6.1. Distinctions of sizes
There are some distinctions between protostellar disks and interstellar cloudlets. A protostellar object often has a flattened dense disk out to 50 - 100 AU and an infalling nebular envelope out to re 2000 AU (re = 1400 AU in Weintraub et al. 1992; re 2000 AU in Heyer et al. 1996; re = 1400 - 10000 AU in Whitney et al. 1997). Here 2000 AU ~ 3 × 1011 km 0.01 pc. Thus, a protostellar disk (0.001 - 0.01 pc) differs in size from a cloudlet (0.1 - 1 pc). Second, a protostellar disk will have exactly one protostar (emitting at IR wavelengths), while a cloudlet can have either none, one, or maybe two protostellar disks within a total size < 1 pc. Third, a cloudlet may have associated gas distributed (i) partly in a circumstellar envelope or shell < 200 AU, (ii) partly in a flat halo above and below a protostellar disk < 2000 AU, and (iii) partly in a slightly distant interstellar volume < 1 pc (weakly or not yet gravitationnally bound).
Below, these objects will be lumped together, since their small sizes fit completely inside a radio telescope beam (a 10 arcsec beam corresponds to a size of 0.05 pc at a nearby distance of 1 kpc; or a 0.5 pc size at a distance of 10 kpc).