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Session 62 - Comet Hyakutake Encounter.
Display session, Wednesday, June 12
Great Hall,

[62.14] The Inner Coma of Comet Hyakutake (C/1996 B2)

C. M. Lisse, Y. R. Fernandez, L. M. Woodney, M. F. A'Hearn (U.Md.)

On its way towards perihelion, Comet Hyakutake (C/1996 B2) passed within 0.1 AU of Earth on 1996 March 25.3 UT. This is the closest approach of a comet since 1983. At the time of closest approach the angular resolution of the comet was \sim 75 km/arcsec. Recent observations of OH by Schleicher (1996) require that the nucleus have a radius of at least 1 km in order to sublime enough water. A conservative extrapolation of the observed OH to the time of closest approach implies a total outgassing rate in excess of 10^29 molecules/sec, within 30% of 1P/Halley's rate. The close perigee of such a bright comet occurs extremely rarely, and to take advantage of this we observed the comet with NASA/IRTF for four nights on and immediately before perigee, using the near-IR NSFCAM. Our observations of C/Hyakutake have provided us with a unique chance to study the chemistry and structure of the comet's inner coma and nucleus. \par Our observations mapped the comet's innermost coma in the promising low abundance and exotic species found with our spectroscopic observing program (Woodney et al. 1996), as well as H2O and the 3.4\ \mum organic feature. This allowed us to model the outflow, relate spatial structures to the rotation of the nucleus, derive time scales for photo-creation and destruction of species, and establish parent-daughter relationships among species and the gas and dust phases (cf. Klavetter and Hoban 1992). For a normally bright comet, this would be impossible longward of 2 \mum, where the observations are background limited. However, for C/Hyakutake this was not a constraint because the visual magnitude was V\sim 1 at perigee, with a surface brightness of 15 mag/arcsec^2 at 2500 arcsec from the nucleus. We present here the preliminary results obtained so far from the reduction of our data. \par \medskip References: Klavetter, J. J. and S. Hoban 1992, Icarus 95, 60. Schleicher, D. G. 1996, IAUC 6311. Woodney, L. M. et al. 1996, This meeting.

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