Spectropolarimetry and radiative transfer modeling of three proto-planetary nebulae

We measured the spectrum of polarization for three proto-planetary nebulae (PPNs), IRAS 17411-2411, IRAS 08005-2356, and IRAS 04296+3429, and made model calculations with the dust-scattering Monte Carlo code DIRTY and the dust emission code 2Dust. We show that high levels of polarization in these PPNs correlate with extreme asymptotic giant branch (AGB) superwind mass-loss rates in excess of 10^-4 M⊙ yr^-1. All three objects show evidence for evacuated lobes cleared by collimated fast winds, and two indicate a significant equatorial mass enhancement. Our best-fit models require sharply peaked grain size distributions, suggesting that most of the light is being scattered by grains of a characteristic size in IRAS 17441-2411 and IRAS 08005-2356. IRAS 17441-2411 and IRAS 08005-2356 have lobes with wide opening angles, perhaps produced by deflection of a polar jet from an accreting companion by the AGB superwind. Modeling the spectropolarimetry of IRAS 04296+3429 indicates a point-symmetric, multipolar morphology in the PPN phase. The modeling of spectropolarimetry and other observations of PPNs provides a powerful way to constrain circumstellar morphology and dust parameters.