DEVELOPMENT OF COMA AND TAILS OF C/1996 B2 HYAKUTAKE

Part II: April 1 - July 30, 1996
(version April 15, 1996)
Rob L.W. van de Weg

To arrive at a forecast for the development of the coma and the plasma and dust tails of a comet is not simple, because there are many factors playing a role. Usually only the orbit and the absolute brightness of an object are given. A mistake in the brightness forecast of one magnitude is considered as acceptable. However, this may lead to a coma diameter or a tail length that is more than two times smaller or larger. Nevertheless, it is important for observers to be prepared for what can be expected.

Part I of this forecast was prepared in order to follow the close approach of the earth. Part II is focused on the effects of the perihelion passage. For this forecast the most recent orbital elements by Yeomans were used. Observations of coma and tails indicate an absolute brightness of about h0 = 4.8 magn, which is consistent with the observed brightness. This absolute brightness was used for the results below.

Details of the forecast are given in the table below. Summarizing, it is expected that in the second half of April the diameter of the coma will drop from 1 degree to about 5 arcminutes. The length of the plasma tail is until mid-April about 50 degrees. In the second half of April the plasma tail will shorten rather drastically allthough the gas production increases very fast. The reverse process will happen after perihelion passage. This phenomenon was observed in the last 25 years only for comet Kohoutek in 1973 and comet West in 1976. The dust tail will become more and more dominant in the second half of April. After perihelion passage the coma diameter will grow again to maximum diameter of about 14 arcminutes. The plasma tail will increase again rapidly in length to 20 - 25 degrees around May 17 - 20. After perihelion passage the dust tail of comet Hyakutake is a wide fan of almost 5 degrees long and well separated from the plasma tail. In June and July the comet will become a more and more diffuse object.

The values for the coma diameter and tail lengths assume perfect observational circumstances (limiting magnitude near the comet of about 6.5 magn). Under moderate circumstances very probably less than half of the calculated values would be recorded.

 
date        coma diameter   plasma tail       dust tail
                            length  PA        length  PA
            arcminutes      degrees degrees   degrees degrees   
       

01 apr 1996    59           70       47        5.9     47
06 apr 1996    33           54       47        5.3     49
11 apr 1996    21           49       45        5.2     48
16 apr 1996    14           51       41        5.2     46
21 apr 1996     9.4         26       34        5.3     42
26 apr 1996     5.9         11       23        6.2     35
01 may 1996     4.2          3.5    353        6.4     22
06 may 1996     4.8          2.9    259        6.7      1
11 may 1996     6.5          8.8    228        5.2    345
16 may 1996     8.3         18      220        3.9    236
21 may 1996     9.9         20      216        2.1    226
26 may 1996    11           16      214        2.4    228
31 may 1996    13           12      213        3.0    221
05 jun 1996    13            9.4    212        3.2    220
10 jun 1996    14            7.2    211        3.0    219
15 jun 1996    14            5.6    210        2.9    218
20 jun 1996    14            4.3    209        2.9    217
25 jun 1996    13            3.3    208        2.9    215
30 jun 1996    13            2.5    206        2.9    213
10 jul 1996    12            1.4    198        3.0    206
20 jul 1996    10            0.8    187        3.4    194
30 jul 1996    7.8           0.4    174        3.7    183

PA = Position Angle
     (0 = North, 90 = East, 180 = South, and 270 = West).

Observations of coma diameters and the length of both tails are
very welcome to:
Rob L.W. van de Weg
e-mail: vandeweg@cs.utwente.nl

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