White Dwarf Photospheric Abundances in Cataclysmic Variables. III. Five Dwarf Novae with an Evolved Secondary Donor Star

Abstract

In the last two decades infrared spectroscopy has brought mounting evidence, in the form of weak CO features together with enhanced ^13 C, of the presence of CNO-processed material in the atmosphere of the donor star of some nonmagnetic cataclysmic variables. Some of these donors also exhibit a temperature too high for their binary orbital period, indicating that they evolved off the main sequence before mass transfer began. The ultraviolet spectra of evolved donor systems exhibit strong N v ( λ 1240) and the almost complete absence of C iv ( λ 1550) emission lines. We present here an archival Hubble Space Telescope ultraviolet spectral analysis of five systems containing an evolved donor star. We derive their white dwarf masses, effective temperatures, and photospheric chemical abundances. The [N/C] ratio is very large, of the order 10 ^2 –10 ^3 (in solar units) for the short-period systems V485 Cen, GZ Cet, and QZ Ser, and of the order 20 for the longer-period systems HS 0218 and EY Cyg. Silicon ([S/H]) is solar for GZ Cet and QZ Ser, suprasolar for V485 Cen, and subsolar for HS 0218 and EY Cyg. We also derive abundances of O, Mg, Al, Si, P, S, Ca, and Fe, which vary from system to system. The abundances we derived are consistent with the more evolved nature of the donor star (metal enriched, hydrogen depleted). It is impossible to confirm hydrogen deficiency for these systems, since at these wavelengths (1100–2000 Å) white dwarf spectra show little dependency on the [He/H] ratio, unless it is extremely large ([He/H] ≫ 1).

Keywords

• White dwarf stars
• Dwarf novae
• Cataclysmic variable stars
• Ultraviolet spectroscopy
• Stellar atmospheres