Low-mass protostars and protostellar stages

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 10µm 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 10⁴ 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.