Comet Hyakutake Gas Arcs: First Observational Evidence of Standing Shock Waves in a Cometary Coma
Résumé
We present the discovery and observations throughout the period March 25–April 5, 1996, ofC2andCNarc structures centered on the anti-sunward axis of comet Hyakutake (C/1996 B2). We interpret them as the signatures of the interaction between two supersonic rarefied gas jets: H2O vapor emanating from the comet nucleus on one hand and gas flowing from a secondary source centered on the anti-sunward direction on the other. An upgraded version of the Gasdynamic code described in Crifoet al.(1995, 1997a) is used to model the interaction. To accommodate low gas densities, Navier–Stokes equations are used instead of Euler equations, and photochemical effects are introduced to compute the water-group secondary molecules' distributions. A double-shock H2O structure, characteristic of the interaction between two opposite supersonic flows, is found. Associated with it is an arc-shaped distribution 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. The spatial distribution of their daughter products (e.g., CN) will be similar to that of OH, if their lifetimes are comparable to that of OH. We show that, 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 particular that the solution proposed by Harriset al.(1997) for the companion OH arcs 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 the 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 of the circumnuclear coma by Crifoet al.(1995, 1997a, b).