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

The onset of 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 a universal use of this technique. Inspired by the successful detection of proto-planetary disks using spectro-astrometric observations, we apply this technique here for the first time to search for subarcsecond structures in PNe. Our exploratory study using CRyogenic high-resolution Infra-Red Echelle Spectrograph (CRIRES) 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+/-5mas, as detected in the Br gamma line, whereas the structures found in SwSt 1 have sizes of 230 +/- 29 mas in the [Fe III] line and 130 +/- 21 mas in the Br 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 for the small-sized collimating agents that shape the complex morphologies of extremely axisymmetric PNe.