Modeling of Jets from Comet Hale-Bopp (C/1995 O1): Observations from the Vainu Bappu Observatory

Abstract

Jet and shell structures from Comet Hale-Bopp during the period October 1996 to October 1997 using data from the Vainu Bappu Observatory are modeled. Evolution of jet and shell structures during the period of observations could be attributed to activity from sources near +65 deg, +35 deg, +5 deg, -5 deg, -35 deg, -65 deg in latitude. Although no deliberate attempt was made to place the sources at symmetric latitudes, the best fit shows a remarkable symmetry. Due to changing solar illumination geometry, while only the southern sources were apparently active during 1996, most of the sources appeared active in February 1997 and October 1997. In the 10 April image, under a near pole-on solar illumination, curiously, only the sources at +65 deg, +5 deg, and -5 deg appear to be active and not the source at +35 deg. Pole positions which gave a reasonably good fit to the observed sets of shells varied from 260 deg to 290 deg in right ascension and -50 deg to -65 deg in declination. Around 10 April, while the gas production from the equatorial sources is expected to be low due to near grazing incidence of the Sun light, the fractional area of the high latitude source is limited for a spherically symmetric nucleus. To explain the well-defined shells due to rotation of the jets, we assume that the source at +65 deg occupies 10% of the longitude belt and 10 deg in latitude. Assuming a radius of 35 km for the comet corresponding to the upper limit of H. A. Weaver and P. L. Lamy (1999, Earth Moon Planets, in press) and Z. Sekanina (1999b, Earth Moon Planets, in press), the area of 56 km^2 for this source could account for 29% of the observed total production rate of 4.0x10^30 molecules s^-1 by D. G. Schleicher et al. (1997, Bull. Am. Astron. Soc. 29, 1033). The intricate shell patterns during February-May 1997 could not be exactly replicated using the model of a single nucleus. Complexity of the shell structures can be better explained using the binary model of Comet Hale-Bopp suggested by Sekanina (1999a, Earth Moon Planets, in press; 1998b, Astrophys. J. 509, L133-L136) with sizes of 70 and 30 km across. Approximate estimates of the longitude of the sources active between February and May 1997 are presented. The lower limits on the dust to gas mass production ratio are estimated to be, 4.8+/-1.1, 3.4+/-1.0, and 6.2+/-1.3 on 18 February, 10 April, and 2 May, 1997, respectively.