HST MID-ULTRAVIOLET SPECTROSCOPY OF COMET 46P WIRTANEN DURING ITS APPROACH TO PERIHELION IN 1996-1997/

We have used the Hubble Space Telescope Faint Object Spectrograph to o bserve the mid-UV spectrum of the ROSETTA mission target comet, 46P/Wi rtanen, on three dates during the comet's apparition in 1996 and early 1997. During this time the comet moved inward from a heliocentric dis tance near 2.7 AU to 1.3 AU. The first measurement (22.7 July 1996) de tected only reflected solar continuum; based on these measurements, we set an upper limit on the nuclear radius of 1.45 km. A second measure ment (25.9 Aug 1996) detected this reflected continuum, as well as wea k OH (0-0) emission; the third measurement (15.2 Jan 1997) detected th ree OH bands at high signal-to-noise (the 0-0, the 1-0, and the 1-1), along with emission features from the CS radical and the CO2+ ion; upp er limits on their emission brightness and production rate of C-2 was set. Our key findings are as follows: The data indicate that Q(H2O) va ries like R-4.9 +/- 0.25 inside 2.5 AU, and that the CS2/H2O productio n ratio at 1.3 AU is close to 3 x 10(-4). We also found that the 2600- 3200 Angstrom mid-UV coma color slope is more neutral (similar to 25%/ 1000 Angstrom) than typical comets (similar to 40-70%/1000 Angstrom, b ut is steeper than Wirtanen's own visible-wavelength color slope. Wirt anen's CS2/H2O production ratio is near the low end of the range obser ved to date. A f rho was estimated from the data as a measure of dust production; we found A f rho (2950 Angstrom) values of 15, 18, and 15 cm, respectively, for the three FOS datasets, after correction for the estimated flux contribution of the nucleus. Interestingly, we found t hat production of dust, to which the UV is sensitive, did not appear t o vary by more than a factor of about two with heliocentric distance o ver the 2.7 to 1.3 AU range, despite a factor of 50 variation in the p roduction of H2O. Wirtanen appears to be a small, but relatively norma l comet. However, Wirtanen's ratio of dust (i.e., A f rho) to H2O prod uction is smaller than in > 90% of the 85 comets in A'Hearn et al.'s ( 1995) large sample.