Chemical and Physical Conditions in Molecular Cloud Core DC 000.4-19.5
  (SL42) in Corona Australis
  Chemical reactions in starless molecular clouds are heavily dependent on
interactions between gas phase material and solid phase dust and ices. We have
observed the abundance and distribution of molecular gases in the cold,
starless core DC 000.4-19.5 (SL42) in Corona Australis using data from the
Swedish ESO Submillimeter Telescope. We present column density maps determined
from measurements of C18O(J=2-1,1-0) and N2H+(J=1-0) emission features.
Herschel data of the same region allow a direct comparison to the dust
component of the cloud core and provide evidence for gas phase depletion of CO
at the highest extinctions. The dust color emperature in the core calculated
from Herschel maps ranges from roughly 10.7 to 14.0 K. This range agrees with
the previous determinations from Infrared Space Observatory and Planck
observations. The column density profile of the core can be fitted with a
Plummer-like density distribution approaching n(r) ~ r^{-2} at large distances.
The core structure deviates clearly from a critical Bonnor-Ebert sphere.
Instead, the core appears to be gravitationally bound and to lack thermal and
turbulent support against the pressure of the surrounding low-density material:
it may therefore be in the process of slow contraction. We test two chemical
models and find that a steady-state depletion model agrees with the observed
C18O column density profile and the observed N(C18O) versus AV relationship.
