Comet 103P/Hartley 2 at perihelion: gas and dust activity

Abstract:
The comet 103P/Hartley 2, target of the EPOXI mission (NASA), was supposed to be observed for 3 days around its perihelion, from October 27 to 29, 2010, but photometric data were obtained only on October 27 and 29, 2010. On both dates, the comet visibility was not optimal due to its proximity to the Moon, as projected on the plane of the sky, whereas on October 28, the comet could not be observed at all. Aims. The goal of the campaign was to give ground support to the EPOXI mission by establishing a baseline of activity at perihelion to be compared with in situ activity observed by the space mission about 7 days later on Nov. 4, 2010. We aimed to assess gas and dust production rates, to study the gas and dust coma morphology, to investigate the behaviour of the refractory component by analysing the dust colour variations with date and with projected cometocentric distance, rho, and to determine the slope of the surface brightness profiles, B, as a function of rho. Methods. Long-slit spectra and optical broad-and narrowband images were acquired with the instrument ACAM mounted on the William Herschel Telescope (WHT) at La Palma Observatory. We investigated the evolution of the dust coma morphology from the images acquired with specific continuum cometary filters (in the blue and red wavelength region) with image-enhancing techniques. We studied (1) the gas and dust production rates; (2) the dust radial brightness profiles; (3) the profiles of the CN, C(2), C(3) and NH(2) column densities, and (4) the CN and C(3) coma morphologies. The dust and gas profiles were azimuthally averaged, as well as measured in both the E-W direction (similar to Sun-antisolar direction) and in a direction defined by the slit orientation at PA 70 to 250 degrees. Results. The morphological analysis of the dust coma reveals only one structure. Aside from the dust tail in the west direction, a bright jet is detected in images acquired on October 27 at 03:00-04:00 UT. This jet turns on and off and it is not clearly detected at any time on the images obtained during October 29. This structure is enhanced by making use of the radial renormalization and the Larson-Sekanina method. It is also confirmed by the distortion of the isophotes at the same position angle (PA). The Af rho parameter, a proxy to the dust production rate, and the gas (CN, C(3), C(2), and NH(2)) production rate, Q(i), have been measured at perihelion, r(h) approximate to 1.058 AU. The quotient Q(C2) /Q(CN) similar to 1.3 places 103P/Hartley 2 as a typical comet in terms of long-chain hydrocarbon abundance. The gas-to-dust mass ratio is similar to 3-6, indicating that 103P/Hartley 2 is a relatively gas-rich comet. At perihelion, Af rho, as measured in a circular aper! ture of similar to 4700 km ranges from similar to 60 cm in the blue to similar to 110 cm in the red, which indicates an overall change in the optical properties of the dust grains. On the other hand, the Af rho is rather stable in the innermost coma when it is computed from the spectroscopic measurements within several continuum spectral ranges from 482-685 nm. Both 2D dust colour maps and profiles in the directions imposed by the slit indicate that there are variations with rho with a trend towards bluer dust colour with increasing rho. This could indicate sublimation of ices as the cameras on board the EPOXI mission have shown. The average dust reddening is similar to 24%/100 nm. The azimuthally averaged surface brightness profiles of the continuum from the broad band images can be well fitted with m similar to 1 in the tail direction, whereas in the opposite direction the dust profiles are much flatter at rho <= 10 000 km. The azimuthally averaged profiles of the come! t images acquired with the blue and red continuum cometary fil! ters sho w a nominal behaviour of log B similar to -m log rho with m similar to 1.