DEVELOPMENT OF COMA AND TAILS OF C/1996 B2 HYAKUTAKE

Part I: January 1 - April 11, 1996
(version March 20, 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.

What has been changed since version February 25, 1996?

New orbital elements by Yeomans were used.
Herman Mikuz pointed out that there was a systematic difference between expected and observed coma diameters and tail lengths. Indeed, observed tail lengths were about 80% longer and observed coma diameters were 20% larger. This is caused by the fact that a pessimistic absolute brightness for the comet (h0 = 5.5 magn) was used. Observations of coma and tails indicate an absolute brightness h0 = 4.8 magn, which is consistent with the observed brightness. This absolute brightness was used for the results below.
The other change is that the second part including expectations for the development of coma and tails after April 11 will be posted separately.

Details of the forecast are given in the table below. Summarizing, it is expected that comet C/1992 B2 Hyakutake shows on March 25 a coma with a diameter of 2.5 degrees. The maximum length of the plasma tail may be about 90 degrees on March 27 - 29. However, it will be very difficult to observe the full length of the plasma tail.Because the tail will be strecthed out so far the contrast between plasma tail and sky background becomes very low. Further, the expected plasma tail length is based on the expected water production rate (the nucleus of a comet consists of 80 % water-ice) which is often highly variable.

The dust tail will remain rather unconspicious in March and begin April. Besides, plasma and dust tail will be seen superimposed as projected on the sky in March and April. The conditions for the visibility of the dust tail will improve highly end April and in May.

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   
       
02 jan 1996     2.5          0.03   273        0.03   283
07 jan 1996     3.2          0.05   274        0.05   284
12 jan 1996     3.9          0.07   276        0.07   284
17 jan 1996     4.6          0.11   277        0.10   284
22 jan 1996     5.5          0.16   278        0.13   285
27 jan 1996     6.4          0.23   279        0.15   285
01 feb 1996     7.5          0.33   280        0.19   286
06 feb 1996     8.7          0.47   281        0.22   286
11 feb 1996    10            0.68   282        0.28   286
16 feb 1996    12            0.97   282        0.35   286
21 feb 1996    14            1.4    283        0.45   286
26 feb 1996    17            2.1    283        0.60   286
02 mar 1996    21            3.0    283        0.81   285
07 mar 1996    27            4.7    283        1.2    284
12 mar 1996    36            7.5    281        1.7    282
17 mar 1996    55           13      274        2.2    274
21 mar 1996    91           23      256        2.8    256
22 mar 1996   106           28      248        3.0    250
23 mar 1996   123           35      237        3.1    239
24 mar 1996   141           47      226        3.4    228
25 mar 1996   151           63      217        3.6    217
26 mar 1996   146           79      207        3.9    206
27 mar 1996   130           89      150        4.2    150
28 mar 1996   111           92       55        4.4     55
29 mar 1996    93           89       48        4.7     48
30 mar 1996    79           83       47        5.0     47
01 apr 1996    59           70       47        5.3     47
06 apr 1996    33           54       47        5.1     49
11 apr 1996    21           49       45        4.8     48
 
PA = Position Angle
     (0 = North, 90 = East, 180 = South, and 270 = West).
 
Part II of this version of this forecast can be expected before
April 16.
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.
 
Reference: R.L.W. van de Weg, 'Predicting and analysing the tail
development of comet P/Halley', in the Proceedings of the
International Halley Seminar, Genk (Belgium), p. 91-99 (1987).  
****
P.S. I have plans to publish a paper on the improved model as
used here (where? suggestions?), after this comet has gone. Up
till this publication I will not provide improved formulas and
algorithms.
****

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