THE SPITZER c2d SURVEY OF LARGE, NEARBY, INTERSTELLAR CLOUDS. XI. LUPUS OBSERVED WITH IRAC AND MIPS

We present c2d Spitzer IRAC observations of the Lupus I, III, and IV dark clouds and discuss them in combination with optical, near-infrared, and c2d MIPS data. With the Spitzer data, the new sample contains 159 stars, 4 times larger than the previous one. It is dominated by low- and very low mass stars, and it is complete down to M approximate to 0.1 M-circle dot. We find 30%-40% binaries with separations between 100 and 2000 AU with no apparent effect on the disk properties of the members. A large majority of the objects are Class II or III objects, with only 20 (12%) Class I or flat-spectrum sources. The disk sample is complete down to debris -like systems in stars as small as M approximate to 0.2 M-circle dot and includes substellar objects with larger IR excesses. The disk fraction in Lupus is 70%-80%, consistent with an age of 1-2 Myr. However, the young population contains 20% optically thick accretion disks and 40% relatively less flared disks. A growing variety of inner disk structures is found for larger inner disk clearings for equal disk masses. Lupus III is themost centrally populated and rich, followed by Lupus I with a filamentary structure and by Lupus IV, where a very high density core with little star formation activity has been found. We estimate star formation rates in Lupus of 2-10 M-circle dot Myr(-1) and star formation efficiencies of a few percent, apparently correlated with the associated cloud masses.