The symbiotic star CH Cygni. The broad Lyα emission line explained by shocks

Context: In 1985, at the end of the active phase 1977-1986, a broad (4000 km s-1) Lyα line appeared in the symbiotic system CH Cygni that had never been observed previously. Aims: In this work we investigate the origin of this anomalous broad Lyα line. Methods: We suggest a new interpretation of the broad Lyα based on the theory of charge transfer reactions between ambient hydrogen atoms and post-shock protons at a strong shock front. Results: We have found that the broad Lyα line originated from the blast wave created by the outburst, while the contemporary optical and UV lines arose from the nebula downstream of the expanding shock in the colliding-wind scenario.