Comparison between the best-fit chemical ages (black solid line; the formula labeled) and the ages from a modified free-fall collapse model with different B values for a 1 M_⊙ cloud core (dotted, dashed, and dash-dotted lines). The gray region indicates an area twice that of the fit ages in order to cover the best-fit data points (black crosses) taken from Fig. B.1. The red vertical lines indicates a density of 7 × 10⁴ cm⁻³.