EUVE Science Highlights -- October 9, 2000
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Preliminary Results from EUVE-Led 
Super-Coordinated EX Hya Observation
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  o  EUVE led a six-satellite and 14-ground-based telescope campaign
     to observe the star EX Hya from 2 May through 15 June 2000.
  o  The spectrum obtained (see next slide) represents the best
     spectrum of an interacting binary yet obtained by EUVE.
  o  Initial results from the raw spectrum show over 30 emission
     lines due to highly ionized Iron and Neon. Temperature
     estimates are near 1,000,000o K.
  o  Full analysis will allow temperature and density determinations
     for the emitting plasma within EX Hya on spacial scales equal
     to 20% the radius of the white dwarf primary.

[fig]

Notes:
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This science highlight was submitted by Dr. Steve B. Howell of the 
Planetary Science Institute.

Based on paper:  Howell, S.B. and Sirk, M.M., ApJ, to be submitted.



Nature of Soft X-ray Emissions in Comets Revealed with EUVE
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  o  Recent analysis of X-ray detection in six out of nine EUVE
     comet observations shows that X-rays in comets is a common
     phenomenon.
  o  The EUVE spectra of comets revealed for the first time - before
     the recent coordinated EUVE/Chandra observation of comet LINEAR
     S4 - emissions of multiple charged ions that are brought to
     comets by the solar wind. 
  o  The main puzzle has been solved due to the unprecedented
     quality of the EUVE spectra. 
  o  The EUVE instrumental dispersion for extended sources is better
     than all other X-ray and EUV observatories, including Chandra
     (by a factor of 9).
     
Notes:
------
This science highlight was submitted by Vladimir Krasnopolski of the 
Physics Department, Catholic University of America.

Based on papers: 1)  EUVE Search for X-Rays From Comets Encke, 
Mueller (C/1993 A1), Borrelly, and post-perihelion Hale-Bopp, V. A. 
Krasnopolsky, M. J. Mumma, and M. J. Abbott, Icarus, accepted 
January 2000. 2) Spectroscopy of Comet Hyakutake at 80-700 A: 
Evidence for Solar Wind Charge Transfer, V. A. Krasnopolsky and M. 
J. Mumma, Science, submitted February 2000.

Soft X-ray emission from comets was discovered in the EUVE and ROSAT 
observations of Comet Hyakutake in 1996. The Comet was a more 
efficient emitter of soft X-rays than the Moon by a factor of 
80,000. Results of nine EUVE observations of comets have been 
published, and X-rays were detected in six of them. It is clear that 
cometary X-rays is a general phenomenon.

However, the nature of this phenomenon was puzzling, and a few 
excitation processes were suggested for its explanation. Recent 
analysis of the EUVE spectra of comets revealed for the first time - 
before the coordinated EUVE/Chandra observation of comet LINEAR S4 - 
 emissions of multiple charged ions that are brought to comets by 
the solar wind (see Figure). Those ions get electrons from neutral 
molecules and atoms in comets and emit photons in the X-ray and 
extreme ultraviolet (EUV) regions of the spectrum. 

Therefore, the main puzzle has been solved, first of all due to the 
unprecedented quality of the EUVE spectrum (see Figure) and similar 
spectra in the ranges of 80-180 and 300-700 Å. While most sources of 
X-rays and EUV emission are point-like, comets are extended sources. 
The basic factor which determines the quality of  X-ray and EUV 
spectra of extended sources is the instrument dispersion. For 
extended sources, the EUVE instrumental dispersion is better than on 
all other X-ray and EUV observatories, including Chandra (by a 
factor of 9).