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The onset of the asymmetry in planetary nebulae (PNe) occurs during the shorttransition between the end of the asymptotic giant branch (AGB) phase and thebeginning of the PN phase. Sources in this transition phase are compact andemit intensely in infrared wavelengths, making high spatial resolutionobservations in the infrared mandatory to investigate the shaping process ofPNe. Interferometric VLTI IR observations have revealed compelling evidence ofdisks at the cores of PNe, but the limited sensitivity, strong observationalconstraints, and limited spatial coverage place severe limits on the universaluse of this technique. Inspired by the successful detection of proto-planetarydisks using spectro-astrometric observations, we apply here for the first timethis technique to search for sub-arcsecond structures in PNe. Our exploratorystudy using CRIRES (CRyogenic high-resolution Infra-Red Echelle Spectrograph)commissioning data of the proto-PN IRAS 17516-2525 and the young PN SwSt 1 hasrevealed small-sized structures after the spectro-astrometric analysis of thetwo sources. In IRAS 17516-2525, the spectro-astrometric signal has a size ofonly 12 mas, as detected in the Brackett-gamma line, whereas the structuresfound in SwSt 1 have sizes of 230 mas in the [Fe III] line and 130 mas in theBrackett-gamma line. The spectroscopic observations required to performspectro-astrometry of sources in the transition towards the PN phase are lesstime consuming and much more sensitive than VLTI IR observations. The resultspresented here open a new window in the search of the small-sized collimatingagents that shape the complex morphologies of extremely axisymmetric PNe.

CRIRES-VLT high-resolution spectro-astrometry as a tool in the search of small structures at the cores of Planetary Nebulae