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? 1. New orbital elements by Yeomans were used. 2. 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. 3. 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. (Received at ESO on March 21, 1996)