EVIDENCE FOR THE EARLY-ONSET OF ASPHERICAL STRUCTURE IN THE PLANETARY-NEBULA FORMATION PROCESS - SPECTROPOLARIMETRY OF POST-AGB STARS

We present optical spectropolarimetry of 31 post-asymptotic giant bran ch (post-AGB) stars, objects that are believed to represent the first phases of the transition from the AGB to the planetary nebula stage. 2 4 of these objects are found to be intrinsically polarized. We group t hese objects into four classes based on their observed polarization pr operties, P(lambda) and theta(lambda), and discuss possible explanatio ns for the observed behaviors. Type 1 objects display high levels of p olarization and large position angle rotations. Type 2 objects lack la rge position angle rotations, but have polarizations too large to be a ttributed to foreground interstellar material. Type 3 objects show pos ition angle rotations and polarization changes across TiO absorption f eatures; and finally, Type 4 are objects in which the observed polariz ation can be entirely attributed to interstellar effects. The currentl y popular paradigm of planetary nebula morphology of a dense torus plu s bipolar lobes can explain the P and theta behavior of the Type 1 and Type 2 objects. However, a large number of the objects exhibit time-v ariable P and theta. This implies that their morphologies may not be s table, 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 pla netary nebula stage. We compare the polarization properties of the pos t-AGB stars to the morphological characteristics of evolved planetary nebulae. Our results indicate that the nebular morphology may originat e at an early evolutionary stage. We do not observe any correlation be tween chemistry (O-rich vs C-rich) and polarization class in the AGB s tars. 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 sta rs in our sample requires a high binary star frequency.