Comet Hyakutake gas arcs: First observational evidence of standing shock waves in a cometary coma

We present the discovery and observations throughout the period March 25-Ap ril 5, 1996, of CL and CN are structures centered on the anti-sunward axis of comet Hyakutake (C/1996 B2), We interpret them as the signatures of the interaction between tao supersonic rarefied gas jets: H2O vapor emanating f rom the comet nucleus on one hand and gas flowing from a secondary source c entered on the anti-sunward direction on the other. An upgraded version of the Gasdynamic code described in Crifo et al. (1995, 1997a) is used to mode l the interaction. To accommodate low gas densities, Navier-Stokes equation s are used instead of Euler equations, and photochemical effects are introd uced to compute the water-group secondary molecules' distributions. A doubl e-shock H2O structure, characteristic of the interaction between two opposi te supersonic flows, is found. Associated with it is an are-shaped distribu tion of OH, In view of the dominance of the collisions between and with H2O molecules, the distribution of any: other primary molecules with lifetimes comparable to that of H2O (e.g., HCN) will be identical to that of H2O, Th e spatial distribution of their daughter products (e.g., CN) will be simila r to that of OH, if their lifetimes are comparable to that of OH. We show t hat, given the limitations of the observations and of the modeling method, it is not possible to derive a unique solution in terms of secondary source properties and of near-nucleus night-side production. We show in particula r that the solution proposed by Harris er al, (1997) for the companion OH a rcs is only one of the possible solutions, and that, in conflict with what those authors find, it does result in a standing shock structure between th e nucleus and the source. The successful simulations of the arcs presented here constitute the first observational evidence for the formation of shock waves in neutral: cometary atmospheres, originally predicted by Kitamura ( 1990) and subsequently advocated as an essential process in the formation o f the circumnuclear coma by Crifo et al. (1995, 1997a, b), (C) 1998 Academi c Press.