The Lupus Clouds

The Lupus clouds compose one of the main low-mass star forming complexes within 200 pc of the Sun. They contain four main star forming sites, including the rich T Tauri association in Lupus 3. They are located in the Scorpius-Centaurus OB association, whose massive stars are likely to have played a significant role in the evolution and perhaps the origin of the complex. The entire variety of objects related to the various stages of early stellar evolution are represented in Lupus, including some of the best studied T Tauri stars like RU Lup. The determination of many properties of the clouds, as well as their associated stellar population, are limited by the uncertainty with which the distance is known. We argue that a single value of the distance is probably inadequate to be representative of the entire complex, and that depth effects are likely to be significant, as expected from its large extent on the plane of the sky. The total mass of molecular gas in the complex is a few times 10^4 M_⊙. The most distinctive property of its stellar population is the outstanding abundance of mid M-type pre-main sequence stars. Some likely substellar objects have been identified as well, particularly thanks to mid-infrared observations carried out with the Spitzer Space Observatory. A widely distributed young low-mass population of weak-line T Tauri stars, identified by its X-ray emission, is observed in the direction of Lupus but it is probably related to the Gould Belt rather than to the star forming complex. A few individual objects of particular interest are briefly reviewed: the Herbig Ae/Be stars HR 5999 and HR 6000 that dominate the Lupus 3 cloud, the classical T Tauri star RU Lup, the EXor class prototype EX Lup, the very low luminosity, outflow-driving sources HH 55 and Par-Lup3-4, the extreme emission-line star Th 28, and the binary object GQ Lup composed of an ordinary T Tauri star with a substellar companion of possible planetary mass.