Thickness along the line of sight through the core L1251A estimated from the measured column densities N_NH3 and number densities n_H2 shown in Fig. 3. The mean abundance ratio of [ NH₃ ]/[ H₂ ] = 4.6 × 10^-8 (Dunham et al. 2011) is fixed for the whole filament. The (0, 0) map position is as in Fig. 1. We note that for the beam filling factor η< 1, the thickness is smaller (see Sects. 3 and 3.1 for details).