The Structure and Dynamics of the Proto-Planetary Nebula M1-92

We present high-resolution (10) maps of ¹³CO J = 2-1 in the proto-planetary nebula M1-92, Minkowski's Footprint, obtained with the Institut de Radioastronomie Millimétrique (IRAM) interferometer at Plateau de Bure. We confirm the main components found in our previous works: a central disklike condensation, a bipolar double-shell structure with axial outward velocity increasing with the distance to the star, and two opposed features at the tips of the nebula where the maximal deprojected velocity is attained, ~70 km s^-1. The major quality of the present data allows us to estimate the very small width of the double-shell walls, ~06 (2 × 10¹⁶ cm), and the diameter of the central disk, 2''-3'' (10¹⁷ cm). The whole structure is probably the remnant of the previous asymptotic giant branch (AGB) shell after being shocked by the bipolar post-AGB jets. The mass of the molecular envelope is about 0.9 M_☉, and its kinetic momentum and energy (released by the wind interaction) are ~3 × 10³⁹ g cm s^-1 and ~7 × 10⁴⁵ ergs, respectively. Since the interaction time must be significantly smaller than the age of the nebula, 900 yr, these figures imply very energetic post-AGB jets that cannot be driven by radiation pressure. We also notice that the inner disklike structure is too large for collimating the very narrow post-AGB jets. We propose that reaccretion of material, ejected during the previous AGB phase, is the most likely mechanism to explain the strongly bipolar and very energetic post-AGB ejections.