Detailed phase function of comet 28P/Neujmin 1 ^*

¹ European Southern Observatory, Casilla 19001, Santiago, Chile
² Institut d'Astrophysique de Marseille, Traverse du Siphon, 13376 Marseille Cedex 12, France
³ Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
⁴ Osservatorio Astronomico di Torino, Strada Osservatorio 20, 100025 Pino Torinese (TO), Italy

Corresponding author: C. Delahodde, catherine.delahodde@astrsp-mrs.fr

Received: 11 April 2001
Accepted: 10 July 2001

Abstract

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 ( 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 hrs, with a full amplitude of 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, , and , are similar with those of D-type asteroids and other comet nuclei. The phase function obtained for the nucleus has a linear slope of mag deg^-1, less steep than that of the mean C-type asteroids.