HST Observations of the Nucleus Fragment 73P/Schwassmann-Wachmann 3-C During its Close Approach to Earth in 2006

Details HST Observations of the Nucleus Fragment 73P/Schwassmann-Wachmann 3-C During its Close Approach to Earth in 2006 HST Observations of the Nucleus Fragment 73P/Schwassmann-Wachmann 3-C During its Close Approach to Earth in 2006

Sep 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006dps....38.0601t&link_type=abstract

American Astronomical Society, DPS meeting #38, #06.01; Bulletin of the American Astronomical Society, Vol. 38, p.489

Astronomy and Astrophysics

Astronomy

5

Scientific paper

Fragment C of 73P/Schwassmann-Wachmann 3 (73P) was first detected with the HST-WFPC2 on 26 November 2001 when its heliocentric and geocentric distances were 3.26 AU and 2.34 AU, respectively. At that time, we found a very active nucleus with an effective radius of 0.68±0.04 km (Toth et al., 2005: Icarus 178, 235-247). The comet's favorable apparition in April-May 2006 offered an exceptional opportunity to re-observe this nucleus at close range. Our observations took place on 10 April when the comet was 1.24 AU from the Sun, 0.29 AU from the Earth, and at a phase angle of 31°. They were performed with the HRC mode of the ACS with three standard filters, F475W ("B"), F555W ("V") and F625W ("R"), as well as with the broadband F606W (broad "V") to search for possible orbiting fragments. Fragment C was highly active, and the nucleus and the coma contributed almost equally in the central pixels. Assuming a spherical body with a geometric albedo of 0.04 and a linear phase coefficient of 0.04 mag/deg for the R-band, we derived an effective radius of 0.41±0.02 km. The light curve suggest that we were seeing the varying cross-section of an elongated body and we determined a = 0.57±0.08 km, b = c = 0.31 ±0.02 km, the minimum axial ratio 1.8±0.3, and a rotational period of 3.7±0.2 hr. Its color is characterized by (V-R) = 0.57±0.11 and (B-V) = 1.16±0.20, slightly outside the main trend among cometary nuclei, but comparable to a few of them. We were unable to detect any fragment in the vicinity of the C fragment on our deep HST images. H.A.W. acknowledges support by NASA through grant GO-10625 from the STScI, S.C.L. acknowledges support from the UK Particle Physics and Astronomy Research Council, and I.T. acknowledges support from CNRS, France.

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