Catherine E. Delahodde

Detailed phase function of comet 28P/Neujmin 1

In 2000, comet 28P/Neujmin 1 was an ideal candidate for a phase function study: during its April opposition it reached a phase angle of α = 0.8◦, and previous observations at similar heliocentric distances (r ≥ 4 AU) indicated that the comet was likely to be inactive. We observed this comet using ESO’s NTT and 2.2m telescopes at La Silla, on 6 epochs from April to August 2000, covering α = 0.8–8◦. These data were combined with a large set of data from 1985 to 1997. In order to disentangle the rotation effects from the phase effects, we obtained complete rotation coverage at opposition, providing us with a lightcurve template. We have obtained an improved rotation period of 12.75 ± 0.03 hrs, with a full amplitude of ∆m = 0.45 ± 0.05 mag, which yields an axis ratio lower limit of 1.51 ± 0.07. For each subsequent epochs, we have obtained enough rotation coverage to re-synchronize the lightcurve fragments with the template, and determine the magnitude change caused by the phase function. The average colours of the nucleus, V −R = 0.454± 0.050, and R− I = 0.413± 0.050, are similar with those of D-type asteroids and other comet nuclei. The phase function obtained for the nucleus has a linear slope of 0.025 ± 0.006 mag deg−1, less steep than that of the mean C-type asteroids.

VLT observations of comet 46P/Wirtanen

Comet 46P/Wirtanen, prime target of ESAs 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 ′ 40 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 Afp 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 Afp value of the comet was 6.5 ′ 2 cm (equivalent to a dust production rate of about 1 kg/s).