[Context]: The comet 67P/Churyumov-Gerasimenko, target of the Rosetta Mission (ESA) was monitored from January to April 2009 during its last perihelion passage before the Rosetta spacecraft encounters the comet nucleus in May 2014. Photometric data were obtained only from January 25 to March 19, 2009 and they were used to monitor the comet gas and dust activity. Non-photometric data are considered for analysing the evolution of the dust coma morphology. [Aims]: The goal of the campaign was to characterize the comet activity evolution as it approaches the Sun. We aimed to assess gas and dust production rates shortly before perihelion and after perihelion, as well as to follow the evolution of the dust coma morphology during this passage. [Methods]: Long-slit spectra and optical broadband images were acquired with the instrument CAFOS mounted at the 2.2. m telescope at Calar Alto Observatory (CSIC-MPG), and with the camera at the 1-m telescope of the Lulin Optical Observatory in Taiwan. We investigated the evolution of the dust coma morphology from the R Johnson images with image enhancing techniques. When possible, we studied the dust and gas production rate, the radial profiles of the dust brightness azimuthally averaged and in the Sun-antisunward direction, and the Sun-antisunward profiles of the CN, C2, C3, and NH2 column densities. [Results]: The morphological analysis of the dust coma reveals that structures indeed exist. Aside from the dust tail in the SW direction, four more small-scaled structures have been detected by using the adaptive Laplacian filtering, the radial renormalization and the Sekanina-Larson method. These structures are also confirmed by the distortion of the isophotes at the same position angles (PA). During January and February, a faint structure could be seen at PA 330°, whereas this feature seems to have disappeared after perihelion. Additionally, the broad structure at PA ~45° seemed to split into two narrower ones on February 26, to become only a considerably fainter one at PA ~75° during March. The Afρ values show considerable scatter in the few days we could measure them. There is a clear increase when the comet approaches the perihelion, however it is not possible to conclude where the peak dust activity is reached when the comet is at or beyond the closest distance to the Sun. The CN, C2, C3, and NH2 production rates, Q, have been obtained at rh 1.25 AU. These Q’s are very similar to the ones derived from previous passages around perihelion. The decrease in the perihelion distance in 2009 (1.24 AU in 2009 vs. 1.30 AU in 1982 and 1995) has not induced a noticeable increase either in the gas production rates or in the dust production rates, as measured from the Afρ parameter. The azimuthally averaged surface brightness profiles of the continuum from the broad band images can be well fitted with −1.92 ≤ m ≤ −1.35 in log B − log ρ representation for ρ ≤ 100000 km projected cometocentric distance from the R broadband images. On the other hand, when the fit is done in the sun-antisunward direction from the long-slit spectra at ρ ≤ 25000 km, the slope m ≈ −1.

Peer reviewed

8 páginas.