Line width vs. impact parameter {#sdv-b}
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Quiescent starless cores mapped in (1,1) and (1–0) lines typically show a “velocity coherent” zone of nearly constant line width ($\Delta v$) within the half-maximum contour map, followed by a $\Delta v$ rise at larger distances from core center (e.g. Goodman et al. [@gbw98]). There are however exceptions to this general trend, as pointed out by Caselli et al. ([@cbm02]). In particular, L1544 shows a significant increase of  and  (but not H$^{13}$CO$^+$ and DCO$^+$ ) line widths toward the center (factor of 1.5 within $\sim$ 50; Caselli et al. [@cas02a]). This increase has been interpreted as evidence of infall in the central few thousands AU, where CO and related species (such as H$^{13}$CO$^+$ and DCO$^+$) are heavily depleted. Indeed, the line–width increase is consistent with models of ambipolar diffusion (see Sect. \[skinematics\]).

The same trend has been observed in L1521F using  and lines, and in Fig. \[dv\_b\] we show the results obtained in (1–0) lines (the linewidth corrected for optical depth, or the intrinsic linewidth, is plotted). The figure also shows two theoretical predictions which will be discussed in Sect. \[skinematics\]. The decrease of the intrinsic line width with impact parameter in L1521F, 