IRAS 17423-1755 (Hen 3-1475) revisited: an O-rich high-mass post-Asymptotic Giant Branch star

TL;DR

This study reclassifies IRAS 17423-1755 as an oxygen-rich high-mass post-AGB star, using high-resolution Spitzer spectra to clarify its chemistry and evolutionary stage, revealing water ice and nebular ionization features.

Contribution

First high-resolution spectrum confirming the O-rich nature of IRAS 17423-1755, correcting previous C-rich classification, and providing insights into its evolutionary status.

Findings

Confirmed O-rich chemistry via silicate absorption

Identified water ice as the cause of the 3.1 um feature

Detected nebular [Ne II] emission indicating ionization

Abstract

The high-resolution (R=600) Spitzer/IRS spectrum of the bipolar proto-planetary nebula (PPN) 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 um 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 um absorption feature seen in the Infrared Space Observatory (ISO) 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 um C2H2, 14.0 um HCN, etc.) or the classical polycyclic aromatic hydrocarbon infrared emission bands. Thus, the strong 3.1 um absorption band toward IRAS 17423-1755 has to be identified as water ice. In addition, a [Ne II] nebular emission line at 12.8 um is clearly detected, indicating that the ionization of its central region may be already started. The spectral energy distribution in the infrared (2-200 um) 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.