The Sun’s Birth Environment: Context for Meteoritics

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 $\sim {10}^2$ $\sim 10^{2}$ stars, or a region like the Carina Nebula with $\sim {10}^6$ $\sim 10^{6}$ 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 ²⁶Al. 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 $\approx 50-90$ $\approx 50 - 90$ 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 $G_0\sim 30-3000$ $G_{0} \sim 30 - 3000$ during its birth and first $\approx 10$ $\approx 10$ Myr of evolution. Based on the orbits of Kuiper Belt objects, the Solar System was subsequently exposed to a stellar density $\approx 100M_{\odot }{\mathrm{pc}}^{-3}$ $\approx \, 100 \, M_{\odot } \, {\mathrm{pc}}^{-3}$ for $\approx 100$ $\approx 100$ 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 $\times {10}^5$ $\times 10^{5}$ 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 ²⁶Al.