Intense extreme-ultraviolet source in Cetus

We report the detection of a second extrasolar extreme ultraviolet (EUV) (100--1000 A) object with the EUV telescope aboard the Apollo-Soyuz mission. The observed intensity in the 170--620 A band is 3 x 10-9 ergs cm-2 s-1, similar to that of HZ 43, the first known EUV star. The spectrum is significantly steeper than HZ 43, implying Tapproximately-less-than90,000 K. The positional error box of 3.8 deg2 includes Feige 24, a binary system consisting of a very blue DA white dwarf and a dMe star. A grid of LTE pure-hydrogen, high-surface-gravity model atmospheres, in combination with the EUV data and optical spectrophotometry of the DA, is used to derive the stellar parameters. We find that T/sub eff/=60,000 K, R/sub asterisk/=17,000 (d/100 pc) km, and L=7(d/100 pc)2 L/sub sun/ provides a satisfactory fit to the lambda>170 A data. Under the assumptions of this model, pure hydrogen atmospheres with white dwarf gravities predict fluxes for lambda<170 A which are in excess of the observations. Possible resolutions of this discrepancy are considered, such as the presence of trace amounts of atmospheric helium or ionized interstellar helium. The maximum density of neutral interstellar hydrogen implied simply by the detection of a finite EUV flux is n/sub H/=0.01 (100 pc/d) cm-3