least at the onset, to separate into rings. Because radial gradients are neglected in the base disk structure, the manner in which the instability evolves to finite amplitude is unknown, but it could prevent the disk from reaching a quasistatic structure altogether. We conclude that convection may not be the dominant source of turbulence needed to evolve young solar/stellar nebulae, and may in fact be a disruptive mechanism in disk structure.

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