Cosmic symplectite recorded irradiation by nearby massive stars in the solar system's parent molecular cloud

The Sun's astrophysical birth environment affected the formation and composition of the Solar System. Primitive meteorites display mass-independent oxygen isotope anomalies that were likely caused by ultraviolet (UV) photochemistry of CO gas-phase molecules, either (i) in the outer solar nebula by light from the young Sun or (ii) in the parent molecular cloud by light from nearby stars. However, measurements of oxygen isotopes alone cannot unambiguously constrain the UV spectrum of the source responsible for the photochemistry. Sulfur, with four stable isotopes, can be used as a more direct probe of the astrophysical environment of mass-independent photochemistry. Here, we report the in situ isotopic analysis of paired oxygen and sulfur isotope systematics in cosmic symplectite (COS), magnetite-pentlandite intergrowths, in the primitive ungrouped carbonaceous chondrite Acfer 094. We show that COS grains contain mass-independent sulfur isotope anomalies (weighted means of Δ³³S = +3.84 ± 0.72‰ and Δ³⁶S = −6.05 ± 2.25‰, 2SE) consistent with H₂S photochemistry by UV from massive O and B stars close to the Solar System's parent molecular cloud, and inconsistent with UV from the protosun. The presence of coupled mass-independent sulfur and oxygen (Δ¹⁷O = 86 ± 6‰, 2SE) isotope anomalies in COS imply that these anomalies originated in the same astrophysical environment. We propose that this environment is the photodissociation region (PDR) of the Solar System's parent molecular cloud, where nearby massive stars irradiated the edge of the cloud. We conclude that the Sun's stellar neighbors, likely O and B stars in a massive-star-forming region, affected the composition of the Solar System's primordial building blocks.