IUE observations of R Aquarii (M7 + pec) have been obtained in low dispersion in order to study its circumstellar emission. Strong permitted, semiforbidden, and forbidden emission lines are identified that are superposed on a bright ultraviolet continuum. From our analysis we deduce that the strong emission-line spectrum that involves C III], C IV, Si III], [0 II], and [0 III] probably arises from a dense compact nebula the size of which is comparable to the binary system of which R Aqr is the primary star. Low-excitation emission lines of Fe II, Mg II, 0 1, and Si II suggest the presence of a warm chromosphere (T ~ 10,000 K) in the primary M7 late type giant. We identify the secondary as a white dwarf, comparable to or somewhat brighter than the Sun, since such a star can produce enough ionizing photons to excite the continuum and emission-line spectrum and yet be sufficiently faint to escape detection by direct observation. The UV continuum observed is attributed to Balmer recombination and not to blackbody emission from the hot companion. The general spectral properties of R Aqr between 1200 A and 3200 A are discussed in the context of our model for the circumstellar nebula, the companion, and the mass-loss rate of the primary star.
Michalitsianos, A.G., Kafatos, M., Hobbs, R.W. (1980) IUE Observations of Circumstellar Emission from the Late Type Variable R Aquarii (M7 + pec), Astrophysical Journal, 237: 506-512. doi: 10.1086/157895