  A multitransitional study with the BIMA interferometric array was carried out
toward the starless core found in the L673 region, in order to study the
small-size structure of the cores detected with previous single--dish
observations, which provides us with a test of the predictions of the chemical
model of Taylor et al. (1996; 1998). We detected emission in the CS (2-1), N2H+
(1-0), and HCO+ (1-0) lines. Several clumps of size ~0.08 pc were found for
each line distributed all over the region where previous single-dish emission
was found (Morata et al. 1997). Each molecular transition traces differently
the clump distribution, although in some cases the detected clumps are
coincident. The distribution of the N2H+ emission and the single-dish NH3
emission are coincident and compatible with an origin in the same gas. The
large fraction of missing flux measured for the CS (2-1) transition can be
explained if the cloud is formed by a clumpy and heterogeneous medium. Four
positions were selected to derive the abundance ratios [N2H+/CS] and [HCO+/CS]
from the molecular column density determinations, and to compare them with the
values predicted by the chemical model. The model was able to explain the
interferometric observations, and, in particular, the chemical differentiation
of the detected clumps and the coincidence of the NH3 and N2H+ emissions. The
lack of HCO+ towards the two selected positions that trace the more evolved
clumps cannot be accounted for by the model, but it is possibly due to strong
self-absorption. We propose a classification of the studied clumps according to
the stage of chemical evolution indicated by the molecular abundances.
