AN ALL-SKY CATALOG OF FAINT EXTREME-ULTRAVIOLET SOURCES

We present a list of 534 objects detected jointly in the Extreme Ultra violet Explorer (EUVE) 100 Angstrom all-sky survey and in the ROSAT X- Ray Telescope 0.25 keV band. The joint selection criterion permits use of a low count rate threshold in each survey. This low threshold is r oughly 60% of the threshold used in the previous EUVE all-sky surveys, and 166 of the objects listed here are new EUV sources, appearing in neither the Second EUVE Source Catalog nor the ROSAT Wide Field Camera Second Catalog. The spatial distribution of this all-sky catalog show s three features: an enhanced concentration of objects in Ursa Major, where the Galactic integrated H I column reaches its global minimum; a n enhanced concentration in the third quadrant of the Galaxy (l(II) fr om 180 degrees to 270 degrees) including the Canis Major tunnel, where particularly low H I columns are found to distances beyond 200 pc; an d a particularly low number of faint objects in the direction of the f ourth quadrant of the Galaxy, where nearby intervening H I columns are appreciable. Of particular interest is the composition of the 166 det ections not previously reported in any EUV catalog. We offer prelimina ry identifications for 105 of these sources. By far the most numerous (81) of the identifications are late-type stars (F, G, K, M), while 18 are other stellar types, only five are white dwarfs (WDs), and none a re extragalactic. The paucity of WDs and extragalactic objects may be explained by a strong horizon effect wherein interstellar absorption s trongly limits the effective new-source search volume and, thereby, se lectively favors low-luminosity nearby sources over more luminous but distant objects.