The ionization equilibrium of the gas in the shells surrounding Herbig Ae/Be stars has been studied. The opaque circumstellar dust clumps which could be responsible for the brightness variations of these stars were included in the consideration.
The column density of the ions C I, C II, Ca I, Ca II, Ca III, Na I, Na II and the equivalent width of the Na I D2 and Ca II K lines have been calculated. It was found that in the shells without clumps strong lines could originate if the gas density is enhanced, for example, because of a strong deceleration of the wind. But the lines is expected to be slightly shifted from their rest wavelengths.
The lines forming in the clumps at the distances larger than several AU from the star must have variable profiles. They can be either blueshifted or redshifted in dependence on the clump orbit orientation relative to the line of sight. It was shown that the column density and equivalent width were mainly determined by clump opacity, the gas to dust ratio, the element depletion and the velocity of large-scale turbulent motions in the clump. If the size of the clump is large enough, appearance (or strengthening) of absorption lines should be accompanied by stellar brightness variations.
© European Southern Observatory (ESO) 1997
Online publication: June 5, 1998