2013A&A...551A..64B

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. We have obtained VISIR-VLT mid-IR images of a sample of four heavily obscured post-AGB objects that are barely resolved in previous Spitzer IRAC observations in order to analyze their morphology and physical conditions across the mid-IR. The images obtained in four different mid-IR filters have been deconvolved, flux-calibrated, and used to construct RGB composite pictures as well as color (temperature) and optical depth maps that allow us to study the morphology and physical properties of the extended emission of the sources in our sample. We have detected the extended emission from the four objects in our sample and resolved it into several structural components that are greatly enhanced in the temperature and optical depth maps. The morphologies of the sample, as well as their physical conditions, reveal the presence of asymmetry in three young PNe (IRAS15534-5422, IRAS17009-4154, and IRAS18454+0001), where the asymmetries can be associated with dusty torii and slightly bipolar outflows. The fourth source (IRAS18229-1127), a possible post-AGB star, is better described as a rhomboidal detached shell. 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.