CFHT, Current Image of the Week


November 29th, 1999

Satellites, Rings and Arcs around planet Neptune

Each week, discover a new spectacular image obtained at CFHT. Browse the archive

Satellites, Rings and Arcs around planet Neptune
Credit: Image courtesy of C. Roddier, F. Roddier, J.E. Graves, M. Northcott & O. Guyon,
Institute for Astronomy, Hawaii
(See also the refereed paper: Sicardy, B., et al. 1999, Nature, 400, 731.)

This picture is a deep, false-color composite image of Neptune showing faint orbiting objects. The top image is as observed, with the bright Neptune artificially masked to enhance contrast. The bottom panel shows Neptune in silhouette, the orbits of the satellites Proteus, Larissa and Galatea (the light blue lines) and the Le Verrier and Adams rings (white lines). The three satellites appear on the left side as red spots. They are elongated due to their orbital motion during the 600sec exposure. The Adams ring arcs are seen at western elongation (to the right of Neptune), just outside Galatea's orbit.

Why are these observations important?
All four giant planets (Jupiter, Saturn, Uranus & Neptune) exhibit ring systems. However, only Neptune have 'ring arcs', a collection of incomplete narrow rings named Liberté, Égalité, Fraternité and Courage. Interestingly, these 'ring arcs' should be destroyed rapidly (over a few months) by differential motion if they were in complete isolation. Yet, since first discovered by stellar occultations in 1984, they appear to have persisted. Gravitational resonance effects involving the satellite Galatea in particular have been invoked to explain their long lifetime. These observations of the ring arcs obtained at CFHT indicate that the 'arcs' are near, but not within the resonance with Galatea, in contrast to what is predicted and therefore calling for a revision of the models.

Technical description: This image was obtained by co-addition of nine 600-s exposures taken with Hokupa'a, the University of Hawaii adaptive optics system, attached to the Canada-France-Hawaii Telescope (CFHT) on July 6th, 1998. Data were acquired through a narrow-band filter centered on a methane absorption band at 1.72µm, where Neptune is darker because it reflects and scatters less light from the Sun. The image is 12 arcsec across, with a pixel size of 0.035 arcsec. North is up and east is to the left. The Adaptive Optics technique compensates image degradation produced by the Earth's atmosphere. Neptune itself (behind the black mask) was used as a guide source to probe the wave-front aberrations.

next week: Messier 31 & NGC 205

editors: François Ménard & Jean-Charles Cuillandre
[] & []

Copyright © 1999 by CFHT. All rights Reserved.

CFHT is funded by the Governments of Canada and France, and by the University of Hawaii.