Discovery of a white dwarf companion (EUVE J0254-053) to the K0 IV star HD18131

Abstract

New ultraviolet (UV) observations of late-type stars detected in the Extreme Ultraviolet Explorer (EUVE) all-sky survey revealed an unsuspected white dwarf companion to the K0 star HD18131. The International Ultraviolet Explorer (IUE) spectrum show a composite of a white dwarf and a late-type star. The white dwarf dominates the emission below 2000A while the K0 star prevails at longer wavelengths. A model atmosphere analysis of the new ultraviolet spectrophotometry and of the extreme ultraviolet (EUV) photometry reveals a hot, hydrogen-rich (DA) white dwarf (Teff~30000K) that is the most likely source of the EUV emission (EUVE J0254-053). We estimate a distance to the white dwarf of 70-90pc. The K0 star shows a modest level of chromospheric activity with the detection of Mg II h and k emission in the IUE spectrum. Optical spectroscopy revealed that the K0 star is a subgiant (K0 IV). The star is located at a distance of ~70pc, consistent with the estimated distance of the white dwarf. Therefore, it most likely constitutes a physical pair with the white dwarf. Until results of a radial velocity study are made available we cannot establish whether the pair is wide or close. In earlier works the strong EUV emission was attributed to the K0 star; however, our multiwavelength observations show the white dwarf as the most likely source. This discovery has important implications for the EUV white dwarf population survey and, in particular, for the binary frequency.