| Summary: | Using various molecular line and continuum emission criteria, we have selected a
sample of six isolated, dense concentrations of molecular gas, i.e., “cores”, which are
either starless (L694-2, L429, L1517B, and L1689-SMM16) or contain a protostellar
Very Low Luminosity Object (VeLLO) and are currently experiencing gravitational
collapse (L1014 and L1521F). Studying the molecular emission from dense gas tracers
toward this sample of cores will help us gain a more detailed image of the internal
physical conditions of dense cores and their evolution.
We observed the cores in our sample in NH3 (1,1) and (2,2) emission using the
Green Bank Telescope (GBT) and in N2H+ (1−0) emission using the Nobeyama
Radio Observatory (NRO). L429 shows the most complicated structure among the
cores in our sample. Also, the maxima of molecular line integrated intensities and
dust continuum emission toward L429 show a significant offset. The rest of the cores
in our sample are roughly round and the morphologies of line integrated intensities
follow that of the corresponding continuum emission closely. Cores in our sample
have gas kinetic temperatures ∼ 9 − 10 K and therefore show comparable thermal
velocity dispersions. L429 and L1517B are, respectively, the most turbulent and most
quiescent cores in our sample. Finally, L1521F is the most centrally concentrated core
of our sample.
L1689-SMM16 is the least previously studied core in our sample and had not
yet been probed in molecular emission. Jeans and virial analyses made using updated
measurements of core mass and size confirm that L1689-SMM16 is prestellar,
i.e., gravitationally bound. It also has accumulated more mass compared to its corresponding
Jeans mass in the absence of magnetic fields and therefore is a “super-Jeans”
core. The high levels of X(NH3)/X(N2H+) and deuterium fractionation reinforce the
idea that the core has not yet formed a protostar. Comparing the physical parameters
of the core with those of a Bonnor-Ebert sphere reveals the advanced evolutionary
stage of L1689-SMM16 and shows that it might be unstable to collapse. We do not
detect any evidence of infall motions toward the core, however. Instead, red asymmetry
in the line profiles of HCN (1−0) and HNC (1−0) indicates expansion of the
outer layers of the core at a speed of ∼ 0.2 − 0.3 km s−1. For a gravitationally bound
core, expansion in the outer layers might indicate that L1689-SMM16 is experiencing
oscillations.
Radiative transfer modelling of NH3 emission toward L694-2 and L1521F at low
and high spatial resolutions show that the less evolved core, L694-2, is best described
by relatively constant radial profiles of temperature and fractional NH3 abundance.
On the other hand, L1521F, which contains a protostellar VeLLO, is best described
by a radial abundance profile that is enhanced toward the core centre and a radial
temperature profile that decreases toward the core centre. Comparison of our results
with previous studies on L1544, a well-studied starless core, imply that as dense
cores evolve and progress toward the moment of collapse, they become more centrally
concentrated. As a result, the gas temperatures at their centres decrease, leading to
increase in levels of CO depletion factor and increase in NH3 fractional abundance
toward the centre. === Graduate
|
|---|
