EDP Sciences
Free access
Volume 387, Number 3, June I 2002
Page(s) 1107 - 1113
Section The solar system
DOI http://dx.doi.org/10.1051/0004-6361:20020494

A&A 387, 1107-1113 (2002)
DOI: 10.1051/0004-6361:20020494

VLT observations of comet 46P/Wirtanen

H. Boehnhardt1, C. Delahodde1, T. Sekiguchi1, G. P. Tozzi2, R. Amestica1, O. Hainaut1, J. Spyromilio3, M. Tarenghi3, R. M. West3, R. Schulz4 and G. Schwehm4

1  European Southern Observatory ESO, Alonso de Cordova 3107, PO Box 19001, Santiago 19, Chile
    e-mail: hboehnha@eso.org;cdelahod@eso.org;tsekiguc@eso.org;ohainaut@eso.org;ramestic@eso.org
2  Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 50125 Firenze, Italy
    e-mail: tozzi@arcetri.astro.it
3  European Southern Observatory ESO, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
    e-mail: jspyromi@eso.org;mtarengh@eso.org;rwest@eso.org
4  European Space Agency ESA, ESTEC, Keplerlaan, 2200 AG Noordwijk, The Netherlands
    e-mail: rschulz@estec.esa.nl;gschwehm@estec.esa.nl

(Received 4 December 2001 / Accepted 28 March 2002 )

Comet 46P/Wirtanen, prime target of ESA's ROSETTA mission, was successfully observed at the Very Large Telescope Observatory in Chile: on 17 May 1999 with the Test Camera at the Cassegrain focus of the 8.2-m VLT Kueyen telescope and on 8 December 2001 with FORS1 at Unit Telescope 4 Yepun. May 1999: no coma was detected at heliocentric distance r = 4.98 AU. From the measured brightness in the Bessell R-filter, a mean nucleus radius of $555\pm40$ m is derived (for a geometric albedo of 0.04 and a phase darkening of 0.04 mag/deg). The nucleus signal varies during the 2.7 h observing interval and a peak-to-peak amplitude of ~0.38 mag is determined. The measured lightcurve is in agreement with a rotation period of 6-7.5 hours and a ratio of the main nucleus axes of at least 1.4. The non-detection of a coma allows one to put an approximate upper limit for Af $\rho$ of <0.45 cm (suggesting a dust production rate of 0.05 kg/s). December 2001: a weak and condensed coma seems to be present in the seeing disk of the comet at 2.9 AU inbound, causing a higher brightness than expected from the previous size estimates of the nucleus. The colour of the comet appears very red ( V-R spectral gradient ~47%/100 nm). The Af $\rho$ value of the comet was $6.5\pm2$ cm (equivalent to a dust production rate of about 1 kg/s).

Key words: comets: individual: 46P/Wirtanen -- comets: general

Offprint request: H. Boehnhardt, hboehnha@eso.org

© ESO 2002