A Quadrupolar Preplanetary Nebula: IRAS 19475+3119

Our imaging of the preplanetary nebula IRAS 19475+3119 (hereafter I19475) at 0.43 and 0.6 μm with HST reveals a quadrupolar nebula of size about 10.5'' × 4.7'', with two bipolar elongated lobes emanating from the center of the nebula. One of the bipolar lobes shows detailed point-symmetric structure with respect to the central star. A faint, surface brightness-limited, diffuse halo surrounds the lobes. Interferometric observations of the CO J = 1-0 line with OVRO at ~8'' resolution show that the bulk of the emission comes from an unresolved molecular envelope expanding at 15 km s -1 , resulting from the AGB progenitor's dense, slow wind. Weaker emission is seen from a fast bipolar outflow oriented along the longer and more tenuous pair of the sets of lobes. Optical spectroscopy reveals a complex Hα profile with a broad photospheric absorption feature and a narrow inverse P Cygni shaped core; comparison with previous data shows that the core profile shape varies dramatically with time. We find an unresolved source of millimeter-wave continuum emission in I19475, which together with previous submillimeter continuum measurements, implies a very substantial mass (roughly a few times 0.01 M ☉ ) of large (radius ≳1 mm), cold (~30 K) dust grains in I19475. Combining our estimates of the circumstellar mass with the typical mass of a post-AGB star, we find that the main-sequence progenitor of I19475's central star had a mass ≳2.5 M ☉ . We discuss the formation of I19475's quadrupolar nebula in the light of past and current ideas for the dramatic transformation of the morphology and kinematics of mass ejecta as AGB stars evolve into planetary nebulae.