Abstract

Aims. We combined bispectrum speckle interferometry, adaptive optics (AO) imaging polarimetry, and radiative transfer modeling of polarized light to derive various physical properties of the proto-planetary nebula Frosty Leo. Methods. We performed bispectrum K'-band speckle interferometry and H- and K-band imaging polarimetry of Frosty Leo using the ESO 3.6 m telescope and the AO-equipped CIAO instrument of the 8 m Subaru telescope, respectively. Two-dimensional radiative transfer modeling was carried out in order to obtain a quantitative interpretation of our observations. Results. Our diffraction-limited speckle image shows distinct hourglass-shaped, point-symmetric bipolar lobes, an equatorial dust lane, and complex clumpy structures in the lobes. Our polarimetric data display a centro-symmetric polarization vector pattern with P similar to 30-50% in the bipolar lobes and a polarization disk between them. The polarization images also reveal an elongated region with low polarization along a position angle of -45 degrees. The observations suggest that this region has a low dust density and was carved out by a jet-like outflow. Our radiative transfer modeling can simultaneously explain the observed spectral energy distribution, the intensity distribution of the hourglass-shaped lobes, and our polarization images if we use two grain species with sizes of 0.005 = a = 2.0 mu m at latitudes between -2 degrees and +2 degrees, and 0.005 = a = 0.7 mu m in the bipolar lobes. Assuming a distance of 3 kpc, an expansion velocity of 25 km s(-1), and a gas-to-dust mass ratio of 160, we derive a dust mass of the disk of 2.85 x 10(-3) M(circle dot), a gas mass-loss rate of 8.97 x 10(-3) M(circle dot) yr(-1), and a total envelope mass of 4.23 M(circle dot).