data, although the (1–0) extended emission partially hides the features arising from the central region.

[lccccc]{} line & V$_{LSR}$ & $\Delta$V & $\tau$ & $\int T_{\rm{MB}}dV ^b$& T$_{\rm ex}$\
& km s$^{-1}$ & km s$^{-1}$& & K km s$^{-1}$ & K\
(1-0) & 6.472$\pm$0.001&0.299$\pm$0.001&17.9$\pm$0.1&5.859$\pm$0.016&4.94$\pm$0.03\
(3-2) & 6.388$\pm$0.010$^a$&0.290$\pm$0.030& 1.4$\pm$1.0&0.692$\pm$0.075&5.1$\pm$3.7\
(2-1) & 6.505$\pm$0.004&0.268$\pm$0.010& 2.2$\pm$0.4&0.978$\pm$0.026&4.6$\pm$0.9\
(3-2) & 6.507$\pm$0.006&0.222$\pm$0.024& 1.2$\pm$0.9&0.332$\pm$0.029&4.1$\pm$3.2\
\
\

\[linepar\]

Fig.  \[fig1b\] shows that L1521F has in common with several other cores (see also Tafalla et al. [@taf02]; Caselli et al. [@cas02a],[@cas02b]) the property that while the maps of the nitrogen bearing molecules have a similar appearance to those observed in dust emission, maps in rare CO isotopomers such as  show no correlation with the dust. In fact, the distribution of (1–0) and (2–1) is essentially flat over the area we have mapped, suggesting perhaps that the layer sampled in  is either in the background or foreground relative to that seen in dust emission.

We have been able to make some estimate of the optical depths of the observed  lines by comparison with our 9-point  map (20 spacing centred on (-5,-5)). From the integrated intensity ratio of the  and (1–0) lines and assuming an intrinsic abundance ratio \[\]/\[\] of 3.65 (from 