The color dependent morphology of the post-AGB star HD161796

TL;DR

This study uses imaging polarimetry to reveal the highly aspherical dust shell of the post-AGB star HD 161796, showing wavelength-dependent optical thickness and supporting the idea that DUPLEX and SOLE planetary nebulae are the same structure viewed differently.

Contribution

First detailed polarimetric imaging of HD 161796's dust shell, demonstrating wavelength-dependent optical properties and clarifying nebula classification.

Findings

Shell is highly aspherical with equatorial density enhancement

Optical thickness varies with wavelength, changing nebula appearance

Central star is hotter than previously assumed

Abstract

Context. Many protoplanetary nebulae show strong asymmetries in their surrounding shell, pointing to asymmetries during the mass loss phase. Questions concerning the origin and the onset of deviations from spherical symmetry are important for our understanding of the evolution of these objects. Here we focus on the circumstellar shell of the post-AGB star HD 161796. Aims. We aim at detecting signatures of an aspherical outflow, as well as to derive the properties of it. Methods. We use the imaging polarimeter ExPo (the extreme polarimeter), a visitor instrument at the William Herschel Telescope, to accurately image the dust shell surrounding HD 161796 in various wavelength filters. Imaging polarimetry allows us to separate the faint, polarized, light from circumstellar material from the bright, unpolarized, light from the central star. Results. The shell around HD 161796 is highly aspherical. A clear signature of an equatorial density enhancement can be seen. This structure is optically thick at short wavelengths and changes its appearance to optically thin at longer wavelengths. In the classification of the two different appearances of planetary nebulae from HST images it changes from being classified as DUPLEX at short wavelengths to SOLE at longer wavelengths. This strengthens the interpretation that these two appearances are manifestations of the same physical structure. Furthermore, we find that the central star is hotter than often assumed and the relatively high observed reddening is due to circumstellar rather than interstellar extinction.