2003MNRAS.339..125C

The UV and optical spectra from the HST Faint Object Spectrograph and from the Fast Spectrograph for the Tillinghast Telescope (FAST) spectrograph at the Whipple Observatory reported by Kenyon, Proga & Keyes at different phases are analysed, leading to new results about the configuration of AG Pegasi. The FAST spectra contain both Hα and Hβ lines, whose ratio changes with phase indicating that different nebulae contribute to each spectrum. In particular, the spectrum emitted from one of the nebulae must be collision dominated, in order to justify the relatively-high Hα/Hβ. Consistent modelling by the code suma, which accounts for the coupled effect of the photoionization from the hot star and the shock, shows the important role of at least three nebulae: (1) the nebula between the stars, downstream of the shock created by collision of the winds, which propagates in reverse towards the white dwarf; (2) the nebula downstream of the shock expanding in the outskirts of the giant atmosphere; and (3) the shock propagating outwards from the binary system, which is not reached by the hot source radiation. The relative contributions of the three nebulae to the final spectra at different phases are calculated for all the lines. It is found that the contribution to the different lines changes with the viewing angle of the system. The relatively-low pre-shock density and magnetic field adopted in the modelling of the expanding shocks reveal that they are merging with the ISM, thus explaining the slow decline of AG Pegasi.