This thesis examines several regions of star formation selected because they were believed to be the youngest of their type. The sources studied are examples of low, medium
and high-mass star formation, namely B335, NGC2024 and DR21(OH).
High-resolution mapping of the thermal dust emission from B335 shows it to contain
a dense core elongated in a direction perpendicular to the orientation of the bipolar outflow. The core is sufficiently gravitationally bound to be able to collimate an initially isotropic wind in to the observed bipolar structure. The presence of an evolved outflow from a source of such low luminosity and high extinction means that it does not appear to fit the currently popular models for pro to stellar evolution.
CS and C34S J = 7 - 6 molecular line observations show that most of the six dense
cores, FIRl-6, in NGC2024 are too warm to be proto stellar condensations. Significant
CS 7 — 6 emission with excitation temperatures of 20 to 60 K has been found to be associated with all of the cores. Revised mass estimates lie between 2 and 11 M g, and
the corresponding luminosities require the presence of em bedded heating sources. FIR4
contains a highly reddened 2 γm source and a possible reflection nebula, and FIR6 has
a very wide CS 7 — 6 line that is probably the result of the interaction between a very
young outflow and the surrounding dense core.
A chain of dense cores situated in the DR21 cloud has been mapped in 1.3mm
thermal dust emission and the 2 — 1 and 1 — 0 transitions of CS and C34S. The sources
detected have sufficiently high luminosities that the cores must contain embedded young
stars. To account for the location of the masers near the brightest source it seems likely that there is a single 15 Mg star. The CS spectra indicate the presence of infalling
gas along the line-of-sight, and all the spectra arc relatively wide with high-velocity emission present towards the east. They have been fitted by a two component model,
with warm background gas and cooler foreground gas, and the results indicate that
the two components are physically associated. The mass of the cloud core is about 2000Mg, obtained from both the dust emission and the CS data, which together with
the CS linewidths indicates that the cloud is approximately in virial equilibrium.
All three sources are found to be quite young, although it seems likely tha t they
have already begun nuclear burning and so none is a “true protostar.” Suggestions
for future observations of these sources and for the identification of further candidate
protostars are also described.