Near-infrared speckle imaging and AO polarimetry of the bipolar proto-planetary nebula

IRAS 09371+1212 is an oxygen-rich post-asymptotic giant branch (postAGB) star with an hourglass-like bipolar proto-planetary nebula (PPN), and has been well studied since its discovery because of several interesting peculiarities. It is also known as Frosty Leo because its unique, strong peak at 60 μm in the IRAS photometry was initially proposed to be due to emissions from water ice which condensed onto dust grains in the circumstellar envelope (Forveille et al. 1987). Rouan et al. (1988) identified the absorption feature attributed to water ice at 3.1 μm and found the optical depth τ3.1μm of ∼3.3. This value is comparable to that of the PPN OH 231.8+4.2 (τ∼2.5, Smith et al. 1988), but exceptionally larger than other oxygen-rich evolved stars (Meyer et al. 1998). The distinct bipolarity is remarkable. Rouan et al. (1988) first resolved the two bipolar lobes toward the south and north with a 2. 4 separation in the JHKL bands with the 3.6 m CFH telescope. Adaptive optics imaging detected the bright central star (Beuzit et al. 1994; Roddier et al. 1995). Dougados et al. (1990) and Scarrott & Scarrott (1990) performed imaging polarimetry in the JK bands and BVRI bands, respectively. The polarimetric data showed a centrosymmetric vector pattern in the bipolar lobes and a so-called polarization disk between them. This polarization signature implies the presence of an enhanced equatorial dust concentraion seen edge-on. While it has an axisymmetric appearance <5 from the central star, several complex structures such as ansae and jets have been detected (Sahai et al. 2000). It is thought that the formation of a bipolar nebula requires the factor of additional effects besides the superwind and the interacting stellar wind (Kwok 1993). Widely accepted interpretations argue that the most possible mechanism is interaction with binary companions (e.g. Balick & Frank 2002). High-resolution imaging and polarimetry with radiative transfer modeling is an important approach to provide us unique science results and a better understanding of when and how the mechanism works to shape the circumstellar dust shell.