Fractional mass enclosed within a sphere of radius r = R for deprojected Sérsic models of different intrinsic axis ratios. From left to right: enclosed M_sph is plotted as a function of log radius (relative to the projected 2D effective radius, R_e), assuming intrinsic axis ratios of q₀ = 1, 0.4, 0.2, respectively. The colored curves denote the enclosed mass profiles for Sérsic indices from n = 0.5 to n = 8 (yellow to purple). The vertical lines denote R = R_e (grey dashed) and R = 1.3R_e (≈r_{1/2, mass, 3D} for q₀ = 1; grey dash-dotted), and the horizontal colored lines denote the fraction of the mass enclosed within r = R_e for n = 1, 4 (lime, teal dashed, respectively) and 50% of the total mass (grey dashed dotted). For q₀ = 1, the half-mass 3D spherical radius is indeed r_{1/2, mass, 3D} ≈ 1.3R_e regardless of n, as in Ciotti (1991). For flattened (i.e., oblate) systems, the half-mass 3D spherical radius is smaller, and approaches R_e as q₀ decreases. See also Fig. 2.