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Discovery of a white dwarf companion (EUVE
J0254­053) to the K0 IV star HD18131
S. Vennes, M. Mathioudakis, J.G. Doyle, J.R. Thorstensen
and P.B. Byrne
New ultraviolet (UV) observations of late­type stars detected in the Extreme Ul­
traviolet Explorer (EUVE) all­sky survey revealed an unsuspected white dwarf
companion to the K0 star HD18131. The International Ultraviolet Expoloer
(IUE) spectrum show a composite of a white dwarf and a late­type star. The
white dwarf dominates the emission below 2000 A while the K0 star prevails at
longer wavelengths. A model atmosphere analysis of the new ultraviolet (EUV)
photometry reveals a hot, hydrogen­rich (DA) white dwarf (Teff 30,000 K) that
is the most likely source of the EUV emission (EUVE J0254­053). Assuming
0.6 M fi for the white dwarf, we estimate a distance to the white dwarf of 80
\Sigma 20 pc. 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 67 pc, 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 white
dwarf population survey at EUV wavelenths and in particular, for the binary
frequency.
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