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H. A. Weaver (JHU/APL), M. F. A'Hearn (UMD), C. Arpigny (U. Liège), M.R. Combi (U. Michigan), P.D. Feldman (JHU), M.C. Festou (Obs. Midi-Pyrénées), G.-P. Tozzi (Oss. Astro. Arcetri)
We report the first detection of atomic deuterium (D) emission in a comet. On three different dates (2004 April 24.3, 26.9, and 28.8 UT), the D Lyman-\alpha line at 1215.34~Å\ was clearly detected (S/N\sim6) during observations of comet C/2001~Q4 (NEAT) using the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST). During that time, the comet's heliocentric distance varied from 1.04~AU to 1.00~AU, the geocentric distance ranged from 0.54~AU to 0.39~AU, and the water production rate was \sim1.9 \times 10^{29} s^{-1}. The STIS echelle grating E140H was used with a 0.\!''2 \times 6'' slit, which provides a velocity resolution of \sim5.5 km s^{-1} for a spatially uniform emission, to measure the D line simultaneously with the atomic hydrogen (H) Lyman-\alpha line, both of which were cleanly separated from the terrestrial airglow H and D emissions. Since the observations were made with a relatively small aperture, detailed modeling is required to convert the observed D and H column densities into a cometary D/H ratio, which can then be compared to the deuterium abundance in other comets, in the earth's water, and in the interstellar medium. The derivation of the H production rate is relatively straightforward, but uncertainty in the photolysis pathways for HDO, the presumed primary source of D in the inner coma, and of the spatial distribution of D, make the derivation of an accurate D production rate problematic. We present our current best estimate for the D/H ratio in C/NEAT and discuss the prospects for improving the accuracy of that determination. We also briefly report on an HST search for D emission from C/2002~T7 (LINEAR), conducted during 2004 June 6-7 UT.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.