2.4 lower) spatial resolution[^1], and consistent perhaps with the idea that  is abundant in a shell outside but close to the dust peak. However, given that the LVG method assumes homogeneous conditions, the LVG–derived densities are averages along the line of sight, thus lower values are expected when compared to the continuum data analysis, which takes into account the core density structure.

The deuterium fractionation is directly estimated from the $N(\NTDP )/N(\NTHP )$ column density ratio ($\equiv$ $R_{\rm deut}$), and the $R_{\rm deut}$ map in L1521F, assuming CTEX conditions, is shown in Fig. \[Dfrac\_map\]. $R_{\rm deut}$ ranges between $\sim$ 0.02 at the core edge to 0.1 in a region about 20 in size and centered on the dust peak position. The peak value of $R_{\rm deut}$ is about a factor of 2 smaller than that found in L1544 (Caselli et al. [@cas02b]). We note that the column density of  is similar in the two cores, and that the factor of 2 of difference in deuterium fractionation is due to the (factor of 2) larger  column density in L1544. This suggests that, although the two cores are similar in structure, L1521F is probably slightly less evolved than L1544 (see Sect. \[sdiscussion\]).

