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I. Toth (Konkoly Observatory, Hungary), P. L. Lamy (Laboratoire d'Astronomie Spatiale, France), H. A. Weaver (Johns Hopkins University)
The nucleus of 73P/Schwassmann-Wachmann 3 (73P/SW3), a Jupiter-Family comet, has non-tidally broken into at least three components A, B and C, in autumn 1995. Fragment B was detected with the Planetary Camera 2 of the Hubble Space Telescope on 26 November 2001. i.e., after the comet's perihelion passage on 27.9 January 2001, when it was at 3.25 AU from the Sun, 2.33 AU from the Earth, and at a solar phase angle of 7.4o. The high spatial resolution of the PC2 allowed to separate the signal of the faint nucleus fragment from that of its active coma, and we measured its R magnitude from our images taken with the F675W filter. Assuming a spherical body whose geometric albedo is 0.04 and a linear phase coefficient of 0.04 mag/deg for the R band, we derived an effective radius of 0.68±0.04 km. The short time span of our HST observations did not allow to determine its shape but the lower limit of its axial ratio a/b (assuming a prolate spheroid) is ~1.16. From the pre-breakup radius of the original nucleus of 1.3 km determined by Boehnhardt et al. (2002: EMPl 90, 131), we found that the fractional volume of fragment B is about 14%. Its size of a few hundred meters, typical of fragments of other split comets, is probably too large for a primordial building block and indicate that it still is a conglomerate; further disintegration of this fragment remains possible. Its level of activity is remarkably high in spite of a heliocentric distance of ~3.2 AU, just beyond the limit for sublimation of water ice (2.8 AU). We measured an Af\rho of 19.6-23.2 cm but the determination of a dust production rate is precluded by the absence of gas production rates at the time of our observation. Ground-based observations performed in 2001 by Boehnhardt et al. (2002: EMPl 90, 131) indicate that another fragment (E) has already disappeared, and we conclude that this may well be short-term fate of fragment B as it may not survive its next perihelion passage in 2006. The tau-Herculids meteor swarm associated to 73P/SW3 will likely experience an increase of activity.
This work was supported by the CNRS, CNES, and Université de Provence, France, and by the Hungarian Academy of Science through grant No. 9871.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.