Photometrical analysis of the Neck-Line Structure of Comet Halley

Abstract

In this paper we apply the quantitative analysis of Neck-Line Structures (NLS; M. Fulle and G. Sedmak, Icarus 74, 383–398 (1988) ) to the dust streamer and the real antitail which were detected in images of Comet Halley during April, May, and June, 1986 ( Z. Sekanina, S. M. Larson, G. Emerson, E. F. Helin, and R. E. Schmidt, Astron. Astrophys. 187, 645–649 (1987) ), and which were best explained by the NLS model ( M. Fulle, Astron. Astrophys. 181, L13-L14 (1987) ). The results concern the dust size distribution at very large sizes, the size-dependent dust ejection velocity from the inner coma. and the anisotropy of dust ejection. All the results are related to the time interval 1986, Jan 10–18. The size distribution which best fits the NLS isophotes is characterized by a power index u ≈ −4.0 for sizes between 0.2 and 2mm, and u ≈ −0.4 for sizes between 2 and 10 mm. Therefore we confirm the strong excess of large dust suggested by J. A. M. McDonnell et al. ( Astron. Astrophys. 187, 719–741 (1987) ) to explain the Giotto data, although these authors obtained a value u ≈ −3.0 for sizes between 0.2 and 10 mm on 1986, Mar 14. These differences of the u values are in agreement with the results of M. Fulle, C. Barbieri, and G. Cremonese ( Astron. Astrophys. 201, 362–372 (1988) ), which showed large variations of the size distribution between January and March. The dust ejection velocity can be expressed by v = 0.20 × (1 − μ) 1 7 ( km sec −1), where 1 − μ is the ratio between the solar radiation pressure force and the gravitational force, in agreement with the results obtained for Comet Bennett ( M. Fulle and G. Sedmak, Icarus 74, 383–398 (1988) ).