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Comet Machholz (C/2004 Q2): morphological structures in the inner coma and rotation parameters

Extensive observations of comet C/2004 Q2 (Machholz) were carried out between August 2004 and May 2005. The images obtained were used to investigate the comet's inner coma features at resolutions between 350 and 1500 km/pixel. A photometric analysis of the dust outflowing from the comet's nucleus and the study of the motion of the morphological structures in the inner coma indicated that the rotation period of the nucleus was most likely around 0.74 days. A thorough investigation of the inner coma morphology allowed us to observe two main active sources on the comet's nucleus, at a latitude of +85° \pm 5° and +45° \pm 5°, respectively. Further sources have been observed, but their activity ran out quite rapidly over time; the most relevant was at latcom. = 25° \pm 5°. Graphic simulations of the geometrical conditions of observation of the inner coma were compared with the images and used to determine a pole orientation at RA=95° \pm 5°, Dec=+35° \pm 5°. The comet's spin axis was lying nearly on the plane of the sky during the first decade of December 2004.