Electron fraction, X(e), as a function of H₂ density. The KDE of the underlying distribution was obtained from the data. The contour levels are drawn starting at 0.5σ and progressing outward in steps of 0.5σ where the σ levels are equivalent to that of a bivariate normal distribution. The analytic relations from McKee (1989) and Caselli et al. (2002b) as well as the result from chemical modeling of L1544 (Redaelli et al. 2021) are shown with orange, blue, and green curves, respectively. The analytic relations shown here do not take into account the local generation of cosmic rays.