A study of H₂ emission in three bipolar proto-planetary nebulae: IRAS 16594-4656, Hen 3-401, and Rob 22

We have carried out a spatial-kinematic study of three proto-planetary nebulae, IRAS 16594-4656, Hen 3-401, and Rob 22. High-resolution H₂ images were obtained with NICMOS on the Hubble Space Telescope, and high-resolution spectra were obtained with the Phoenix spectrograph on Gemini-South. IRAS 16594-4656 shows a "peanutshaped" bipolar structure with H₂ emission from the walls and from two pairs of more distant, point-symmetric faint blobs. The velocity structure shows the polar axis to be in the plane of the sky, contrary to the impression given by the more complex visual image and the visibility of the central star, with an ellipsoidal velocity structure. Hen 3-401 shows the H₂ emission coming from the walls of the very elongated, open-ended lobes seen in visible light, along with a possible small disk around the star. The bipolar lobes appear to be tilted 10°-15° with respect to the plane of the sky, and their kinematics display a Hubble-like flow. In Rob 22, the H₂ appears in the form of an "S"-shape, approximately tracing out the similar pattern seen in the visible. H₂ is especially seen at the ends of the lobes and at two opposite regions close to the unseen central star. The axis of the lobes is nearly in the plane of the sky. Expansion ages of the lobes are calculated to be ∼1600 yr (IRAS 16594-4656), ∼1100 yr (Hen 3-401), and ∼640 yr (Rob 22), based on approximate distances.