2009A&A...505..825B

Observations of the CO J(1-0) 115GHz and J(2-1) 230GHz lines in comet C/1995 O1 (Hale-Bopp) were performed with the IRAM Plateau de Bure interferometer on 11 March, 1997. The observations were conducted in both single-dish (ON-OFF) and interferometric modes with 0.13km/s spectral resolution. Images of CO emission of between 1.7 and 3'' angular resolution were obtained. The ON-OFF and interferometric spectra show a velocity shift with sinusoidal time variations related to the Hale-Bopp nucleus rotation of 11.35h. The peak position of the CO images moves perpendicularly to the spin axis direction in the plane of the sky. This suggests that a CO jet is present, is active night and day with about the same extent, and is spiralling according to the nucleus rotation. The high quality of the data allows us to constrain the characteristics of this CO jet. We developed a 3D model to interpret the temporal evolution of CO spectra and maps. The CO coma is represented as the combination of an isotropic distribution and a spiralling gas jet, both of nucleus origin. The analysis of the spectra and visibilities obtained from the interferometric data shows that the CO jet contains ∼40% of the total CO production and is located close to the nucleus equator at a latitude ∼20° north. Our inability to reproduce all observational characteristics shows that the true structure of the CO coma is more complex than assumed, especially within the first thousand kilometres from the nucleus. The presence of another moving CO structure, faint but compact and possibly created by an outburst, is identified.