EDP Sciences
Free access
Volume 376, Number 2, September II 2001
Page(s) 672 - 685
Section The solar system
DOI http://dx.doi.org/10.1051/0004-6361:20011028

A&A 376, 672-685 (2001)
DOI: 10.1051/0004-6361:20011028

Detailed phase function of comet 28P/Neujmin 1

C. E. Delahodde1, 2, K. J. Meech3, O. R. Hainaut1 and E. Dotto4

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

(Received 11 April 2001 / Accepted 10 July 2001 )

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$^{\circ}$, and previous observations at similar heliocentric distances ($r \geq 4$ 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$^{\circ}$. 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 $12.75 \pm 0.03$ hrs, with a full amplitude of $\Delta m = 0.45 \pm 0.05$ mag, which yields an axis ratio lower limit of 1.51 $\pm$ 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, $V-R = 0.454 \pm 0.050$, and $R-I = 0.413 \pm 0.050$, are similar with those of D-type asteroids and other comet nuclei. The phase function obtained for the nucleus has a linear slope of $0.025 \pm 0.006$ mag deg-1, less steep than that of the mean C-type asteroids.

Key words: comets: general: surface properties -- comets: individual: P/Neujmin 1 -- techniques: photometric

Offprint request: C. Delahodde, catherine.delahodde@astrsp-mrs.fr

SIMBAD Objects in preparation

© ESO 2001