European Southern Observatory

http://www.eso.org/
A Spectrum of Comet Hale-Bopp
11 September 1995

• Press Photo 27/95 [87K] shows such a spectrum, after it has been subjected to preliminary image processing. It is based on a Hale-Bopp spectrum, obtained in the morning of September 5, 1995, by visiting astronomer Birgitta Nordström (Copenhagen University Observatory, Denmark) with a CCD on the Boller & Chivens spectrograph at the ESO 1.5-metre telescope at La Silla. The exposure lasted 30 minutes and the slit was placed in the East-West direction. Hector Vega (ESO) assisted during this observation.

• As can be seen, there is no sign of CN and CO+ emission lines at the indicated wavelengths in this spectrum. They would have shown up as white lines, extending above and below the otherwise "flat" spectrum. The sensitivity of this observation was apparently not sufficient to detect such emissions, if present at all. Indeed, this equipment was not optimized for this particular kind of observation which was made in the course of another observing programme, concerned with galactic stars of low metal abundance.

European Southern Observatory

http://www.eso.org/
25 August 1995

• The ESO observations are of many different types and have involved many observers. At the 15-metre Swedish-ESO Submillimetre Telescope (SEST), Albert Nummelin, Anne-Marie Lagrange and Thierry Forveille searched on August 3-4 and 9-10 for emissions from the CO molecule. According to one theory, CO gas may possibly be the driving agent that is responsible for `lifting' dust particles off a comet's nucleus when it is more than about 750 million kilometres from the Sun. However, no emission from CO was seen to the sensitivity limit of these observations, thus placing important constraints on the proposed mechanism.

• Normally, CN is one of the first gaseous molecules to be detected in the coma of comets approaching the Sun. For instance, in Comet Halley, emissions from CN were first seen at a heliocentric distance of about 725 million kilometres. It would therefore be of great interest to learn whether CN is already now present in the coma of Comet Hale-Bopp. Spectroscopic observations with the ESO 1.5-metre telescope were performed by Anne-Marie Lagrange, Jean Luc Beuzit, Stephane Guisard and Pierpaolo Bonfanti on August 3-4 and 9-10. They have now been reduced and do not show any such emission. At the present distance of the comet from the Sun, the temperature is too low for water ice (the major component of cometary nuclei) to evaporate efficiently, and with the non-detection of CO and CN, the driving gas that has produced the well visible dust cloud around the nucleus of Comet Hale-Bopp is still unknown.