STAR-FORMATION BY INFALL OF HIGH-VELOCITY CLOUDS ON THE GALACTIC DISK

We review the observational data on prominent nearby star-formation re gions like the Chamaeleon, rhoOph, Orion and Taurus-Auriga-Perseus, wh ich stand out of the galactic plane, to show that there are strong arg uments in favor of a common explanation for the displacement of these molecular clouds with respect to the galactic plane in terms of impact of high-velocity clouds on the gas of the galactic disk. We propose t hat star formation by infall of high velocity clouds is a dominant pro cess in the solar neighborhood. With a simple hydrodynamical model we show that recently formed stars tend to separate from the gas in which they formed, since they are no longer subject to hydrodynamical braki ng or acceleration, and tend to form aligned groups of stars in a sequ ence of ages. The model reproduces many aspects of the morphology of t he cloud complexes studied, as well as the relative position of young clusters of stars associated with them, and their age sequence. Our mo del constitutes an alternative to the traditional sequential star form ation model. A number of implications of this new interpretation of th e evolution of the nearby molecular clouds are briefly discussed.