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Astron. Astrophys. 335, L30-L36 (1998)

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Letter to the Editor

HST mid-ultraviolet spectroscopy of comet 46P/Wirtanen during its approach to perihelion in 1996-1997

S. Alan Stern 1, Joel Wm. Parker 1, Michel C. Festou 2, Michael F. A'Hearn 3, Paul D. Feldman 4, Gerhard Schwehm 5, Rita Schulz 5, Jean-Loup Bertaux 6 and David C. Slater 1

1 Southwest Research Institute, Boulder, Colorado, USA
2 Observatoire Midi-Pyrénées, Toulouse, France
3 University of Maryland, College Park, Maryland, USA
4 The Johns Hopkins University, Baltimore, Maryland, USA
5 ESA Space Science Department, Noordwijk, The Netherlands
6 Service d'Aéronomie, Verrières le Buisson, France

Received 21 October 1997 / Accepted 29 January 1998

Abstract

We have used the Hubble Space Telescope Faint Object Spectrograph to observe the mid-UV spectrum of the ROSETTA mission target comet, 46P/Wirtanen, on three dates during the comet's apparition in 1996 and early 1997. During this time the comet moved inward from a heliocentric distance near 2.7 AU to 1.3 AU. The first measurement (22.7 July 1996) detected only reflected solar continuum; based on these measurements, we set an upper limit on the nuclear radius of 1.45 km. A second measurement (25.9 Aug 1996) detected this reflected continuum, as well as weak OH (0-0) emission; the third measurement (15.2 Jan 1997) detected three 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 [FORMULA] ion; upper limits on their emission brightness and production rate of C2 was set. Our key findings are as follows: The data indicate that Q(H2O) varies like [FORMULA] inside 2.5 AU, and that the CS2/H2O production ratio at 1.3 AU is close to [FORMULA]. We also found that the 2600-3200 Å mid-UV coma color slope is more neutral ([FORMULA]25%/1000 Å) than typical comets ([FORMULA]40-70%/1000 Å, but is steeper than Wirtanen's own visible-wavelength color slope. Wirtanen's CS2/H2O production ratio is near the low end of the range observed to date. [FORMULA] was estimated from the data as a measure of dust production; we found [FORMULA] (2950 Å) 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 that production of dust, to which the UV is sensitive, did not appear to vary by more than a factor of about two with heliocentric distance over the 2.7 to 1.3 AU range, despite a factor of 50 variation in the production of H2O. Wirtanen appears to be a small, but relatively normal comet. However, Wirtanen's ratio of dust (i.e., [FORMULA]) to H2O production is smaller than in [FORMULA]% of the 85 comets in A'Hearn et al.'s (1995) large sample.

Key words: comets: general – comets: individual: Wirtanen – ultraviolet: solar system

Send offprint requests to: S. Alan Stern

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© European Southern Observatory (ESO) 1998

Online publication: June 18, 1998
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