The symbiotic system AG Draconis. Soft X-ray bremsstrahlung from the nebulae

TL;DR

This paper models the emission spectra of the symbiotic system AG Draconis, revealing how shock interactions produce soft X-ray bremsstrahlung and explaining observed spectral features during outbursts.

Contribution

It introduces a detailed model linking wind collisions, shock velocities, and spectral emissions, enhancing understanding of outburst phenomena in symbiotic systems.

Findings

Soft X-ray bremsstrahlung originates from nebulae behind reverse shocks.

Outburst debris absorbs soft X-ray flux, affecting spectral lines.

Dust and molecules trap elements, influencing spectral and brightness variations.

Abstract

The modeling of UV and optical spectra emitted from the symbiotic system AG Draconis, adopting collision of the winds, predicts soft X-ray bremsstrahlung from nebulae downstream of the reverse shock with velocities > 150 km/s and intensities comparable to those of the white dwarf black body flux. At outbursts, the envelop of debris, which corresponds to the nebula downstream of the high velocity shocks (700-1000 km/s) accompanying the blast wave, absorbs the black body soft X-ray flux from the white dwarf, explains the broad component of the H and He lines, and leads to low optical-UV-X-ray continuum fluxes. The high optical-UV flux observed at the outbursts is explained by bremsstrahlung downstream of the reverse shock between the stars. The depletion of C, N, O, and Mg relative to H indicates that they are trapped into dust grains and/or into diatomic molecules, suggesting that the collision of the wind from the white dwarf with the dusty shells, ejected from the red giant with about 1 year periodicity, leads to the U-band fluctuations during the major bursts.