High-Resolution Near-Infrared Imaging and Polarimetry of Four Proto-Planetary Nebulae

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High-resolution near-infrared Hubble Space Telescope (HST) NICMOS (F160W, F222M) images and polarization (2 mum) observations were made of four bipolar proto-planetary nebulae (PPNs): IRAS 17150-3224, 17441-2411, 17245-3951, and 16594-4656. The first three of these are viewed nearly edge-on, and for the first time the central stars in them are seen. Color maps reveal a reddened torus between the bipolr lobes in the edge-on cases, with bluer lobes. The polarization values are high, with maximum values ranging from 40% to 80%. The polarization patterns are basically centrosymmetric, with some deviations in the low-polarization equatorial regions. For IRAS 17150-3224, circumstellar arcs are seen at 1.6 mum, along with a newly discovered loop in the equatorial region. Bright caps are seen at the end of the lobes, indicating that they are not open-ended. A distinct point-symmetric pattern is seen in the strengths of the polarization vectors, especially in IRAS 17150-3224. HST NICMOS observations provide a valuable complement to the WFPC2 visible images in deriving the basic structure of bipolar PPNs.