EUVE Observations of the He-Rich DO White Dwarf MCT 0501-289: Evidence for Very Low ISM Column Density

We report the discovery by the Extreme Ultraviolet Explorer (EUVE) of a remarkably bright extreme ultraviolet source associated with a very low density region of the interstellar medium (ISM). This exceptional object has been detected, during EUVE calibration pointing, in all four filter bandpasses (centered at 100, 200, 450, and 600 Angstroms), with a marked intensity increase towards longer wavelengths. We have identified the source with a peculiar He-rich DO white dwarf in the Montreal-Cambridge-Tololo survey of southern hemisphere blue stars, MCT 0501-289. We determined from the long wavelength filter count rates a neutral hydrogen column density in the range 3-6 times 10(17) cm(-2) for a range of effective temperatures of 60,000--90,000 K\@. The corresponding photometric distance, 90--100 pc, results in a rather tenuous mean density in the line of sight of n(H I) = 0.001-0.002 cm(-3) . This object is only 2(deg) away from a previously identified bright EUV source, the hot DA white dwarf MCT 0455-281. Together these two objects constitute an important source of ionizing radiation in the ISM and offer a clue to the origin of this exceptionally low H I density. Although their energy input in the ISM is too small to have contributed significantly to the removal of the neutral hydrogen, these objects emit sufficient radiation to maintain a Stromgren sphere of at least 100 pc radius. The occurrence of an important event (SN) in the past (t = --10(6) years) could have swept out neutral hydrogen, and the white dwarfs could have delayed recombination and maintained the evidence of this highly energetic event. We also report the nondetection of a third very hot white dwarf, MCT 0458-302, located only 3(deg) away from the pair. This third object is 2(m) fainter, and probably delineates the maximum extent of the low density region. This work has been supported by NASA contracts NAS5-30180 and NAS5-29298.