This paper discusses the origin of the redshifted absorption components observed in the Na D lines of some Herbig Ae/Be stars. We have computed non-LTE models of the thermal and ionization structure of gas clouds of different density, column density and chemical composition, from the solar one to that typical of CI-chondrites. The redshifted absorption lines can only form in small, dense, infalling gas clumps at distances ≲ 10 R, from the star. If the gas has solar chemical composition, then the clump size must be L ∼ 1011 cm (about R(Black star sign)) and the density nH ≳ 3 × 1012 c-3. These conditions can be produced at the base of a column of gas falling into the star from a circumstellar accretion disk along magnetic lines. In this case, an accretion rate Macc ≳ 3 × 10-7 M⊙ yr-1 and a stellar magnetic field of about 600 Gauss are required. As the gas metallicity increases, less dense clouds are required to fit the Na D observations. In the extreme case of a gas cloud resulting from the evaporation of CI-chondrite meteorites, it is nH ≳ 5 × 108 cm-3, L ∼ 1011 cm. The mass of the cloud is therefore of the order of 1020 gr, and the parental body must have a radius of 20 km at least. These results show that both scenarios, i.e, magnetospheric accretion and evaporation of star-grazing planetesimal bodies, are in principle possible.