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

We review the observational data on prominent nearby star-formation regions like the Chamaeleon, rhoOph, Orion and Taurus-Auriga-Perseus, which stand out of the galactic plane, to show that there are strong arguments 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 that star formation by infall of high velocity clouds is a dominant process 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 braking or acceleration, and tend to form aligned groups of stars in a sequence of ages. The model reproduces many aspects of the morphology of the cloud complexes studied, as well as the relative position of young clusters of stars associated with them, and their age sequence. Our model constitutes an alternative to the traditional sequential star formation model. A number of implications of this new interpretation of the evolution of the nearby molecular clouds are briefly discussed.