Primordial heavy-element mass fraction vs. mass. The panel in the top right corner shows the atmospheric metallicity vs. time for these curves, created with the analytic model from KH24 (for details, see Appendix G). The orange curves are consistent with Z_atm = 0.12 ± 0.02 at (3.2 ± 1.6)  ×  10⁸ yr (bold mock data point). The red curves are in addition consistent with with Z_atm = 0.1 ± 0.02 at (3.2 ± 1.6)  ×  10⁶ yr and Z_atm = 0.15 ± 0.02 at (3.2 ± 1.6)  ×  10⁹ yr (thin mock data points).