The Sun’s Birth Environment: Context for Meteoritics

Abstract Meteorites trace planet formation in the Sun’s protoplanetary disk, but they also record the influence of the Sun’s birth environment. Whether the Sun formed in a region like Taurus-Auriga with 10^2 ∼ 10 2 stars, or a region like the Carina Nebula with 10^6 ∼ 10 6 stars, matters for how large the Sun’s disk was, for how long and from how far away it accreted gas from the molecular cloud, and how it acquired radionuclides like 26 Al. To provide context for the interpretation of meteoritic data, we review what is known about the Sun’s birth environment. Based on an inferred gas disk outer radius 50 - 90 ≈ 50 − 90 AU, radial transport in the disk, and the abundances of noble gases in Jupiter’s atmosphere, the Sun’s molecular cloud and protoplanetary disk were exposed to an ultraviolet flux G₀ 30 - 3000 G 0 ∼ 30 − 3000 during its birth and first 10 ≈ 10 Myr of evolution. Based on the orbits of Kuiper Belt objects, the Solar System was subsequently exposed to a stellar density \, 100 \, M \, pc^-3 ≈ 100 M ⊙ pc − 3 for 100 ≈ 100 Myr, strongly implying formation in a bound cluster. These facts suggest formation in a region like the outskirts of the Orion Nebula, perhaps 2 pc from the center. The protoplanetary disk might have accreted gas for many Myr, but a few 10^5 × 10 5 yr seems more likely. It probably inherited radionuclides from its molecular cloud, enriched by inputs from supernovae and especially Wolf-Rayet star winds, and acquired a typical amount of 26 Al.