Evidence for interacting gas flows and an extended volatile source distribution in the coma of comet C/1996 B2 (Hyakutake)

Science

Volumn 277, Issue 5326, 1997, Pages 676-681

  Harris, Walter M ^a   Combi, Michael R ^b   Honeycutt, R Kent ^c   Mueller, Béatrice E A ^d   Scherb, Frank ^e  

^a UNIVERSITY OF WISCONSIN MADISON   (United States)

^b UNIVERSITY OF MICHIGAN   (United States)

^c INDIANA UNIVERSITY   (United States)

^d NATIONAL OPTICAL ASTRONOMY OBSERVATORY   (United States)

^e Wisconsin Electric Machines and Power Electronics Consortium   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

ARTICLE; ASTRONOMY; GAS FLOW; IMAGE ANALYSIS; PRIORITY JOURNAL; VOLATILIZATION;

COSMIC DUST; GASES; ICE; METEOROIDS; WATER;

EID: 0030869029     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.277.5326.676     Document Type: Article

References (43)

1. All ephemeris data were supplied by D. Yeomans using the Naval Ancillary Information Facility program.
2. S. M. Larson, Z. Sekanina, D. Levy, S. Tapia, M. Senay, Astron. Astrophys. 187, 639 (1987).
3. H. U. Keller et al., ibid., p. 807.
4. E. P. Mazets et al., ibid., p. 699.
5. The WIYN [Wisconsin, Indiana, Yale, NOAO (National Optical Astronomical Observatories)] observatory is a 3.5-m telescope located at the Kitt Peak National Observatory (east longitude = -111°36.0′, latitude 31°57.8′, and altitude of 2120 m). The telescope employs active optical techniques to achieve ground-based seeing as good as 0.4″.
6. 3 contamination was present in the CN filter bandpass).
7. D. G. Schleicher, R. L. Millis, D. J. Osip, S. M. Lederer, Bull. Am. Astron. Soc. 28, 1089 (1996).
8. See W. Huggins [C.R. Acad. Sci. 10, 26 (1881)] for discussion of the CN band and C. Fehrenbach and C. Arpigny [ibid. 277B, 569 (1973)] for the OH band.
9. See W. Huggins [C.R. Acad. Sci. 10, 26 (1881)] for discussion of the CN band and C. Fehrenbach and C. Arpigny [ibid. 277B, 569 (1973)] for the OH band.
10. Standard processing for WIYN images includes subtraction of instrument bias and division by a flat field taken the same day as the observation. Images taken before September 1996 include an additional processing stage to account for nonlinearity at very low light levels that were present in the WIYN imager charge-coupled device at the time of these observations. The correction algorithms were supplied courtesy of T. Von Hippel (University of Wisconsin) and D. Silva (NOAO).
11. To emphasize small-scale spatial structure, each image was divided by a smoothed version of itself. The smoothed image was created by the application of a 4 arc sec (20 pixel) median filter. The remainder after this division consists of detailed features of the near-nuclear region, with a spatial scale smaller than the filter, that are difficult to detect above the bright background of the coma.
12. +, in our different approximations to the standard bandpasses used in comet observations. Our method of assuming that the projected dust jets within 250 km of the nucleus were devoid of CN or OH emission gives consistent results, but it can lead to an oversubtraction of the dust in the areas where gas and dust emission regions overlap. The importance of the oversubtraction for the shape and intensity of the arc features was minimal, because these structures are present only in the CN or OH images. We examined the effect of subtracting different percentages (±25% from best fit) of the dust emission from gas images, and, whereas the detailed structures in gas emission in the difference image are highly dependent on the accuracy of the subtraction, it did not affect the general differences in spherical symmetry between gas and dust discussed here.
13. D. C. Jewitt and K. J. Meech, Astrophys. J. 317, 992 (1987).
14. -1 over large spatial scales.
15. M. R. Combi and A. H. Delsemme, Astrophys. J. 237, 1026 (1980).
16. W. M. Harris, F. Scherb, M. R. Combi, B. E. A. Mueller, Bull. Am. Astron. Soc. 28, 1088 (1996).
17. At angles ≳100° from the sunward direction, an asymmetry begins to assert itself in the volatile radial distribution that favors the antisunward hemisphere, with the degree of asymmetry greatest at larger distances from the nucleus. Rather than reflecting a deviation from radial symmetry in the volatile source distribution, the additional signal appears to be from an unrelated local density enhancement associated with outgassing material in the tail. Whereas this feature becomes dominant at large angles from the sunward direction, it does not contribute at angles smaller than ∼100°, and can not explain the difference in overall symmetry between gas and dust.
18. Wide-field observations of the radial distribution of neutral oxygen [OI] (6300 Å) emission from Hyakutake with the narrow band Wisconsin H-alpha Mapper (WhαM) instrument at the Pine Bluff Observatory showed nearly perfect spherical symmetry in the coma out to more than 0.5° (L. M. Haffner, personal communication).
19. Y. Kitamura, Icarus 66, 241 (1986).
20. A. Korosmezy and T. I. Gombosi, ibid. 84, 118 (1990).
21. A. H. Delsemme and A. Wenger, Planet. Space Sci. 18, 709 (1970).
22. T. Mukai, S. Mukai, H. Fechtig, E. Grün, R. H. Giese, Adv. Space Res. 5, 339 (1985); M. S. Hanner, Icarus 47, 342 (1981).
23. T. Mukai, S. Mukai, H. Fechtig, E. Grün, R. H. Giese, Adv. Space Res. 5, 339 (1985); M. S. Hanner, Icarus 47, 342 (1981).
24. See A. H. Delsemme and D. C. Miller [Planet. Space Sci. 19, 1259 (1971)] for discussion of C/1960-II Burnham; M. F. A'Hearn, C. H. Thurber, and R. L. Millis [Astron. J. 82, 518 (1977)], M. F. A'Hearn and J. J. Cowan [Moon Planets 23, 41 (1980)], and K. Saito, S. Isobe, N. Nishioka, and T. Ishii [Icarus 41, 351 (1981)] for C/1975n West; and R. L. Newburn, J. F. Bell, and T. B. McCord [Astron. J. 86, 469 (1981)] for P/Ashbrook-Jackson.
25. See A. H. Delsemme and D. C. Miller [Planet. Space Sci. 19, 1259 (1971)] for discussion of C/1960-II Burnham; M. F. A'Hearn, C. H. Thurber, and R. L. Millis [Astron. J. 82, 518 (1977)], M. F. A'Hearn and J. J. Cowan [Moon Planets 23, 41 (1980)], and K. Saito, S. Isobe, N. Nishioka, and T. Ishii [Icarus 41, 351 (1981)] for C/1975n West; and R. L. Newburn, J. F. Bell, and T. B. McCord [Astron. J. 86, 469 (1981)] for P/Ashbrook-Jackson.
26. See A. H. Delsemme and D. C. Miller [Planet. Space Sci. 19, 1259 (1971)] for discussion of C/1960-II Burnham; M. F. A'Hearn, C. H. Thurber, and R. L. Millis [Astron. J. 82, 518 (1977)], M. F. A'Hearn and J. J. Cowan [Moon Planets 23, 41 (1980)], and K. Saito, S. Isobe, N. Nishioka, and T. Ishii [Icarus 41, 351 (1981)] for C/1975n West; and R. L. Newburn, J. F. Bell, and T. B. McCord [Astron. J. 86, 469 (1981)] for P/Ashbrook-Jackson.
27. See A. H. Delsemme and D. C. Miller [Planet. Space Sci. 19, 1259 (1971)] for discussion of C/1960-II Burnham; M. F. A'Hearn, C. H. Thurber, and R. L. Millis [Astron. J. 82, 518 (1977)], M. F. A'Hearn and J. J. Cowan [Moon Planets 23, 41 (1980)], and K. Saito, S. Isobe, N. Nishioka, and T. Ishii [Icarus 41, 351 (1981)] for C/1975n West; and R. L. Newburn, J. F. Bell, and T. B. McCord [Astron. J. 86, 469 (1981)] for P/Ashbrook-Jackson.
28. See A. H. Delsemme and D. C. Miller [Planet. Space Sci. 19, 1259 (1971)] for discussion of C/1960-II Burnham; M. F. A'Hearn, C. H. Thurber, and R. L. Millis [Astron. J. 82, 518 (1977)], M. F. A'Hearn and J. J. Cowan [Moon Planets 23, 41 (1980)], and K. Saito, S. Isobe, N. Nishioka, and T. Ishii [Icarus 41, 351 (1981)] for C/1975n West; and R. L. Newburn, J. F. Bell, and T. B. McCord [Astron. J. 86, 469 (1981)] for P/Ashbrook-Jackson.
29. Solar heating rapidly increases the temperature of the icy grains until they reach equilibrium or become hot enough to vaporize and break up. This is why larger, more reflective grains last longer than small dirty ones. Once a grain reaches its critical temperature, it begins to evaporate with greater spherical symmetry than the nucleus. When integrated over an entire halo of icy grains, this process produces a more spherically symmetric coma for a comet than a nucleus source, even if the IGH is itself nonspherical. This mechanism also accounts for the lack of a spherically symmetric dust halo. The gas liberated from the icy grains will expand adiabatically away at ∼0.8 km/s (or roughly twice the thermal speed of water at the vaporization temperature of 190 K); however, there is not enough molecular drag near the smaller grains to accelerate any dust that is produced. The dust will therefore remain in the vicinity of its parent grain until accelerated antisunward into the tail by radiation pressure rather than out into the spherical coma.
30. J. K. Harmon et al., Bull. Am. Astron. Soc. 28, 1195 (1996).
31. M. F. A'Heam, Proceedings of the 1996 Asteroid, Comet, and Meteor Conference, in press.
32. 2 and v ∼ 0.8 km/s as estimates for the collision cross section and radial flow velocity). This corresponds to a mean free path of 335 km (compared with the measured 240 km of the arc) at the r = 1000 km distance of the arc center point, and to 14,200 km at r = 6500 km. The scale of these values is consistent with the observed central width and widening of the large arc.
33. col = 13 km) is smaller than the width of the observed structures. A hydrodynamic model solution is indicated under these conditions.
34. M. R. Combi, Icarus 123, 207 (1996).
35. G. P. Tozzi, G. B. Field, E. Mannucci, P. Patriarchi, R. M. Stanga, Bull. Am. Astron. Soc. 28, 1089 (1996); G. P. Tozzi, personal communication.
36. G. P. Tozzi, G. B. Field, E. Mannucci, P. Patriarchi, R. M. Stanga, Bull. Am. Astron. Soc. 28, 1089 (1996); G. P. Tozzi, personal communication.
37. C. Laffont, P. Rousselot, J. Clairemidi, G. Moreels, ibid., p. 926.
38. + filter showed no evidence of either feature, and this argues against the possibility that either an ion or dust arc was present.
39. O increases to the point that outgassing becomes a significant force.
40. Successive runs of the DSMC model indicated that the contrast of these interaction regions is a maximum for a sun-comet-Earth (SCE) angle near 90°, and that visibility drops rapidly away from this geometry. The arc seen on 26 March would have been undetectable for viewing geometries of 45° > SCE > 135°.
41. 2 away from the nucleus. As a result, volatile sources close to the nucleus will generate interaction regions with more compact dimensions than more distant sources.
42. An arc associated with the secondary condensation at 3500 km would have a width at its apex of more than 3000 km, with its surface brightness reduced by a factor of 12 relative to what it would have been at the location of the bright arc. Assuming that the brightness of the visible dust condensation accurately mirrors gas production (15%), then this source would produce an arc with 1% of the surface brightness of the brighter source. This would not have been detectable in our images.
43. We acknowledge the generous contribution of time and effort by the staff at the WIYN observatory and, in particular, D. Sawyer for supporting such a large unexpected observing program. We also thank J. Percival for work in modifying the WIYN control system to permit tracking of a moving source, F. Roesler for donation of an OH filter, D. J. Lien for many useful conversations on the topic of halo debris, and J. Wisniewski for assisting in the reduction of the images. Supported by the WIYN consortium universities and NOAO, by NASA grants NAGW-3319, W18963, and NAG5-647 to the University of Wisconsin, and by NASA grant NASW-1907 to the University of Michigan.