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We investigate the spatial distribution, the space velocities and agedistribution of the pre-main sequence (PMS) stars belonging to Ophiuchus, Lupusand Chamaeleon star-forming regions (SFRs), and of the young early-type starmembers of the Scorpius-Centaurus OB association. These young stellarassociations extend over the galactic longitude range from 280 deg. to 360deg., and are at a distance interval of around 100 and 200 pc. This study isbased on a compilation of distances, proper motions and radial velocities fromthe literature for the kinematic properties, and of basic stellar data for theconstruction of Hertzsprung-Russel diagrams. Although there was no well-knownOB association in Chamaeleon, the distances and the proper motions of a groupof 21 B- and A-type stars, taken from the Hipparcos Catalogue, lead us topropose that they form a young association. We show that the young early-typestars of the OB associations and the PMS stars of the SFRs follow a similarspatial distribution, i.e., there is no separation between the low and thehigh-mass young stars. We find no difference in the kinematics nor in the agesof these two populations studied. We analyze the different scenarios for thetriggering of large-scale star-formation that have been proposed up to now, andargue that most probably we are observing a spiral arm that passes close to theSun. The alignment of young stars and molecular clouds and the average velocityof the stars in the opposite direction to the Galactic rotation agree with theexpected behavior of star formation in nearby spiral arms.Comment: 14 pages, 14 postscript figures, accepted for publication in A&

Formation scenarios for the young stellar associations between galactic longitudes l $\mathsf{= 280\degr{-}360\degr}$