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[38.08] Water hot-band emission in comets near 2.9-microns: A comparison of model predictions and measurements from ground-based observatories

N. Dello Russo (Catholic University of America/NASA-GSFC), M. J. Mumma, M. A. DiSanti (NASA-GSFC), E. L. Gibb (NAS-NRC/NASA-GSFC), K. Magee-Sauer (Rowan University), B. P. Bonev (University of Toledo/NASA-GSFC), R. J. Barber, J. Tennyson (University College London)

Water is the dominant ice in comets, and its sublimation controls the release of other volatiles within 3-4 AU of the sun. Accurate measurements of H₂O abundances are important for determining the relative volatile fractions in comets since the volatile activity of a comet and the abundances of minor species are often expressed relative to H₂O production. Targeting water lines from the ground at infrared wavelengths is a proven method for obtaining accurate H₂O production rates since in general multiple lines can be observed simultaneously. In addition, observations from the ground allow coverage over a range of heliocentric distances. To avoid the atmospheric extinction that prevents the detection of strong H₂O lines from fundamental bands, we target (weaker) lines seen in non-resonance fluorescence. The detection of these lines requires no specific geocentric Doppler-shift for the comet. This approach has been successfully used to detect water lines from the ground in comets since the early 1990s. The 2.9-micron spectral region is particularly diagnostic since a dense grouping of strong lines can be detected from several different hot-bands.

We have developed line-by-line temperature-dependent fluorescence models for seven hot-bands near 2.9-microns. Here we compare our models to high-resolution comet data obtained with the Near Infrared Spectrometer (NIRSPEC) at Keck 2 and the Cryogenic Echelle Spectrometer (CSHELL) at the NASA IRTF (R ~ 20,000 for both instruments). We discuss H₂O production rates, rotational temperatures, and ortho-to-para ratios for several comets within our current database (e.g. C/1999 H1 (Lee), C/1999 S4 (LINEAR), C/2001 A2 (LINEAR), and C/2002 C1 (Ikeya-Zhang)), and we predict line intensities and detectability limits for H₂O in comet 2P/Encke in November 2003.

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, 35 #4
© 2003. The American Astronomical Soceity.