EXPANSION OF HYDROGEN-POOR KNOTS IN THE BORN-AGAIN PLANETARY NEBULAE A30 AND A78

Abstract

We analyze the expansion of hydrogen-poor knots and filaments in the\nborn-again planetary nebulae A30 and A78 based on Hubble Space Telescope (HST)\nimages obtained almost 20 yr apart. The proper motion of these features\ngenerally increases with distance to the central star, but the fractional\nexpansion decreases, i.e., the expansion is not homologous. As a result, there\nis not a unique expansion age, which is estimated to be 610-950 yr for A30 and\n600-1140 yr for A78. The knots and filaments have experienced complex dynamical\nprocesses: the current fast stellar wind is mass loaded by the material ablated\nfrom the inner knots; the ablated material is then swept up until it shocks the\ninner edges of the outer, hydrogen-rich nebula. The angular expansion of the\nouter filaments shows a clear dependence on position angle, indicating that the\ninteraction of the stellar wind with the innermost knots channels the wind\nalong preferred directions. The apparent angular expansion of the innermost\nknots seems to be dominated by the rocket effect of evaporating gas and by the\npropagation of the ionization front inside them. Radiation-hydrodynamical\nsimulations show that a single ejection of material followed by a rapid onset\nof the stellar wind and ionizing flux can reproduce the variety of clumps and\nfilaments at different distances from the central star found in A30 and A78.\n