On the Production and Kinematics of Sodium Atoms in the Head of Comet Hale-Bopp

The interpretation of the presence and extent of the atomic sodium observed in comet Hale-Bopp raises a number of questions related to the production, motion, and lifetime of these Na atoms under the action of the solar radiation near 1 AU. Here we report on data which pertain to the study of sodium in the head of the comet, yielding information on the Na emission in the coma out to ~ 0.15 x 10(6) km on the sunward side, to ~ 0.2 x 10(6) km tailwards. The observations were carried out at the Observatoire de Haute-Provence, with the 1.93 m telescope and the ELODIE spectrometer, which is fed by a pair of optical fibers with 2-arcsec apertures separated by 1.8 arcmin. In particular, spectra were obtained on 15-17 April, 1997, near the discovery date of the sodium tail at the La Palma Observatory. These spectra (resolution ~ 7 km/s) were used to determine Na D line profiles, as well as the relative intensities of the emissions corresponding to the two positions observed simultaneously. The main outcome of our first analysis concerns the profiles recorded some distance away from the optical centre, which all show velocity shifts and asymmetric line broadening in the direction of anti-sunward velocities. Some evidence is found to suggest the existence of an extended or multi-source production of sodium atoms, at least in the coma. Another series of spectra secured just before perihelion, 25-27 March, was analysed in the same way as the April spectra. Comparison between the two sets of data provides a nice illustration of the marked influence of the heliocentric radial velocity, not only upon the strength of the fluorescence ("Swings effect"), but also upon the motion of the emitting atoms, which are accelerated by the radiation pressure to widely different degrees as their excitation wavelengths fall at different places within the Fraunhofer Na D line profiles.