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M. A. DiSanti (Catholic U./NASA-GSFC), M. J. Mumma (NASA-GSFC), N. Dello Russo (Catholic U./NASA-GSFC), K. Magee-Sauer (Rowan U.), R. Novak (Iona College), T. W. Rettig (Notre Dame U.)
The release of carbon monoxide was studied in comets Hale-Bopp and Hyakutake through emission in the v=1-0 fundamental band (near 4.7 microns) using the NASA-IRTF facility echelle spectrometer (CSHELL). The 0.2 arcsec pixel size offered seeing-limited spatial resolution along the 30 arcsec slit length with sufficiently high spectral resolving power (2-3x104) to map the line-by-line CO excitation and distribution in the coma.
Our Hale-Bopp observations span a large range in heliocentric distance (4.1 AU pre-perihelion to 3.1 AU post-perihelion), permitting isolation of native and distributed CO contributions. Each CO component had an abundance of 12 percent relative to water; this amount of native CO ice is consistent with the comet's formation in the giant planets region of the proto-solar nebula. The distributed source became active relatively abruptly within 2 AU pre-perihelion, exhibiting behavior consistent with thermal destruction of a precursor organic species. Our measured heliocentric dependence for each component is consistent with insolation-limited release, in agreement with our observed dependence for Q(H2O) and also with the behavior of the dust mass loss rate based on millimeter observations. Dust production based on our measured thermal continuum showed a steeper heliocentric slope, consistent with increasing grain fragmentation in the coma with decreasing R.
Observations of Hyakutake between R = 1.06 and 0.64 AU pre-perihelion also showed the signature of dual CO sources. Relative CO abundances and spatial distributions will be compared and contrasted for the two comets.
This work is supported through the NASA Planetary Astronomy Program under grant NAG5-7905.