Chondrules as Astrophysical Objects 2017 (LPI Contrib. No. 1975) 2015.pdf

COMPLEMENTARITY AND THE FORMATION OF PARENT BODIES: A WINDOW ON DUST COAGULATION. A. Hubbard and M.-M. Mac Low, Department of Astrophysics, American Museum of Natural History ([email protected]; [email protected])

Introduction: Abundances in bulk are matrix agglomerations would signal a different adhe- roughly solar, despite large differences between chon- sion regime than mere agglomerations of sub-micron drules and matrix, and varying fractions presolar grains. [1,2,3]. This complementarity implies that the chon- Parent body assemblage (direct sweep-up): A drules and matrix within a given chondrite derive from small can sweep up free-floating dust a common reservoir of material, ruling out multiple grains with its geometrical cross section as long as the reservoir explanations. Further, chondrule sizes in dust grains are large enough not to be affected by the chondrites are quite heterogenous [4], ruling out care- gas flow around the body. Matrix grains were presum- fully tuned models for parent body formation. Thus, ably similar to chondrule precursor grains, and thus complementarity also constrains how chondrite parent comparable in mass to , while having vol- bodies were assembled, and from what pieces: if the ume filling factors of perhaps 12% [9]. The matrix parent body assemblage process filtered out or prefer- grains associated with CO chondrites’ small chon- entially included either chondrules or matrix, the corre- drules would then interact with the gas flow around lation between the two would be destroyed. exceeding a mere 10 km in radius. Complementarity: The bulk elemental and iso- In this scenario, complementarity allows chon- topic abundances of chondritic meteorities are flat drule-matrix agglomerations as in the SI case, but it across chondrite classes [1,2,3], even though the sub- also allows independent matrix and chondrule grains components both have and are present in quite differ- so long as neither population is significantly settled ent abundances. Thus chondrules and matrix within a compared with the other (which would upset the local given chondrite were co-genetic, drawn from a single complementarity). reservoir of near-solar composition in a fashion that Parent body assemblage (pebble ): preserved that composition. This also restricts the de- Once the planetesimal grows sufficiently, the gas flow lay between chondrule formation and chondrite assem- around the body can both trap and deflect dust grains bly to being shorter than the viscous timescale in the of appropriate sizes [10]. The heterogeneity of chon- disk [5]. drule sizes and chondrule-to-matrix ratios, combined Parent body assemblage (Streaming Instability): with ’s aerodynamical sorting requires The Streaming Instability (SI, [6]) allows direct for- the production of condrule-matrix agglomerations to mation of chondrite parent bodies through the gravita- maintain complementarity, as in the case of the SI. tional collapse of dense dust clouds in the Solar Nebu- Conclusions: The need to maintain complementa- la. Once gravitational collapse begins, it is too late to rity through chondrite parent body assembly implies mix in either matrix or chondrule material, so both that chondrules were not strongly settled compared must already be in the cloud. Given instances of the SI with non-thermally processed material (aka pro- select for dust grains with specific aerodynamical matrix). It further strongly suggests the formation of stopping times, so given the spread in chondrule size intermediate chondrule-matrix agglomerations larger ranges and chondrule-to-matrix ratios between chon- than chondrule precursors, suggesting that dust coagu- drites, complementarity requires both that the dust lation in the presence of chondrules is different than in clouds were made up of chondrule-matrix agglomera- their absence. That suggests that chondrule or chon- tions, and that the clouds were representative of the drule-like objects provide a path to the formation of overall solid mass reservoir. terrestrial planets. Naked chondrules are too small to trigger the SI, so the need for chondrule-matrix agglomerations is not References: [1] Bland et al. (2005) PNAS, 102, unexpected [7]. More surprising would be for the SI 13755-13760. [2] Hezel D. C. and Palme H. (2010) generated dust cloud to be representative of the overall Earth & Planet. Sci. Letters, 294, 85-93. [3] Budde G. solid mass reservoir. The chondrules and especially the et al. (2016) PNAS, 113, 2886-2891. [4] Friedrich, J. chondrule-matrix agglomerations would be expected to M. et al. (2015) Chemie der Erde-Geochemistry 75, have been significantly settled compared to free- 419-443. [5] Goldberg A. et al. (2015) MNRAS, 452, floating matrix material. A settled population of large 4054-4069. [6] Johansen A. et al. (2007) Nature, 448, dust grains can trap smaller grains [8], but this requires 1022-1025. [7] Hubbard A. and Ebel D. S. (2015) Ica- adhesion, and chondrule precursors presumably sam- rus, 245, 32-37. [8] Krijt, S. and Ciesla, F. J. (2016) pled the largest free-floating grains. Thus, chondrule- ApJ, 822, 111. [9] Güttler C. et al. (2010) A&A, 513, Chondrules as Astrophysical Objects 2017 (LPI Contrib. No. 1975) 2015.pdf

A56. [10] Ormel, C. W. and Klahr, H. H. (2010) A&A 520, A43.