Magnetic field strength as a function of the number density. The blue and orange line indicate the magnetic field strength – density relation expected from gravitational collapse for the cases of k = 1/2 and k = 1/3 based on Eq. (1). Ambipolar diffusion is assumed to set in at a density of 10¹⁰ cm⁻³, while recoupling is assumed to occur at densities of 10¹⁴ − 10¹⁷ cm⁻³ following Nakano (1988). We subsequently assume an approximately spherical contraction with B ∝ n^2/3. The indigo dots show the equipartition field strength for fully convective models, the violet dots for models with a radiative core. Green triangles are surface magnetic field strength observed in intermediate-mass T Tauri stars from the sample of Lavail et al. (2017).