TRAPPIST monitoring of comet C/2012 F6 (Lemmon)^⋆

Institut d’Astrophysique et Géophysique, Université de Liège, allée du 6 Août 17, 4000 Liège, Belgium
e-mail: cyrielle.opitom@ulg.ac.be

Received: 10 July 2014
Accepted: 8 October 2014

Abstract

We report the results of the long-term narrowband photometry and imaging monitoring of comet C/2012 F6 (Lemmon) with the robotic TRAPPIST telescope (La Silla Observatory). Observations covered 52 nights pre- and post-perihelion between December 11, 2012, and June 11, 2013 (perihelion: 24 March, 2013). We followed the evolution of the OH, NH, CN, C₃, and C₂ production rates computed with the Haser model as well as the evolution of the A(θ)fρ parameter as a proxy for the dust production. All five gas species display similar slopes for the heliocentric dependence. An asymmetry about perihelion is observed, the rate of brightening being steeper than the rate of fading. The chemical composition of the comet’s coma changes slightly along the orbit: the relative abundance of C₂ to CN increases with the heliocentric distance (r) below −1.4 au and decreases with r beyond 1.4 au while the C₃-to-CN ratio is constant during our observations. The behavior of the dust is different from that of the gas, the slope of the heliocentric dependence becoming steeper in early February, correlated to a change in the visual lightcurve slope. However, the dust color does not vary during the observations. The application of several enhancement techniques on the images revealed structures in the CN, C₃, and C₂ images. These features imply the existence of one or several active zone(s) on the comet nucleus. The shape of the structures is similar in these three filters and changes from a roughly hourglass shape in December and January to a corkscrew shape in February and March. The structures in the continuum filters (sampling the dust) are not correlated to those observed for the gas. During several full nights in February, we observed changes in the CN and C₂ structures that repeated periodically because of the nucleus rotation, our derived rotational period being of 9.52 ± 0.05 h.