The symbiotic star CH Cygni. II. The broad Ly alpha emission line explained by shocks

TL;DR

This paper explains the origin of a broad Ly alpha emission line in the symbiotic star CH Cygni as resulting from shock-induced charge transfer reactions at a blast wave created by an outburst.

Contribution

It introduces a novel interpretation of the broad Ly alpha line based on shock physics and charge transfer reactions, linking it to the blast wave from the star's outburst.

Findings

Broad Ly alpha line originated from the blast wave created by the outburst.

Optical and UV lines arose from the nebula downstream of the shock.

Charge transfer reactions explain the broadening of the Ly alpha line.

Abstract

Context. In 1985, at the end of the active phase 1977-1986, a broad (4000 km/s) Ly alpha 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 Ly alpha line. Methods. We suggest a new interpretation of the broad Ly alpha 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 Ly alpha 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.