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N. Dello Russo, M. A. DiSanti (CUA/GSFC), K. Magee-Sauer (Rowan University), E. Gibb (NRC/GSFC), M. J. Mumma (GSFC)
Water is the dominant ice in most (perhaps all) comets and its sublimation controls the release of other volatiles within 3-4 AU of the sun. For this reason, the volatile activity of a comet and the abundances of minor species are often expressed relative to water production. Severe atmospheric extinction makes direct detection of water in comets difficult from ground-based observatories — its strong transitions terminate in the ground vibrational level that is highly populated in Earth's atmosphere, causing absorption of the incident photons. To avoid such extinction, we target water lines in non-resonance fluorescence — direct absorption of sunlight excites molecules from the lowest vibrational level (000) to a higher vibrational level, followed by cascade into an intermediate level that is not significantly populated in the atmosphere.
This approach has been used to successfully detect water in ten comets using lines from hot-bands in the 2, 2.9, and 5-micron regions. Detection of water lines from several different hot-bands helps in the determination of accurate water production rates and allows temperature-dependent fluorescence models for individual hot-bands to be tested for internal consistency. The 2.9-micron region is particularly diagnostic since a dense grouping of strong lines can be detected from several hot-bands.
We have been able to perform high-resolution studies of parent volatiles at infrared wavelengths with the Near Infrared Spectrometer (NIRSPEC) at Keck 2 and the Cryogenic Echelle Spectrometer (CSHELL) at the NASA IRTF. Both instruments provide spectral resolution sufficient to resolve individual ro-vibrational lines (R ~ 20,000). Here we present water production rates, rotational temperatures, and ortho-to-para ratios in Oort cloud comets C/1999 H1 (Lee), C/1999 S4 (LINEAR), C/2001 A2 (LINEAR), and C/2002 C1 (Ikeya-Zhang) derived from hot-band emissions near 2.9-microns using NIRSPEC and CSHELL.
This work was supported by the NASA Planetary Atmospheres Program (NAG5-10795), and the NASA Planetary Astronomy Program (RTOP 693-344-32-30-07).
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.