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

Chemical abundances studies of cataclysmic variables have revealed high nitrogen to carbon ratios in a number of 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 $M_{\rm wd}=0.77^{+0.16}_{-0.13}M_{\odot}$ with a temperature of $27,750 \pm 1000$~K for TU Men, and a WD mass $M_{\rm wd} \sim 0.80 \pm 0.15$ with a temperature of $\sim 30,000 \pm 1000$~K for SS Aur. However, the analysis of a FUSE spectrum for SS Aur gives to a higher temperature $\sim 33,375 \pm 1875 $~K, yielding to a higher WD mass $\sim 1 \pm 0.25 M_{\odot}$, 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.