INFALL IN HERBIG-AE BE STARS - WHAT NA-D LINES TELL US/

This paper discusses the origin of the redshifted absorption component s observed in the Na D lines of some Herbig Ae/Be stars. We have compu ted non-LTE models of the thermal and ionization structure of gas clou ds of different density, column density and chemical composition, from the solar one to that typical of CI-chondrites. The redshifted absorp tion lines can only form in small, dense, infalling gas clumps at dist ances less than or similar to 10 R(star) from the star. If the gas has solar chemical composition, then the clump size must be L similar to 10(11) cm (about R(star)) and the density n(H) greater than or similar to 3 x 10(12) cm(-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 M(acc) grea ter than or similar to 3 x 10(-7) M. yr(-1) and a stellar magnetic fie ld of about 600 Gauss are required. As the gas metallicity increases, less dense clouds are required to fit the Na D observations. In the ex treme case of a gas cloud resulting from the evaporation of CI-chondri te meteorites, it is n(H) greater than or similar to 5 x 10(8) cm(-3), L similar to 10(11) cm. The mass of the cloud is therefore of the ord er of 10(20) gr, and the parental body must have a radius of 20 km at least. These results show that both scenarios, i.e, magnetospheric acc retion and evaporation of star-grazing planetesimal bodies, are in pri nciple possible.