LYMAN ALPHA EMISSION IN SPECTRA OF HERBIG AE STARS - AN INDICATION OF ACCRETION

Lyman alpha emission lines have been detected in the IUE spectra of several Herbig Ae stars, but not in those of Herbig Be stars. The shapes and positions of these lines are not consistent with those expected from a chromosphere and transition region (TR), which may be the origin of the other far UV emission lines in the spectra of these stars. The close similarity with the Ly alpha profile of 17 Lep, a low mass binary system in its first mass transfer phase, that of AE Aqr, a cataclysmic variable, and those of T Tauri stars suggests that the Ly alpha emission of the Herbig Ae stars is formed by recombination of infalling matter. The accretion rates derived from the Ly alpha fluxes of 17 Lep, AE Aqr and T Tauri stars are in good agreement with those obtained from other observations of these stars. The derived accretion rates of the Herbig Ae stars are similar to those obtained for T Tauri stars but 3 orders of magnitude lower than those of FU Ori and Z CMa. We have used the profile shapes to constrain the geometry of the infall. The corresponding accretion energies are amply sufficient to balance the radiative losses by the other UV emission lines, which are probably produced by a chromosphere and TR, and to drive the outflow from the young stars.