Proto-planetary nebulae as explosions: Bullets versus jets and nebular shaping

Many proto-planetary nebulae ( PPNs) appear as narrow collimated structures sometimes showing multiple, roughly aligned lobes. In addition, many PPN flows have been shown to have short acceleration times. In this paper we explore whether jet or "bullet'' ( a massive clump) models fit the observations of individual collimated lobes adequately by comparing simulations of both radiatively cooled ( stable) jets and bullets. We find that the clump model is favored over jets because (1) it leads to greater collimation of outflows, (2) it accounts better and more naturally for ringlike structures observed in the PPN CRL 618, and (3) it is more successful in reproducing the Hubble-flow character of observed kinematics in some PPNs. In addition, bullets naturally account for observed multipolar flows, since the likely MHD launch mechanisms required to drive outflows make multiple nonaligned jets unlikely. We also find that the bow shock heads of bullets take on a V-shaped configuration, whereas bow shock heads of jets are more U-shaped. The differences in these configurations occur on a linear scale corresponding to an angular size of the order of similar to 3 '' - sufficiently large to suggest a viable means of distinguishing bullets from jets in observations. We argue that PPN outflows may be driven by explosive MHD launch mechanisms such as those discussed in the context of supernovae (SNe) and gamma-ray bursts (GRBs).