The Evolving Structure of AG Pegasi, Emerging from the Interpretation of the Emission Spectra at Different Phases

Abstract

A model of AG Peg is presented, focusing on the physical conditions in the emission nebulae. The model accounts in a consistent way for photoionization by the star and ionization by shocks. The SUMA code is used in the calculations of the spectra. We show that two regions contribute to the UV and optical line spectra. The broad lines are emitted from photoionized gas surrounding the hot star, while narrow lines are emitted by shocked gas—which is also reached by the ionizing flux from the hot star—near the red giant. At an early phase ( = 2.33) the emitting region is between the stars but already very close to the red giant. The nebula surrounding the hot star is not exactly symmetric. At a later phase ( = 7.05) the shock created by head-on-back collision of the winds propagates outward and slightly accelerates throughout the giant atmosphere characterized by a decreasing slope of the density. The strong shock caused by head-on collision of the winds from the two stars cannot be recognized in the UV-optical line spectrum. The spectral energy distribution of the continuum is well fitted by blackbody emissions from the stars. The fit of the optical-UV observed continuum at the early phase presents some problems connected with the reddening correction of the data. On the other hand, the fit of the data at a later phase is consistent with the parameters deduced by the modeling of the system. The far-IR data are well fitted by reradiation by dust, which could not be recognized without modeling, and indicate a dust-to-gas ratio similar to the Galactic one.