Astronomical constraints on nebular temperatures: Implications for planetesimal formation

Motivated by recent observations of T-Tauri stars and the interpretation of these observations in terms of the properties of circumstellar disks, we d erive internal (midplane) temperatures for disks around mature (age similar to 1 Ma) T-Tauri stars. The estimates are obtained by combining published results for disk masses, sizes, accretion rates, and surface temperatures. For 26 stars (for which adequate data are available), we derive midplane te mperatures at 1 AU primarily in the range 200-800 K, and 100-400 K at 2.5 A U. It is likely that the solar nebula, at the same stage of evolution, cont ained planetesimals and objects destined to become meteorite parent bodies. Observations of young stellar objects at earlier stages of evolution (age similar to 0.1 Ma) imply that accretion rates were, on the average, at leas t two orders of magnitude greater than the 10(-8) M./year rates typical for mature T-Tauri stars. Such high values would result in midplane temperatur es at or near the silicate vaporization temperature in the terrestrial plan et region. If cooling of the solar nebula from such a hot epoch was responsible for es tablishing the pervasive elemental fractionation patterns found in chondrit ic meteorites, then objects in the asteroid belt must have grown rapidly (w ithin 0.1 Ma) to sizes of similar to 1 km, a conclusion consistent with cur rent theories of planetesimal formation. However, the fact that primitive m eteorite parent bodies escaped being melted by the decay of Al-26 then impl ies that further growth of at least some objects was essentially delayed fo r 2 Ma or more. Such a diminished growth rate appears to be consistent with simulations of the dynamics of solid bodies in the asteroid belt. Other hypotheses seem less attractive. One might assume that the final cool ing occurred only after the decay of Al-26 (i.e., more than a million years after calcium-aluminum rich inclusion formation), or that Al-26 was not ub iquitous in the early solar system. But the first of these conjectures is i ncompatible with astronomical observations of T-Tauri systems, and the seco nd appears to be contradicted by the evidence for Al-26 in diverse meteorit ic components. The remaining alternative would then appear to be that, desp ite a lack of supporting evidence, chondritic fractionation patterns reflec t the net effect of many local heating and cooling events and have nothing to do with global nebular cooling. We conclude that the most plausible hypo thesis is that both nebular cooling and coagulation of solids to kilometer- sized objects occurred rapidly and that a substantial number of planetesima ls in the asteroid belt remained smaller than a few kilometers in radius fo r at least 2 Ma.