Recent Science Highlights

1 October 1995

(following information provided by Dr. Pierre Chayer)

Dr. J. Dupuis, Dr. S. Vennes, Dr. S. Bowyer, Dr. A.K. Pradhan, and Dr. P. Thejll, performed a study of the local interstellar medium (LISM) by using the extreme ultraviolet (EUV) spectra of the hot DA white dwarfs Feige 24, G191-B2B, MCT 0455-2812, HZ 43, GD 71, and GD 153. All these spectra were obtained with the Extreme Ultraviolet Explorer (EUVE) observatory, and they showed clearly the photoionization edge of interstellar He I at 504 A.

• Dr. Dupuis and his collaborators analyzed the EUV continuum spectra of these stars with a grid of pure-hydrogen model atmospheres and, in the case of Feige 24, G191-B2B and MCT 0455-2812, with a grid of models incorporating new opacities of C, N, O, and Fe from the Opacity Project. They obtained new EUV-continuum based estimates of effective temperatures for their sample stars and, for the three high-metallicity white dwarfs, they established the effects of heavy element blanketing on the EUV continuum of these objects and on the determination of effective temperatures.
• They found that the ratio of the neutral column densities of hydrogen and helium in the LISM is similar for five out of six stars in the sample but that Feige 24 may have a substantially different value. The Feige 24 results suggest that the ionization state in the LISM is inhomogeneous. Their measurements favor a higher degree of ionization of helium relative to hydrogen and also indicate that hydrogen in the LISM is predominantly neutral in at least two lines of sight.