Ultraviolet Temporal Variability of the Peculiar Star R Aquarii

This paper is a comprehensive study of all far-UV spectra obtained with the International Ultraviolet Explorer (IUE) satellite of the symbiotic star R Aquarii (R Aqr) covering a 13 yr period between 1979 and 1992. We have analyzed the ultraviolet emission for the compact H II region and the NE and SW extended jetlike filamentary structures. The results of our ultraviolet analysis indicate that the H II region and features A, B, and D of the NE jet are increasing in intensity, while the SW counterjet, feature A\u27, has declined in intensity. We have found much stronger evidence that the high ionization lines of N v λλ1238, 1240 and He II λ1640 in the H II region and NE jet have steadily increased over the entire period of IUE observations. This result could indicate that the ionized region is increasing in excitation. We have found that the observations of the NE jet are consistent with a decreasing electron temperature and increasing line-emitting region, which suggest that the strengthening of emission-line fluxes may be a result of an expanding emitting volume in a post-shocked region. Low-resolution absolute line intensities and wavelengths for the most prominent emission lines in the λλ1200-3200 wavelength range have been tabulated. In addition, we present plots of the total ultraviolet flux (λλ1200-3200) and absolute line intensities as a function of time to discern the temporal variability of R Aqr. These plots indicate that the far-UV emission from the NE jet is not variable on a ~1.5 yr timescale as suggested by Kafatos, Michalitsianos, & Hollis. Moreover, we find that the variations in the far-UV emission in R Aqr are not coupled to the 386 day pulsation period of the Mira. Finally, successive ionization levels of carbon, nitrogen, oxygen, and silicon of the NE jet are compared with each other to probe the nature of the ionization source in the NE jet. High- and low-resolution ultraviolet spectra were used to calculate the properties of the emission nebula and hot component. We determined electron densities of ne = 4 x 10^5 cm^-3 (H II region) and ne = 6 x 10^4 cm^-3 (NE jet), electron temperatures of Te ~ 20,000 K (H II region) and a high value of Te ≥ 26,000 K between 1982 May and 1983 December, to a low value of Te ≤ 18,000 K from observations during the period of 1986 December and 1989 June (NE jet), and also line-emitting regions of L = 6.3 x 10^14 cm (H II region) and L = 1.5 x 10^15 cm (NE jet). The He II λ1640 modified Zanstra method yielded an effective temperature of T* = 61,000 K for the hot component. Estimates of the number of ionizing photons beyond the Lyman limit, blackbody parameters, and possible accretion disk parameters for the hot component are also given