The discovery of two new bipolar proto-planetary nebulae: IRAS 16594-4656 and IRAS 17245-3951

Abstract

We report the discovery of two new, bipolar proto-planetary nebulae (PPNs). Both are cool IRAS sources for which we have confirmed optical counterparts by our 10 μm observations. Ground-based visible and infrared photometry was combined with the IRAS photometry and spectroscopy to produce their spectral energy distributions (SEDs). These SEDs look like those of other PPNs, in particular those of bipolar PPNs. The central stars of both objects are highly reddened and have color temperatures ∼3000-4000 K. The nebulosities are dominated by scattered light, not emission lines as in planetary nebulae. IRAS 16594-4656 appears to possess the 21 μm emission feature seen previously in a dozen carbon-rich PPNs, along with the 8 μm polycyclic aromatic hydrocarbon feature. Published millimeter-wave observations support the notion that it is carbon-rich, while IRAS 17245-3951 appears to be oxygen-rich. These facts confirm that these two objects are PPNs in transition between the asymptotic giant branch and planetary nebula phases. Hubble Space Telescope imaging reveals that they are indeed bipolar nebulae. IRAS 17245-3951 clearly displays two lobes separated by a dust lane; thus it is viewed nearly edge-on. Two jetlike features are seen in the southern lobe of IRAS 17245-3951, similar to the base of the searchlight beams seen in AFGL 2688. IRAS 16594-4656 appears to be a bipolar nebulae viewed at an intermediate orientation, and both the lobes and the central star can be seen. IRAS 16594-4656 therefore gives us our first clear example of the apparent morphology of a bipolar PPN viewed at an intermediate orientation. The addition of these objects to the list of bipolar PPNs confirms that such bipolar morphologies develop early in post-AGB evolution.