Astron. Astrophys. Suppl. Ser. 136, 245-256

Dust continuum imaging of C/1995 O1 (Hale-Bopp):

Rotation period and dust outflow velocity

J. Warell - C.-I. Lagerkvist - J.S.V. Lagerros

Send offprint request: Johan.Warell@astro.uu.se

Astronomiska Observatoriet, Box 515, SE-75120 Uppsala, Sweden

Received March 12; accepted November 4, 1998

Abstract:

Dust continuum imaging of comet C/1995 O1 (Hale-Bopp) was carried out with the Swedish Vacuum Solar Telescope (SVST) on La Palma, Spain, during the period April 21-25, 1997. CCD observations were made with $\lambda~550\ \rm nm$ and $\lambda~830\ \rm nm$ filters, with band-passes excluding major cometary gaseous emissions. Images were reduced according to standard procedure, aligned, averaged, navigated and enhanced with azimuthal renormalisation, rotational and temporal derivative and unsharp masking techniques.

The rotation period of the nucleus was determined to $11\hbox{$.\!\!^{\rm h}$}46$ and the mean projected dust outflow velocity to $0.41\ \rm km\ s^{-1}$measured over a radius range of $5\,000~-~45\,000\ \rm km$. No difference in quasi-simultaneous dust morphology between the filter wavelengths was seen. Up to five shell envelopes were visible in the sun-ward side of the coma out to a radius of $80\,000\ \rm km$, separated by a projected distance of $\sim$$15\,000-20\,000\ \rm km$ and spiralling towards smaller radii in the direction of local evening. Outer boundaries of shells were more sharply defined than inner, with shell intensity maxima occurring at $\sim$$35\hbox{$^\circ$}$ past the sun-comet line in the direction of rotation. Small scale inhomogeneities of size $1\,000-2\,000\ \rm km$ were seen at radii $\le 20\,000\ \rm km$. Two superposed shell systems with a relative lag angle of $\sim$$55\hbox{$^\circ$}$ were evident during the period of observation.

The sense of rotation of dust emission features indicates that the north pole of the nucleus was directed towards the Earth. The shape of the incomplete spiral shell pattern indicates that the angle between the line-of-sight and the rotation axis of the nucleus was not very large.

Key words: comets: individual: c/1995 01 Hale-Bopp -- techniques: image processing