Evidence for the early onset of aspherical structure in the planetary nebula formation process: Spectropolarimetry of post-AGB stars

Abstract

We present optical spectropolarimetry of 31 post-asymptotic giant branch (post-AGB) stars, objects that are believed to represent the first phases of the transition from the AGB to the planetary nebula stage. 24 of these objects are found to be intrinsically polarized. We group these objects into four classes based on their observed polarization properties, rho (lambda) and theta (lambda), and discuss possible explainations for the observed behaviors. Type 1 objects display high levels of polarization and large position angle rotations. Type 2 objects lack large position angle rotations, but have polarizations too large to be attributed to foreground interstellar material. Type 3 objects show position angle rotations and polarization changes across TiO absorption features; and finally, Type 4 are objects in which the observed polarization can be entirely attributed to interstellar effects. The currently popular paradigm of planetary nebula morphology of a dense torus plus bipolar lobes can explain the rho and theta behavior of the Type 1 and Type 2 objects. However, a large number of the objects exhibit time-variable rho and theta. This implies that their morphologies may not be stable, but rather evolving or transient structures. Regardless of any specific model for the morphology, our main result is that aspherical structure appears very early in the transition from the AGB to the planetary nebula stage. We compare the polarization properties of the post-AGB stars to the morphological characteristics of evolved planetary nebulae. Our results indicate that the nebular morphology may originate at an early evolutionary stage. We do not observe any correlation between chemistry (O-rich vs C-rich) and polarization class in the AGB stars. We examine the formation of aspherical planetary nebulae in the context of binary star evolution. If bipolar geometry is a consequence of binary star evolution, the number of highly polarized post-AGB stars in our sample requires a high binary star frequency.