2001A&A...376..672D

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≥4AU) 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±0.03hrs, with a full amplitude of Δm=0.45±0.05mag, 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±0.006mag/deg, less steep than that of the mean C-type asteroids.