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).