Recent Science Highlights

15 November 1994

• The paper "A Volume-Limited Survey of High Galactic Planetary Nebulae with EUVE" (Fruscione, A. et al. 1994; CEA publication #609) will appear in the February issue of the Astrophysical Journal. It reports the detection of seven Planetary Nebulae (PN) during the EUVE all-sky survey, two of which are newly discovered EUV sources. The emission from the objects is consistent in all cases with that of a point source and therefore is most probably originating from the PN central stars. From the observed values of the EUV count rates and from assumption about the absorption occuring in the interstellar medium between the Earth and the stars, the authors derive temperatures of about 100,000 degrees for the emitting stars.
• The prototype eclipsing binary Algol was observed by the EUVE spectrometers over nearly 1.5 orbital periods. Dr. R. Stern and collaborators, in an abstract submitted for the 1995 meeting of the American Astronomical Society, report that emission lines from high ionization states of iron, formed at high temperature (up to 20 million degrees), are clearly detected in the overall spectrum and that a quiescent continuum is present which increases towards shorter wavelengths. Using synthesized spectra of optically thin line and continuum emission, Dr. Stern and collaborators find that the iron in the corona of Algol is underabundant by factors of ~2-4 relative to the Sun, unless an unreasonably large quantity of coronal plasma at temperatures greater than 30 million degrees is present in the quiescent spectrum. The latter possibility is, however, inconsistent with available X-ray data.
• Dr. M. Mathioudakis and collaborators (in a paper recently submitted to Astronomy and Astrophysics) reports the detection of 23 new main-sequence stars observed with EUVE during the all-sky survey, increasing the total number of main-sequence stars detected in the EUV by ~12%. The authors used the EUVE data to computed EUV fluxes (a measure of the star's activity) for a total of 74 main-sequence stars with known rotational period to study the connection between activity and rotation. They find that as seen in the EUV, saturation (the maximum level of activity in a star) in chromospheric fluxes occurs in lower rotational velocities than it does in coronal fluxes. One of the detected stars is the low activity dwarf M star Gl 685. The EUVE observation indicates that this star has a cool corona at less than about 2.5 million degrees and that the most significant amount of radiative losses in the corona of this dwarf star emerges in the EUV.