Hot gas and the origin of the nebular continuum in novae

A featureless blue continuum (with constant f_v) is a defining feature of declining classical novae. The fact that f_v is constant into the infrared, and the absence of a Balmer jump, suggests that this continuum originates in hot tenuous gas. The electron temperature and density of the hot gas phase of classical nova QU Vul 1984 are estimated from ground-based optical and IUE ultraviolet observations. This region has a temperature of 10⁵-10⁶ K, and a density in the range of 2 × 10⁴ to 3 × 10⁵ cm^-3. In addition, a colder, denser shell composed of many "clumps" or condensations is present, and it is responsible for most of the optical-to-ultraviolet emission lines. The hot gas, however, may be much more massive and could dominate the energetics of the ejecta. The total returned mass could be as high as 10^-4 and 4 × 10^-3 M_⊙, depending on conditions in the hot gas. The large mass of the shell, in combination with the high metallicity found in this nova, supports the idea that novae may be an important contributor to the chemical enrichment of the interstellar medium.