Evolution of two axes of the ellipsoid for different tidal field strengths and perturbation densities. The tidal field strength is characterized by μ_c = 5 (top) and μ_c = 30 (bottom). On the left, the initial density of the structure is equal to the critical density, ρ_HC, computed by Hennebelle & Chabrier (2008). Thus, the axis does not evolve in the absence of an external tidal field (dotted black line). On the right, the density is given as ρ = 0.9ρ_HC + 0.1ρ_c. Depending on the strength of the tidal field, the three axes may or may not collapse: if the tidal field is not too strong, the c axis collapses after a period of expansion at the beginning of the evolution.