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

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5. Discussion

The overall extended sodium distribution in the inner coma of comet Hale-Bopp is clearly not in agreement with a pure nuclear source combined with free radial outflow and tailward acceleration by solar resonance fluorescence. In this case an extent of only a few 103 km sunward would be expected. An extended Na cloud can be caused by the existence of an extended source, in addition to a nucleus source, or by hindering the effective tailward acceleration on the Na atoms.

A possible external source for Na could be a molecular parent (sublimated either directly from the nucleus, or from dust particles), or release of Na directly from dust grains (e.g. Huebner 1970; Oppenheimer 1980), for example by sputtering. This would be in agreement with the implication of low velocities for the Na parent from high-resolution Doppler shift measurements (Arpigny et al. 1998). The sudden decrease in Na intensity on the sunward coma side could then give an indication for the destruction scale length of the Na parent. Observations of Na originating from dust particles in the large-scale dust tail (Cremonese et al., 1997b; Fitzsimmons et al. 1997) support the link of sodium to the dust. In addition, the general sunward/antisunward asymmetry in the coma found for Na and dust, but not for NH2, would be also in agreement with release of sodium from dust particles.

A comparison of tailward features in Na and [FORMULA] in comet Halley has led to suggest an ionic parent for sodium (Combi et al. 1997). In the innermost coma of comet Hale-Bopp strong continuum intensities do not allow us to measure the weak ion emissions in our spectra. A comparison of the [FORMULA] distribution in the cross-tail spectrum on 14 March does not show any similarities to sodium and does not suggest a link between sodium and ionic species.

However, care must be taken when comparing sodium intensities to possible parent molecules, because a strict correlation with spatial structures of its parent is not to be expected. Strong radiation pressure forces act on Na because of its high fluorescence efficiency, but not as efficiently on its parents. The sodium distribution is therefore expected to differ from the neutral and ionic coma. The situation would be further complicated if sodium is a granddaughter product. A release similar to CN could be possible for which sublimation of a parent from small dust particles has been proposed (e.g.: A'Hearn et al., 1986).

The need for an extended source must be compared to other mechanisms which could result in an increase of the sodium cloud. Collisional coupling of Na to water molecules can increase the size of the sodium coma (Combi et al., 1997). However, an estimate of the resulting coupling region in Hale-Bopp (Arpigny et al., 1998) shows that this effect alone is insufficient to explain the total extent of the sodium coma in Hale-Bopp. The spectral irradiance within the sodium line profiles can reach intensity levels comparable to that of the Sun inside the coma, and internal radiative pressure could become important. When taking collisional coupling and internal radiative pressure effects into account, a central source of Na could play a larger role.

The combined effect of a nuclear source with an extended coma source is consistent with the observed spatial sodium distribution. The formation of the observed structures in the long-slit intensity profiles needs to be further investigated through detailed modeling of the sources and dynamics in the coma. This will yield useful information related to the nature of the sources themselves.

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© European Southern Observatory (ESO) 1998

Online publication: May 15, 1998