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 $\sim 10^{2}$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>∼</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> </mml:math> stars, or a region like the Carina Nebula with $\sim 10^{6}$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>∼</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>6</mml:mn> </mml:mrow> </mml:msup> </mml:math> 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 $\approx 50 - 90$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>≈</mml:mo> <mml:mn>50</mml:mn> <mml:mo>−</mml:mo> <mml:mn>90</mml:mn> </mml:math> 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 $G_{0} \sim 30 - 3000$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>G</mml:mi> <mml:mn>0</mml:mn> </mml:msub> <mml:mo>∼</mml:mo> <mml:mn>30</mml:mn> <mml:mo>−</mml:mo> <mml:mn>3000</mml:mn> </mml:math> during its birth and first $\approx 10$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>≈</mml:mo> <mml:mn>10</mml:mn> </mml:math> Myr of evolution. Based on the orbits of Kuiper Belt objects, the Solar System was subsequently exposed to a stellar density $\approx \, 100 \, M_{\odot } \, {\mathrm{pc}}^{-3}$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>≈</mml:mo> <mml:mspace/> <mml:mn>100</mml:mn> <mml:mspace/> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> <mml:mspace/> <mml:msup> <mml:mi>pc</mml:mi> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>3</mml:mn> </mml:mrow> </mml:msup> </mml:math> for $\approx 100$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>≈</mml:mo> <mml:mn>100</mml:mn> </mml:math> 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 $\times 10^{5}$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>×</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>5</mml:mn> </mml:mrow> </mml:msup> </mml:math> 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.