Ultraviolet spectroscopic study of EU UMa and ST LMi from HST and IUE observations

• The ultraviolet continua for EU UMa and ST LMi are approximately the same and show a low level of continuum emission.
• The flux values of C IV 1550 Å for EU UMa are smaller than the flux values for ST LMi by factors of 2–4.
• Both EU UMa and ST LMi have approximately the same form of spectral behavior with phase.
• Variations in mass accretion rates cause changes in density and temperature leading to variations in line fluxes.

We present the first orbit-resolved ultraviolet spectroscopic observations of the two polar systems EU UMa and ST LMi obtained with the Hubble Space Telescope Faint Object Spectrograph (HST FOS) and International Ultraviolet Explorer (IUE) during their intermediate- and low-luminosity states in the period between 1982 and 2003. Different line profiles of the two systems showing variations of line fluxes at different orbital phases are presented. This paper focuses on the C IV emission line at 1550 Å produced in the accretion stream, presenting calculations of spectral line fluxes, ultraviolet luminosities, and accretion rates for the two systems. Our analysis of the spectroscopic data reveals changes with orbital phase of the emission line profiles that correspond to the light curve variations of both EU UMa and ST LMi in the optical and infrared bands. The variations of line fluxes are attributed to variations of both density and temperature as a result of a changing rate of mass transfer from the secondary star to the white dwarf. The ultraviolet luminosity and accretion rate of EU UMa are smaller than the ultraviolet luminosity and accretion rate of ST LMi.