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V. A. Krasnopolsky (Catholic University of America), M. J. Mumma (NASA/GSFC)
The ROSAT and EUVE observations of comet Hyakutake in 1996 revealed a puzzling X-ray emission. That comet appeared to be a more efficient emitter of X-rays than the Moon by a factor of 80,000. Currently our EUVE database includes eight observations of comets, and soft X-ray emissions have been detected in five of those observations. The measured soft X-ray luminosities are proportional to r3/2Qgas with the efficiency of (6.4±0.9)10-5 AU3/2 and therefore favor a gas-related mechanism. The observed behavior of the brightness maxima and spatial distributions of soft X-ray emissions indicate charge transfer of solar wind heavy ions as the only viable excitation process.
However, a final and crucial test for the X-ray excitation process should be made by spectroscopy. We extracted three spectra from our EUVE observations of comet Hyakutake. These spectra cover the ranges of 80-180, 170-360, and 300-700 Å. Due to the close flyby of the comet at 0.1 AU, (1-3)104 photons were collected in each spectrum, that is, more by two orders of magnitude than in the previously published spectra. Despite very good dispersion of the EUVE spectrometers, the great angular extent of the comet reduced the resolving power to 10 in our spectra. The spectra revealed for the first time emission lines of multiple charged ions which present a direct evidence for solar wind charge transfer excitation. The most prominent lines are O4+ 215 Å, C4+ 249 Å, He+ 304 Å, and Ne7+ 448 Å. Some other lines, which are of comparable strength, are blended. The photon luminosity of charge exchange at energy below 100 eV (124 Å) exceeds that above 100 eV by a factor of 2, and the mean total quantum yield is 4 photons per heavy ion. The detected O+ lines at 538/539, 617, and 430/442 Å are excited by photoionization of atomic oxygen similar to that in Earth's dayglow. The observed depletion of neon by more than a factor 2600 relative to the solar abundance confirm the current view that Oort cloud comets formed in the Jupiter-Neptune region of the solar nebula.
The excellent combination of the high signal-to-noise ratio with the good resolution in our spectra can hardly be achieved by any existing nstrument. For example, the dispersion of the Chandra HRC/LETG is smaller by a factor of 9 than that of the EUVE spectrometers, and it is difficult to obtain spectra of comets with high signal-to-noise ratio and resolving power >5 with that instrument. Another instrument onboard the Chandra X-ray observatory, ACIS, has a mean resolving power of 3 in the range of 150-800 eV.
This work was supported by NSF grant AST#9732895 to V. A. Krasnopolsky.
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