Post-t Tauri Stars: a False Problem

We consider the problem of the apparent lack of old T Tauri stars in low-mass star forming regions in the framework of the standard model of low-mass star formation. We argue that the similarity between molecular cloud lifetime and ambipolar diffusion timescale implies that star formation does not take place instantaneously, nor at a constant rate. We conclude that the probability of finding a large population of old stars in a star forming region is intrinsically very small and that the post-T Tauri problem is by and large not existent. Subject headings: ISM: clouds ISM: magnetic fields stars: formation stars: pre-main sequence