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.