eso9620 — Photo Release

Spectrum of Comet Hyakutake (3000-8000 A)

13 March 1996

A complete, optical spectrum of Comet Hyakutake was obtained with the ESO 1.52-m telescope (La Silla Observatory) by Hilmar Duerbeck (ESO) on UT March 8.3, 1996. For this observation, the Boller & Chivens spectrograph with a new UV-sensitive CCD chip was used. The subsequent data reduction was performed by the observer and Stefano Benetti (ESO) at ESO's office in Santiago de Chile.

The spectral region covered with this instrumental set-up ranges all the way from the cut-off of the terrestrial atmosphere at about 3000 A to the near-infrared region around 8000 A. This is the first spectrum of Hyakutake that covers the entire optical spectral region and shows the relative intensity of the major emissions, as well as the underlying continuum.

Please note that, due to lack of suitable UV-lines in the calibration spectrum, there is small wavelength shift at the very left end of the spectrum. The intensity calibration is also not very accurate here. The spectral resolution is about 9 A.

A few of the main lines and bands are identified. The strongest is from the OH radical (molecule) at 3090 A; this line is a direct indication of the current, strong evaporation of water from the cometary nucleus. Because of the low transparency of the terrestrial atmosphere at this ultraviolet wavelength, it is rarely observed from the ground, except in bright comets. The same is true for the NH feature at 3360 A.

The other emission lines are also present in earlier spectra obtained at ESO.

The underlying solar-type spectrum has a maximum at about 5500 A and represents sunlight reflected from the cometary dust grains. Several strong absorption lines are visible, e.g. the H and K lines at 3933 and 3968 A, between the CN and C3 emission bands.

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