2011AJ....141...80M

The high-resolution (R ∼ 600) Spitzer/IRS spectrum of the bipolar protoplanetary nebula (PN) IRAS 17423-1755 is presented in order to clarify the dominant chemistry (C-rich versus O-rich) of its circumstellar envelope as well as to constrain its evolutionary stage. The high-quality Spitzer/IRS spectrum shows weak 9.7 µm absorption from amorphous silicates. This confirms for the first time the O-rich nature of IRAS 17423-1755 in contradiction to a previous C-rich classification, which was based on the wrong identification of the strong 3.1 µm absorption feature seen in the Infrared Space Observatory spectrum as due to acetylene (C₂H₂). The high-resolution Spitzer/IRS spectrum displays a complete lack of C-rich mid-IR features such as molecular absorption features (e.g., 13.7 µm C₂H₂, 14.0 µm HCN, etc.) or the classical polycyclic aromatic hydrocarbon infrared emission bands. Thus, the strong 3.1 µm absorption band toward IRAS 17423-1755 has to be identified as water ice. In addition, an [Ne II] nebular emission line at 12.8 µm is clearly detected, indicating that the ionization of its central region may be already started. The spectral energy distribution in the infrared (∼2-200 µm) and other observational properties of IRAS 17423-1755 are discussed in comparison with the similar post-asymptotic giant branch (AGB) objects IRAS 19343+2926 and IRAS 17393-2727. We conclude that IRAS 17423-1755 is an O-rich high-mass post-AGB object that represents a link between OH/IR stars with extreme outflows and highly bipolar PN.