Post-perihelion coma monitoring of comet Hale-Bopp at ESO

The post-perihelion coma activity of Comet C/1995 O1 Hale-Bopp is monitored at ESO telescopes in La Silla and Paranal since Sept. 1997. Imaging through broadband filters in the visible and near-IR wavelength ranges allows to investigate the evolution of the dust coma, namely the appearance of jets, fans, shells and clouds. Long-term evolution: the comet had a porcupine-like embedded fan coma in autumn 1997 that evolved into a northern fan plus shell pattern in 1998 and remains like this since. Thus, the evolution of the coma structure post-perihelion was similar to that pre-perihelion at about the same heliocentric distances, but is occurred in reversed order. This long-term evolution can be characterized by quasi-continuous dust emission from a few (minimum 4) active regions (producing the fan structures) on the nucleus that is modulated by occasional, repetitive and short-term activity increases (generating shell features in the coma). Outbursts: a number of outbursts and unusual activity patterns occurred in the coma of the comet post-perihelion that are documented through the appearance of complex "palm-tree-like" structures of temporary nature in association with outbursts in the visual lightcurve of the comet and a series of 3 dust clouds resembling "mini-comets" and passing through the northern coma at projected velocities of 30-50m/s. The similarity of coma patterns and cometary viewing geometry from Earth before and after perihelion suggests that some nuclear regions had enhanced long-term activity, possibly driven by super-volatile ices at larger (>10AU) heliocentric distances and that the orientation of the rotation axis of the nucleus did not change much over the past 7 years.