Asymptotic giant branch (AGB) stars appear to lose mass spherically, but many planetary nebulae (PNe) resulting from the spherical AGB mass-loss have non-spherical morphologies. It is likely that the mechanism that causes the heavy AGB mass loss is related to the mechanism that dictates the ejecta's departure from sphericity. Disks are known to exist around evolved, mass-losing AGB and post-AGB stars and are suspected to play a fundamental role in the shapes of PNe. Theoretical work envisions their role as the collimating agent resulting in non-spherical mass-loss. Compact disks have been found in some bipolar PNe, but their role in the shaping process remains unknown. Compact Keplerian disks are found to be common around post-AGB binaries; however, these objects may never develop into PNe as no nebulae are observed. Collimated nebulae shining by reflected light or shock ionisation surround another group of post-AGB stars, known as pre-PNe. Pre-PNe are thought to be the immediate precursors to non-spherical PNe. We have begun the first systematic survey of pre-PNe, with the Very Large Telescope Interferometer (VLTI). We seek to detect disks left behind by the shaping process and compare these disks to those around other post-AGB stars and PNe. At present, three out of four pre-PNe observed have shown evidence of disks in their inner-circumstellar regions. One of them, IRAS 16279-4757 shows evidence for a disk similar to those seen in young PNe in addition to a smaller disk inside this disk. Observations were also carried out for IRAS 17347-3139 and IRAS 17150-3224. The initial results show evidence of a compact source in the centre of these targets as well. We are conducting detailed radiative transfer models to derive disk parameters such as inner and outer radii, scale-height, mass, and inclination. Future observations of more pre-PNe are still to come.
|Number of pages||1|
|Publication status||Published - 2012|
|Event||Meeting of the American Astronomical Society (219th : 2012) - Austin, TX|
Duration: 8 Jan 2012 → 12 Jan 2012
|Conference||Meeting of the American Astronomical Society (219th : 2012)|
|Period||8/01/12 → 12/01/12|