He-3 Constraints

Helium-3 presents a complementary puzzle to deuterium in BBN: standard BBN predicts a primordial ³He/H ratio of roughly 10⁻⁵, and unlike deuterium — which is destroyed in stellar interiors — helium-3 is both produced and destroyed in stars depending on mass and evolutionary state. Low-mass stars produce net ³He in their interiors and can return it to the interstellar medium through planetary nebula ejection, yet galactic chemical evolution models that include this source cannot reproduce the observed flat or slowly increasing ³He abundance with Galactic radius without requiring implausibly high stellar yields. The solar ³He abundance and the protosolar value inferred from meteorites are consistent with local ISM measurements but sit in tension with the BBN prediction when chemical evolution is accounted for, suggesting either the BBN yield is too high or stellar returns are more efficient than models predict.

Successive Collision Theory addresses the helium-3 constraint through the pre-existing stellar populations that contributed to the debris field. In SCT, intermediate-mass stars from the pre-existing pockets had already processed their interiors through the hydrogen-burning shell phases that generate ³He, and many of these stars were in or near their asymptotic giant branch phases at the time of the collision. The collision thermalization disrupted their stellar envelopes and mixed their ³He-enriched material into the interstellar gas of the debris field, contributing a net ³He injection that was not tracked by any post-collision stellar evolution model. This pre-collision ³He injection partially offsets the BBN-predicted primordial abundance by adding processed material before the chemical evolution clock starts in standard models, effectively mimicking a lower net ³He yield from post-collision stellar evolution.

The flat ³He gradient across the Galactic disk — which is the observational puzzle for chemical evolution models — is explained in SCT by the initial homogeneous mixing of pre-existing ³He-rich material throughout the collision debris field before post-collision stellar evolution began. The debris field established a relatively uniform ³He baseline across the full spatial extent of what became the Galactic disk, and subsequent stellar processing modified this baseline similarly across all radii, preserving the flat radial gradient. SCT predicts that the ³He abundance in the most pristine regions of the intergalactic medium — far from any post-collision stellar processing — should approach the BBN prediction from below, reflecting the combined primordial and pre-collision stellar contributions, while regions enriched by post-collision stellar activity should show the small positive gradient expected from standard chemical evolution applied on top of this elevated baseline.