Pre-impact monitoring of Comet 9P/Tempel 1, the Deep Impact target

Abstract

Comet 9P/Tempel 1, the target of the Deep Impact Mission, has been monitored for 6 months aiming at its characterization before the impact experiment. During this period of time, the coma has gone through a slow morphological evolution from a wide structure in the south-western quadrant in mid-February to a porcupine pattern in mid-April and up to seven features identified in June. In addition to this evolution, an arclet in the western coma hemisphere was detected on June 14, related to an outburst event. Interpretation of these features and their evolution seems to indicate the presence of at least 3 or 4 very active regions on the nucleus, consistent with the rotation axis being oriented close to previous solutions found by Belton et al. (2005), or close to the angular momentum vector of the orbital motion of the comet. The value of Afρ varies with heliocentric distance as r h-6.71; slightly enhanced Afρ (above the r h-6.71 curve) was observed from mid-February until the end of March, when fan-shaped structures appeared in the coma for the first time. Somewhere between mid-April to mid-May (i.e. 80 to 60 days before perihelion), the comet peaked in activity. In terms of gas production rates, CN, C2 and C3 have been obtained at rh ∼ 1.7, 1.60 and 1.51 AU, being slightly below those derived from previous passages. Abundance ratios of these species indicate that comet 9P/Tempel 1 is classified as typical in terms of C2 abundance. The surface brightness profiles of the continuum, either azimuthally averaged profiles from the broadband images or in north-south direction from the long-slit spectra can be well fit with -1.9 ≤ m ≤ -1.14 in log B - log p representation. Steeper slopes are obtained at larger rh which might be related to variable dust size distribution with distance from the nucleus due to the radiation pressure dynamics and/or physical processing of the dust grains (sublimation, fragmentation). Normalized color of the dust inside the coma in the north-south direction is measured to be ∼20-30%/100 nm. © ESO 2006.