Astron. Astrophys. 334, L61-L64 (1998)

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.

Fig. 1. Spatial sodium and continuum distribution on 14 March 1997 (3:51 UT, exposure time: 60 s) along the radial direction. a:  Na D2 line intensity (solid line) and continuum distribution (dashed-dotted line) over nucleocentric distance along the sun-comet line. Intensities are normalized to one at maximum intensity. The Sun direction is to the left. b, c: Continuum b and Na D2  c distribution over nucleocentric distance on logarithmic scales. Solid line: sunward, dashed line: tailward direction. A distribution corresponding to an inverse distance law is shown for comparison (dotted line).

Fig. 2. Spatial sodium and continuum distribution on 16 April 1997 (20:22 UT, exposure time: 120s) along the radial direction. a: Na D2 line intensity (solid line) and continuum distribution (dashed-dotted line) over nucleocentric distance along the sun-comet line. The intensities are normalized to one at maximum intensity. The Sun direction is to the left. b, c: Continuum b and Na c distribution over nucleocentric distance along the sun-comet line on logarithmic scales. solid line: sunward, dashed line: tailward direction. A distribution corresponding to an inverse distance law is shown for comparison (dotted line).

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.

Fig. 3. Normalized sodium (solid line) and continuum (dashed-dotted line) distribution on 16 April 1997 (21:19 UT, exposure time: 120s) perpendicular to the radial direction, over nucleocentric distance in the sky plane.

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.

Fig. 4. Normalized sodium (solid line) and continuum (dashed-dotted line) distribution on 14 March 1997 (4:58 UT, exposure time: 120s) perpendicular to the radial direction and offset by 1.6 105 km tailward, over distance in the sky plane. The difference of the position of sodium and dust illustrates the efficient radiative acceleration of the sodium atoms along the antisolar direction in comparison to the more slowly moving dust particles, resulting finally in two seperate tails.

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

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