A Search for Helium in Ultra-Massive White Dwarfs Atmospheres

We have obtained spectra with the Extreme-Ultraviolet Explorer (EUVE) of 4 massive white dwarfs (Mass > 1 solar mass): 2EUVE J0138+25.3, 2EUVE J0237-12.3, 2EUVE J0916-19.7, and 2EUVE J1659+44.0. These observations are part of our ongoing search for helium in ultra-massive white dwarfs (UMWDs) atmospheres. The relatively high turnout of UMWDs in EUV surveys is difficult to explain in terms of canonical stellar evolution of isolated stars. Helium has been detected (log He/H -3.6) in the EUV spectrum of the UMWD GD 50 (Vennes, Bowyer, and Dupuis 1996, ApJ, 461, L103.), the presence of which may be indicative of a merger origin. It is important to investigate whether or not GD 50's abundance peculiarity is common amongst UMWDs. Our analysis of the EUVE spectra reveals that, so far, GD 50 is rather unique. After accounting for uncertainties in stellar parameters and interstellar column densities, we derive upper limits on helium abundance which, in all cases, are lower than GD 50's helium abundance. The non-detection of helium in our sample of UMWDs, however, does not necessarily exclude a merger origin. This work is funded by NASA grant NAG5-6551 and a Cycle 6 UCB/EUVE mini-grant under NASA cooperative agreement NCC5-138.