Abrupt changes in the C IV resonance lines of theta CR B.

Abstract

It is noted that the Theta CrB far-UV spectrum changes from conspicuous presence, to complete disappearance, to reappearance at a higher velocity than the previous one, have occurred with a phase lag of about 3 years after the disappearance of a weak shell phase of short duration which was observed in the visible in 1979. This sequence is interpreted in the framework of the Doazan and Thomas (1982) Be-star atmospheric model, as the dissipation and reformation of successive layers in the outer atmosphere. This dissipation proceeded from the outermost subionized layers to the innermost superionized layers. The C IV lines' reappearance at a higher velocity suggests an increase of mass flux and nonradiative energy flux. If Theta CrB's behavior resembles that of 59 Cyg, this set of circumstances could lead to the formation of a new, cool, H-alpha emitting/absorbing envelope.