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 alpha = 0.8 degrees, and previous observations at similar heliocentric distances (r greater than or equal to 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 alpha = 0.8-8 degrees. 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 Deltam = 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.