FUSE Spectroscopic Analysis of the Slowest Symbiotic Nova AG Peg During Quiescence

We present a far-UV (FUV) spectroscopic analysis of the slowest known symbiotic nova AG Peg that underwent a nova explosion in 1850 followed by a very slow decline that did not end until ∼1996, marking the beginning of quiescence. In 2015 June, when AG Peg exhibited a Z And-type outburst with an optical amplitude of ∼1.5 mag. We used accretion disk and WD photosphere synthetic spectral modeling of a Far-Ultraviolet Spectroscopic Explorer (FUSE) spectrum obtained on 2003 June 5.618. The spectrum is heavily affected by ISM absorption as well as strong emission lines. We dereddened the FUSE fluxes assuming , which is the maximum galactic reddening in the direction of AG Peg. We discuss our adoption of the pre-Gaia distance over the Gaia parallax. For a range of white dwarf surface gravities and surface temperatures, we find that the best-fitting photosphere is a hot WD with a temperature Twd = 150,000 K, and a low gravity log(g) ∼ 6.0-6.5. For a distance of 800 pc, the scaled WD radius is Rwd ∼ 0.06 × R , giving log(g) = 6.67 for a 0.65 M WD mass. The Luminosity we obtain from this model is L = 1729 L . The hot photosphere models provide better fits than the accretion disk models, which have FUV flux deficits toward the shorter wavelengths of FUSE, down to the Lyman limit. Given the uncertainty of the nature of a true symbiotic accretion disk, and, while a very hot low gravity degenerate star dominates the FUV flux, the presence of a steady-state (standard) accretion disk cannot be summarily ruled out.