CoCAM, WHT+UES

The 1997 International Time Project on comet Hale-Bopp was a great success and several major discoveries were made, including the first detection of a neutral gas tail, the observation of "cyanogen shells" and the detailed study of the rotation of the nucleus. Members of the Comet Hale-Bopp European Team participated in this project. This team was formed in early 1996 to co-ordinate European observing efforts because comet Hale-Bopp provided an extraordinary opportunity for scientists to observe a bright comet in great detail with the most advanced instrumentation, including the last generation of digital detectors.

Observations carried out to study the distribution of sodium atoms in comet C/1995 O1 Hale-Bopp led to the discovery of a new type of comet tail. Sodium atoms had previously been seen near the center of other comets, but these observations revealed for the first time a straight tail of sodium 6 degrees long.

The discovery images were taken with the CoCAM wide-field CCD camera, built and operated by staff at the Isaac Newton Group, set up next to the INT. CoCAM consists of a 35-mm camera zoom lens working at f/3.5 and imaging onto a 2220 × 1180 pixel EEV CCD chip, whose pixel size of  22.5m  square corresponds to 26", thereby achieving on the sky a total field of 17° × 9°.

On 16 April members of the European Comet Hale-Bopp Team made several exposures of the comet through a narrow filter that isolates emission from sodium atoms, and to their great surprise they found that these atoms were distributed over an enormous region in and around the comet. Contrary to earlier observations of bright comets near the Sun, the sodium was present not only in the region next to the cometary nucleus, but there were also large amounts in the region of the cometary tails.

Following a careful analysis of the observed distribution of these atoms, the astronomers concluded that comet Hale-Bopp displayed a third type of tail never seen before and consisting of sodium atoms.

Whereas the well-known ion and dust tails so prominently displayed by Hale-Bopp show a large amount of structure, the new sodium tail had a completely different appearance. It takes the form of a long tail approximately 600,000 km wide and 50 million km long, in a direction close but slightly different to that of the ion tail. While the electrically charged particles in the ion tail are accelerated to large velocities by the solar wind (very fast atomic particles emitted by the Sun), the sodium atoms are released from dust grains and then accelerated in the antisolar direction by simple fluorescence. These latter conclusions were achieved thanks to observations with the William Herschel Telescope.

Other interesting features reported by the European Comet Hale-Bopp Team include the spiral-jet and arc structures observed with the Jacobus Kapteyn Telescope in the inner coma of the comet. The astronomers made use of a CN and a blue continuum filter, obtaining an expansion velocity for CN of 1.3 km/s.
 
 

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1.- Discovery image of the sodium tail in Comet Hale-Bopp taken on 16 April. The tail appears as a very straight narrow feature extending from the head of the comet to the upper left. (a) [ Small GIF | GIF | TIFF ] This spectrum corresponds to the sodium tail 3.1° from the nucleus, obtained with the WHT plus the Utrecht Echelle Spectrograph (UES) on April 23.9 UT. Sky lines due to OH and Na are labelled. The cometary NaD lines are clearly observable due to an apparent Doppler shift of 144 km/s from the terrestrial emission (Cremonese et al, 1997, Astrophys J, 490, L200). (b) [ GIF | BMP ] The picture on the right is an image of Comet Hale-Bopp showing the usual ion and dust tails of the comet, taken a few minutes before the discovery of the sodium tail. The dust tail is the broad tail pointing straight upwards, while the ion tail is the filamentary structure to the left. Comparison of the two images shows how the sodium tail has a completely different appearance to the other tails of the comet.  2.- The 28th February saw first images of Hale-Bopp obtained at the INT Prime Focus camera. This image was taken through an RGO-B filter. [ GIF | BMP ] 3.- Filamentary ion tail of comet Hale-Bopp on 9 April. This is a 300 s exposure taken with CoCAM using a narrow band filter centered at 618.5 nanometers to isolate H2O+ emission. [ GIF ] 4.- CoCAM camera. [ JPG | BMP ] 5.- This image was taken by the IAC Hale-Bopp Team at the Auxiliary Port of the WHT on 4 February. A broad-band R filter was used for the observation and an exposure time of just two seconds. A complex series of twisted and spiral jets can be seen in the processed image, with many jets splitting into two, three, or even more parts. [ JPG ] 6.- This Z filter image was taken on 1 March with the WHT. Two dusty shells are visible. [ GIF ] 7.- Sum of four 10 s, three 20 s and five 5 s CoCAM exposures with no filters taken on 19 March. The filaments visible in the dust tail are synchronic bands of material, i.e., ejected at the same time from the nucleus. [ GIF ] 8.- Observations obtained by the European Comet Hale-Bopp Team with the JKT on 5 March revealed both spiral-jet and arc structures. These images were taken through a CN (cyanogen) and a H2O filter with Laplacian filtering applied to the reduced images. The CN image shows a faint arc below the nucleus due to the expansion of a shell of cyanogen-emitting dust. [ JPG ] 9.- 300 s CoCAM image of comet Hale-Bopp taken on 13 April through a H2O+ filter and a 3 s exposure of the inner coma using no filters (inset). Both images show the extraordinary possibilities of the wide field imaging device CoCAM. [ GIF ]

References
 

• G Cremonese et al, 1997, "Comet C/1995 O1 (Hale-Bopp)", IAU circular 6631.
• G Cremonese et al, 1997, "Comet C/1995 O1 (Hale-Bopp)", IAU circular 6634.
• G Cremonese et al, 1997, "Neutral Sodium from Comet Hale-Bopp: A Third Type of Tail", Astrophys J, 490, L199.
• A Fitzsimmons, 1997, "Comet C/1995 O1 (Hale-Bopp)", IAU circular 6638.
• D Pollacco, 1997, "Comet Hale-Bopp: First Light on CoCAM", Spectrum Newsletter, 14, 12.