Near-IR bispectrum speckle interferometry, AO imaging polarimetry, and radiative transfer modeling of the proto-planetary nebula Frosty Leonis

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: murakawa@mpifr-bonn.mpg.de
² Subaru Telescope, 650 North A'ohoku place, Hilo, HI 96720, USA

Received: 6 September 2007
Accepted: 16 July 2008

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~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°. 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 μm at latitudes between -2° and +2°, and 0.005 ≤ a ≤ 0.7 μm 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 2.8510^-3 , a gas mass-loss rate of 8.9710^-3  yr^-1, and a total envelope mass of 4.23 .