VISIR-VLT high- resolution study of the extended emission of four obscured post-AGB candidates

The onset of the asymmetry of planetary nebulae (PNe) is expected to occur during the late asymptotic giant branch (AGB) and early post-AGB phases of low- and intermediate-mass stars. Among all post-AGB objects, the most heavily obscured ones might have escaped the selection criteria of previous studies that detected extreme axysimmetric structures in young PNe. Since the most heavily obscured post-AGB sources can be expected to descend from the most massive PN progenitors, these should exhibit clear asymmetric morphologies. High-resolution observations of these sources should reveal marked bipolar morphologies, confirming the link between progenitor mass and nebular morphology. The heavily obscured sources in our sample do not show extreme axisymmetric morphologies. This is at odds with the expectation of highly asymmetrical morphologies in post-AGB sources descending from massive PN progenitors, which, is otherwise supported by observations of bright mid-IR unobscured sources. The sources presented in this paper may be sampling critical early phases in the evolution of massive PN progenitors, before extreme asymmetries develop.