LOW-MASS PROTOSTARS AND PROTOSTELLAR STAGES - AN OBSERVATIONAL PERSPECTIVE

The use of sensitive receivers on large ground-based radiotelescopes such as the JCMT, the IRAM 30 m MRT, and the VLA has recently yielded significant progress In our observational understanding of low-mass protostars. Submillimeter continuum observations suggest that the youngest stellar objects detected in the near-/mid-IR range - the so-called Class I sources or ''infrared protostars'' - have only residual amounts of circumstellar material and are thus relatively evolved. At the same time, a smaller number of colder and more obscured YSOs - designated ''Class 0'' - characterized by virtually no emission below 10 mu m but strong submillimeter emission have been identified. These Class 0 or ''submillimeter protostars'' have not yet assembled the bulk of their final stellar mass, and correspond to the youngest protostar stage known to date (probable age less than or similar to 10(4) yr). Direct evidence for gravitational infall has been found in some of these sources confirming their protostellar nature. However, most (if not all) Class 0 protostars already drive highly collimated CO outflows.