MPI/ESO 2.2-m + EFOSC II (May 14, 1996)

Six low-dispersion spectra of the Comet Hale-Bopp were recorded from the European Southern Observatory in the vicinity of Munich during the period 9 March to 24 April, 1997. The spectra were obtained by Gerardo Avila, Jesus Rodriguez, Carlos Guirao (avid amateur astronomers from ESO) and Juan M. Alcala (not less and from the Max-Planck Institute for Extraterrestrial Physics) with a home-made spectrograph, attached to an amateur telescope with fibre optics. We present here the spectral tracings [GIF; 30k].

Sodium in Hale-Bopp

Of particular interest in these observations is the evolution of the Sodium emission line observed in the core of the comet. The corresponding spectral region is seen on these spectral tracings [GIF; 12k]. The line is clearly visible on top of the spectral continuum (the reflected sunlight from the dust) in the last two runs (21 and 24 April), just after the discovery of the Na tail (cf. the 23 April Update ).

Note that this sodium doublet (in our spectra it is blended due to the low resolving power) seems to appear already in the first spectrum obtained on March 9, 1997. However, its presence in that spectrum may be due to the background light of the close city of Freising or the Munich Airport which were located at angular distances of about 60 o and 80 o from the comet, respectively. Still, we believe that this is a real emission line from the comet since the exposure times were relatively short (around 80 sec). Unfortunately for this specific observation we have no nearly simultaneous, recorded sky exposure to confirm this assumption. On the following spectra, the Na line is not present (1, 7 and 8 April, 1997).

The equipment

A description of the equipment used for these observations follows:

- Telescope: 25cm Schmidt-Cassegrain, F/6.3 in a Alt-Az mount.

- Fibre optics: 0.2mm core diameter and 4m long. Sky aperture: 21 arcsec.

- Spectrograph: Collimator: Doublet lens with 300mm focal distance and f/6. Grating: 600 grooves/mm 50mm x 50mm blazed at 500nm. Camera optics: Photographic Objective, 100mm focal distance and F/2.2.

- CCD Camera: Hale Research (GB) equipped with a Tektronix CCD 512x512, 27 micron pixel, thermo-electric cooling system. Estimated read out noise: 100 electrons(!).

- Dispersion: 0.43nm/mm. Resolving power: 600 (1nm). Spectral range: 220 nm.

- Hardware and Software: Portable PC 486, 33 MHz, 16Mb RAM, 500Mb Harddisc. Linux 1.2.13 operating system with DOS emulator. The data reduction was performed with ESO-MIDAS packages release 95NOV pl2.0 installed in this PC.

- Data reduction: All the spectra were reduced using the contexts LONG and SPEC of the ESO MIDAS system. During each observation run, spectra of a helium-argon calibration lamp, flat-fields and spectrophotometric standard stars were recorded. Each of the Hale-Bopp comet spectra shown in the accompanying figures, except that on March 9, have been averaged from 3 or 4 individual exposures. In accordance to the sky clearness and altitude, the comet exposures ranged between 80 and 300 seconds. The spectra were also bias and dark-current subtracted. The relative flux correction was performed with the help of Etha Ursae Majoris (HR5191) as standard star. The spectra are not, however, corrected by the Sky Rayleigh dispersion.

Coming back to Hyakutake

The first result, which was already published on the web last year (click here) , shows the `standard' comet spectrum in the visible region. The second shows a relatively intense Na-line.

One important point in this connection is the location of the comet on the day of the observation when it was extremely low in the sky (about 15 o above the horizon!). Under these conditions, the background citylight pollution is particularly high. For this reason, we thought that this peak must have been produced by the Na street lights and unfortunately no `sky exposure' was recorded to evaluate its contribution in the spectrum.

However, on 24 April this year (1997) we took a similar exposure towards the same sky location with a mid-cloudy sky during 20 min and we did not find any Na pollution.

We transmit this result to the astronomical community in the hope that there is somebody somewhere who is able to confirm the presence of this emission line in Hyakutake.