White Dwarf Photospheric Abundances in Cataclysmic Variables. I. SS Aurigae and TU Mensae

Chemical abundance studies of cataclysmic variables have revealed high nitrogen to carbon ratios in a number of cataclysmic variable white dwarfs (based on ultraviolet emission and absorption lines), as well as possible carbon deficiency in many secondaries (based on the absence of infrared CO absorption lines). These indicate that the accreted material on the white dwarf surface and the donor itself might be contaminated with CNO processed material. To further understand the origin of this abundance anomaly, there is a need for further chemical abundance study. In the present work, we carry out a far-ultraviolet spectral analysis of the extreme SU UMa dwarf nova TU Men and the U Gem dwarf nova SS Aur using archival spectra. We derive the mass and temperature of the WD using the recently available DR2 Gaia parallaxes. The analysis of HST STIS spectra yields a WD mass with a temperature of 27,750 ± 1000 K for TU Men and a WD mass M wd ∼ 0.80 ± 0.15 M ⊙ with a temperature of ∼30,000 ± 1000 K for SS Aur. However, the analysis of a FUSE spectrum for SS Aur gives a higher temperature of ∼33,375 ± 1875 K, yielding a higher WD mass of ∼1 ± 0.25 M ⊙ , which could be due to the effect of a second hot emitting component present in the short wavelengths of FUSE. Most importantly, based on the white dwarf far-ultraviolet absorption lines, we find that both systems have subsolar carbon and silicon abundances. For TU Men, we also find suprasolar nitrogen abundance, evidence of CNO processing.