2. Observations and data reduction
Comet Hale-Bopp was observed on 14 March, 1997, at the Observatoire de Haute-Provence, France, using the CARELEC spectrograph (slit length 5.5 arcmin, 1.1 arcsec/pixel, dispersion: 1.8 Å/pixel) at the 1.93m telescope. On 16 April 1997, spectra of comet Hale-Bopp were taken using the ISIS double beam spectrograph (slit length: 4 arcmin, 0.36 arcsec/pixel) at the 4.2m William-Herschel-Telescope (WHT) of the Isaac-Newton-Group telescopes at La Palma, Spain, with a dispersion of 0.4 Å/pixel (see Table 1).
Table 1. The heliocentric distance, , and velocity, , the geocentric distance, , and velocity , and the phase angle, , are given.
Sodium emission is caused by resonant scattering of the incoming solar light. Therefore, the line intensity and acceleration of sodium atoms by solar radiation pressure depend strongly on the Doppler shift of Na emission with respect to the solar Fraunhofer absorption lines. In both observing periods the effective solar flux was relatively similar, resulting in accelerations for Na atoms of 24 cm s-2 and 22 cm s-2 on 16 April and on 14 March, 1997, respectively. However, as the Na atoms accelerate in the coma, they experience a decreasing acceleration in March and an increasing acceleration post-perihelion, caused by the shift of cometary sodium D-emission towards or away from the center of the solar Fraunhofer absorption lines. Therefore, the excitation and dynamics of sodium in the coma of Hale-Bopp can differ somewhat in March and April.
The spectra were bias subtracted, flatfielded and wavelength calibrated. Because of the unfavourable observing conditions for Hale-Bopp at perihelion, the comet could only be observed close to twilight. Determining the sky contribution was therefore difficult. The variable water content of the atmosphere results in contamination of the Na emission lines by telluric water absorptions, which cannot be completely removed. However, because of the different spectral regions and depending on the Doppler shifts, each Na line is affected differently. Therefore, we consider as real all structures which appear always in both emission lines. In March, an absolute flux calibration could be obtained, but in April, non-photometric conditions did not allow us to derive absolute fluxes. To measure Na emission, the underlying continuum by reflected solar light on cometary dust particles was subtracted. For the spectra obtained in March the solar spectrum was taken from a solar atlas (Kurucz et al., 1984), smoothed by convolution with a Gaussian profile to the resolution of the measured spectra. In April, the reflected solar light from the moon was measured.
© European Southern Observatory (ESO) 1998
Online publication: May 15, 1998