Ultraviolet Observations of Comet 96/P Machholz at Perihelion

Abstract

Ultraviolet spectra of Comet 96/P Machholz were obtained during its 2002 perihelion with the UltraViolet Coronagraph Spectrometer instrument on board the SOHO satellite. Emission from H i, C ii, C iii, and O i is detected near the nucleus. The outgassing rate is in line with the value extrapolated from rates at larger distances from the Sun, and abundances of C and O are estimated. Reconstructed images show a nearly spherical cloud of H i Lyα emission and an ion tail seen in C iii. Radiation pressure on the hydrogen atoms produces a modest distortion of the shape of the Lyα cloud as seen from SOHO and Doppler shifts up to 30 km s-1 in the outer parts of the cloud. We estimate a ratio of C to H2O similar to what is observed in other comets, so low carbon abundance does not account for the anomalously low C2 and C3 ratios to NH2 observed at optical wavelengths.