Distinct First- to Second-Peak Yield Ratios and Timescales Reveal a Subdominant Prompt r-process Channel

Stellar abundances reveal nonmonotonic [Y/Eu] and [Sr/Eu] evolution, a systematic decline with increasing [Eu/H] at low metallicity, a minimum at [Eu/H] ∼ −0.3, and then a rise at high metallicity. This behavior requires at least three distinct neutron-capture sources operating on different timescales. We develop a one-zone chemical-evolution model constraining their typical delay times, rates, and yield ratios. Reproducing the observed [Y/Eu] and [Sr/Eu] sequences requires a delayed r-process channel (most likely binary neutron-star (BNS) mergers) dominating Eu production (≳95% of total Eu). A prompt channel preferentially produces first-peak elements with minimal Eu, explaining the increasing [Y/Eu] at decreasing [Eu/H] below [Eu/H] ≲ −2.5, and delayed asymptotic giant branch s-process enrichment with delays greater than t_min = 0.3–0.6 Gyr reproduce the late-time upturn in Y (Sr). Our model quantitatively reproduces all constraints, including the large Δ[Y/Eu] ≈ 0.6 dex variation between the late-time rise [Eu/H] and the minimum value, the location of the minimum at [Eu/H] ∼ −0.3, and late-time rise. The first- to second-peak yield ratios correspond to [Y/Eu] ≈ −0.3 (prompt) and ≈−0.8 (BNS mergers). The observed Δ[Y/Eu] amplitude establishes a model-independent lower limit on the first- to second-peak yield ratio of ≳3.4 between the prompt and delayed channels, ruling out models with similar prompt and delayed yield ratios. These results demonstrate that explaining the observed heavy-element abundance patterns requires multiple channels with distinct nucleosynthetic signatures and operational timescales, providing constraints on the relative rates, delay times, and yield patterns of candidate production sites.