  The onset of the asymmetry in planetary nebulae (PNe) occurs during the short
transition between the end of the asymptotic giant branch (AGB) phase and the
beginning of the PN phase. Sources in this transition phase are compact and
emit intensely in infrared wavelengths, making high spatial resolution
observations in the infrared mandatory to investigate the shaping process of
PNe. Interferometric VLTI IR observations have revealed compelling evidence of
disks at the cores of PNe, but the limited sensitivity, strong observational
constraints, and limited spatial coverage place severe limits on the universal
use of this technique. Inspired by the successful detection of proto-planetary
disks using spectro-astrometric observations, we apply here for the first time
this technique to search for sub-arcsecond structures in PNe. Our exploratory
study using CRIRES (CRyogenic high-resolution Infra-Red Echelle Spectrograph)
commissioning data of the proto-PN IRAS 17516-2525 and the young PN SwSt 1 has
revealed small-sized structures after the spectro-astrometric analysis of the
two sources. In IRAS 17516-2525, the spectro-astrometric signal has a size of
only 12 mas, as detected in the Brackett-gamma line, whereas the structures
found in SwSt 1 have sizes of 230 mas in the [Fe III] line and 130 mas in the
Brackett-gamma line. The spectroscopic observations required to perform
spectro-astrometry of sources in the transition towards the PN phase are less
time consuming and much more sensitive than VLTI IR observations. The results
presented here open a new window in the search of the small-sized collimating
agents that shape the complex morphologies of extremely axisymmetric PNe.
