1996A&A...309..155S

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 ≲10R_* from the star. If the gas has solar chemical composition, then the clump size must be L∼10¹¹cm (about R_*) and the density n_H>3x10¹²cm^–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 {dot}(M)_acc>3x10^–7M_☉ 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 n_H> 5x10⁸ cm^–3, L∼10¹¹ cm. The mass of the cloud is therefore of the order of 10²⁰gr, and the parental body must have a radius of 20km at least. These results show that both scenarios, i.e, magnetospheric accretion and evaporation of star-grazing planetesimal bodies, are in principle possible.