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C.E. Delahodde (University of Central Florida), O.R. Hainaut (European Southern Observatory), E. Dotto (Istituto Nazionale di Astrofisica), H. Campins (University of Central Florida)
133P/Elst-Pizarro is an intriguing object among the small bodies of our Solar System: discovered in 1996 with a typical cometary dust tail, and therefore classified as a periodic comet, its orbit lies in the outer asteroid main belt, within the Themis dynamical family. The one-time event impact hypothesis (Toth, 2000) proposed to explain the 1996 apparition of a comet-like tail has since been ruled out by the discovery of recurrent activity in 2002 (Hsieh et al., 2004). With an interval of almost one orbital period (Porb = 5.6 yrs) between the two reported outbursts (DeltaT ~ 6 yrs), the most natural explanation presented by Hsieh et al. is that Elst-Pizarro experiences seasonal (i.e., obliquity-related) gas-driven mass loss from sublimation of near-surface ice.
We present BVRI observations of 133P/Elst-Pizarro obtained at the ESO 3.6-m NTT between April 1st and May 5th, 2000. Our data are consistent with a dormant phase, as no coma was detected down to 25 mag/sq.arcsec in our deepest R composite image, and a multi-filter combined image of all our runs reveals no extended trail above the sky noise. For each individual run, the brightness variations we observe are consistent with the rotational period Prot = 3.471 hrs found by Hsieh et al., but we lack time coverage to determine and compare the amplitude of the lightcurves. Furthermore, combining our dataset with their lightcurve yields a refined period of 3.47113 ± 0.00001 hrs. We find mean colours V-R = 0.39 pm 0.04, R-I = 0.24 ±0.04, and B-V = 0.63 ±0.06, with no evidence of rotational colour variation; our results are also in very good agreement with the Hsieh et al. reported colours. However, our mean R absolute magnitudes m(1,1,alpha) derived from the lightcurve analysis, are not consistent with their phase curve. We will discuss in this presentation the effect of a change in aspect angle between the 2000 and 2002 observing campaigns in the mean magnitude determination.
Support for this work was provided to by NASA (HC) and by the National Science Foundation (HC and CED).
References:
-- Hsieh, H.H.; Jewitt, D.C; Fernandez, Y.~R. (2004): The Strange Case of 133P/Elst-Pizarro: A Comet among the Asteroids, AJ 127, p.2997
-- Toth, I. (2000): Impact-generated activity period of the asteroid 7968 Elst-Pizarro in 1996: Identification of the asteroid 427 Galene as the most probable parent body of the impactors, A&A 360, p.375
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.