3. Spatial distribution
Sodium emission is observed up to distances 2 105 km sunward in both observing periods (Figs. 1 and 2). In all spectra taken around the nucleus position, the sodium distribution is very asymmetric. In profiles along the sun-comet line the emission is always stronger towards the sunward direction. To compare Na and the dust continuum, both spatial profiles have been normalized to the same maximum intensity in Figs. 1a and 2a.
Sodium emission falls off less steeply than the dust emission and shows a sudden drop in intensity at about 5 104 km and 7 104 km in March and April, respectively (Figs. 1c and 2c). However, Na and dust show a similar general sunward/tailward asymmetry. The asymmetric continuum is indicative of preferred sunward outgassing of volatiles from the nucleus. The asymmetry in sodium reflects asymmetric outgassing from the nucleus itself and/or release from a non-isotropic parent distribution. Asymmetries are also found in spectra taken at the nucleus position with the slit along the direction perpendicular to the sun-comet line (Fig. 3). For both, sodium and continuum emission a simple distribution for outgassing of a parent species from the nucleus (following a distribution, : nucleocentric distance) cannot be applied.
Secondary maxima of the sodium intensity distribution are found sunward. Such maxima are also present when the slit is aligned perpendicular to the sun-comet line (Fig. 3), but not when offset by 2 arcmin tailward (Fig. 4). NH2 emissions present in our spectra do not show secondary maxima and only a relatively weak sunward/tailward asymmetry. However, similar secondary maxima are present in the continuum (Figs. 1b and 2b) where the slit cuts through dust jets and shells, which can be seen in optical images of Hale-Bopp's dust distribution.
Comparing sodium and dust intensities, it is important to note that the maxima present in the dust are weaker and their position and number does not correspond to the sodium emission.
© European Southern Observatory (ESO) 1998
Online publication: May 15, 1998