Probing the physical and chemical structure of the CS core in LDN 673.
  Multitransitional and continuum observations
  High-angular resolution observations of dense molecular cores show that these
cores can be clumpier at smaller scales, and that some of these clumps can also
be unbound or transient. The use of chemical models of the evolution of the
molecular gas provides a way to probe the physical properties of the clouds. We
study the properties of the clump and inter-clump medium in the starless CS
core in LDN 673 by carrying out a molecular line survey with the IRAM 30-m
telescope toward two clumps and two inter-clump positions. We also observed the
1.2-mm continuum with the MAMBO-II bolometer at IRAM. The dust continuum map
shows four condensations, three of them centrally peaked, coinciding with
previously identified sub-millimetre sources. We confirm that the denser clump
of the region, $n\sim3.6 \times10^5$\cmt, is also the more chemically evolved,
and it could still undergo further fragmentation. The inter-clump medium
positions are denser than previously expected, likely
$n\sim1\times10^3$--1$\times10^4$\cmt\ due to contamination, and are chemically
young, similar to the gas in the lower density clump position. We argue that
the density contrast between these positions and their general young chemical
age would support the existence of transient clumps in the lower density
material of the core. We were also able to find reasonable fits of the
observationally derived chemical abundances to models of the chemistry of
transient clumps.
