Ultraviolet spectroscopic investigation of HU Aqr and AN UMa with the data from HST and IUE

We present ultraviolet spectroscopic study of two polar systems, HU Aqr and AN UMa, observed with the Hubble Space Telescope's Space Telescope Imaging Spectrograph (STIS) and Faint Object Spectrograph (FOS) and with the International Ultraviolet Explorer (IUE) during the period 1979--2003, to diagnose the ultraviolet fluxes of C IV 1550 Å and He II 1640 Å emission lines originating in an accretion stream during different orbital phases. The reddening of two systems is determined from the 2200 Å absorption feature. Different spectra for both systems, showing the variations in line fluxes at different orbital phases, are presented. We concentrated on calculating the line fluxes of C IV and He II emission lines. From HST and IUE data, we derived accretion luminosities and accretion rates for the two systems. The average temperature of the accretion stream for HU Aqr and AN UMa is about 5000 K and 6000 K, respectively. Our results show that there are variations in line fluxes, accretion luminosities and accretion rates with time for the two systems. These modulations are attributed to the variations of both density and temperature as a result of a changing rate of mass transfer from the secondary star to the primary star. These results from the HST and IUE observations support the irradiation model producing sufficient ultraviolet flux for orbital modulations.